Document:

Exhibit 10.6

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                          REGISTRATION RIGHTS AGREEMENT

                                 by and between

                         NORTH AMERICAN SCIENTIFIC, INC.

                                       AND

        THE INDIVIDUALS AND ENTITIES LISTED ON THE SIGNATURE PAGES HERETO

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                             Dated as of May 4, 2004
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            This REGISTRATION RIGHTS AGREEMENT, dated as of May 4, 2004 (this
"Agreement"), is entered into by and between North American Scientific, Inc., a
Delaware corporation (the "Acquiror"), and the individuals and entities listed
on the signature page hereto (individually, a "Holder" and collectively, the
"Holders").

                                    RECITALS

            WHEREAS, the Acquiror and AM Capital I, Inc., a Delaware corporation
and wholly-owned subsidiary of the Acquiror ("AM Capital"), and NOMOS
Corporation, a Delaware corporation ("NOMOS"), have entered into an Agreement
and Plan of Merger dated as of October 26, 2003 (as amended from time to time,
the "Merger Agreement"), pursuant to which NOMOS will be merged with and into AM
Capital; and

            WHEREAS, the parties desire to set forth herein their agreements as
to the piggyback registration rights of each of the Holders with respect to such
Holder's shares of Common Stock, as part of and as consideration for the
transactions contemplated by the Merger Agreement;

            WHEREAS, the execution and delivery of this Agreement by the
Acquiror with the Holders is a condition to closing the transactions
contemplated by the Merger Agreement.

            NOW, THEREFORE, in consideration of the premises and the mutual
promises herein made, and in consideration of the representations, warranties
and covenants herein contained, and for other good and valuable consideration,
the receipt and sufficiency of which are hereby acknowledged, the parties hereto
agree as follows:

            1. Definitions.

            As used herein, unless the context otherwise requires, the following
terms have the following respective meanings:

            "Acquiror" has the meaning set forth in the preamble hereof.

            "Agreement" has the meaning set forth in the preamble hereof.

            "Business Day" means any day except Saturday and Sunday and any day
that shall be a U.S. federal legal holiday or a day on which banking
institutions in the State of Delaware generally are authorized or required by
law or other governmental action to close.

            "Commission" means the United States Securities and Exchange
Commission or any other federal agency which shall at some future point in time
be administering the Securities Act.

            "Common Stock" means the common stock, par value $.01 per share, of
the Acquiror.

<PAGE>

            "Exchange Act" means the United States Securities Exchange Act of
1934, as amended, and the rules and regulations promulgated thereunder.

            "Indemnified Party" has the meaning set forth in Section 7(c).

            "Indemnifying Party" has the meaning set forth in Section 7(c).

            "Initial Period" means (i) 18 months from the date hereof with
respect to the Holder, John A. Friede, and (ii) six months from the date hereof
with respect to all other Holders.

            "Holder" or "Holders" has the meaning set forth in the preamble
hereof.

            "Losses" has the meaning set forth in Section 7(a).

            "Maximum Offering Size" has the meaning set forth in Section 3.

            "Merger Agreement" has the meaning set forth in the Recitals hereof.

            "Person" means any individual, corporation, limited liability
company, association, partnership, trust or any other entity or organization,
including any government entity.

            "Proceeding" means an action, claim, suit, investigation or
proceeding (including, without limitation, an investigation or partial
proceeding, such as a deposition), whether commenced or threatened.

            "Prospectus" means the prospectus included in any Registration
Statement (including, without limitation, a prospectus that includes any
information previously omitted from a prospectus filed as part of an effective
registration statement in reliance upon Rule 430A promulgated by the Securities
Act), as amended or as supplemented by any and all prospectus and all other
amendments and supplements to the prospectus, including supplements and as
amended by any and all post-effective amendments and including all material
incorporated by reference or deemed to be included by reference in such
prospectus.

            "Registrable Securities" means (i) the shares of Common Stock or
(ii) any security into or for which such Common Stock has been converted or
exchanged in connection with a combination of shares, recapitalization, merger,
consolidation or otherwise; provided, however, that Registrable Securities shall
not include any securities (A) the sale of which shall have been registered
pursuant to the Securities Act and which shares have been sold pursuant to such
registration or (B) that shall have been sold pursuant to Rule 144.

            "Registration Expenses" means all expenses incurred in effecting any
registration pursuant to this Agreement including, without limitation, all
registration, qualification and filing fees, printing expenses, escrow fees,
fees and disbursements of counsel for the Company and one special counsel for
the Holders, blue sky fees and expenses and expenses of any regular or special
audits incident to or required by any such registration, qualification and
filing.

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            "Registration Statement" means any registration statement under the
Securities Act on an appropriate form (which form shall be available for the
sale of the Registrable Securities in accordance with the intended method or
methods of distribution thereof and shall include all financial statements
required by the Commission to be filed herewith) which covers Registrable
Securities pursuant to the provisions of this Agreement, including the
Prospectus included in such registration statement, amendments (including
post-effective amendments) and supplements to such registration statement, and
all exhibits to and all material incorporated by reference in such registration
statement.

            "Rule 144" means Rule 144 under the Securities Act, as such Rule may
be amended from time to time, or any similar rule or regulation hereafter
adopted by the Commission.

            "Securities Act" means the United States Securities Act of 1933, as
amended, and the rules and regulations promulgated thereunder.

            "Selling Expenses" means all underwriting discounts, selling
commissions and stock transfer taxes applicable to the sale of Registrable
Securities and fees and disbursements of counsel for any Holder (other than the
fees and disbursements of one special counsel to the Holders included in
Registration Expenses).

            "Selling Holders" means Holders offering Registrable Securities for
sale pursuant to a registration effected under this Agreement.

            "Shelf Registration Statement" means any shelf Registration
Statement pursuant to Rule 415 under the Securities Act, or any similar rule or
regulation hereafter adopted by the Commission, on Form S-3 (or any substitute
form that may be adopted by the Commission).

            "Underwritten Registration" or "Underwritten Offering" means a
registration in connection with which securities of the Acquiror are sold to an
underwriter for reoffering to the public pursuant to an effective registration
statement.

            2. Piggyback Registration.

            (a) If the Acquiror proposes to file a Registration Statement (other
than on a registration statement on Form S-4 or S-8 (or any substitute form that
may be adopted by the Commission)) with respect to an offering of Common Stock
for its own account pursuant to an Underwritten Offering, then the Acquiror
shall provide written notice of such proposed filing to each Holder as soon as
practicable (but in no event less than 20 days before the anticipated filing
date of such Registration Statement). If within 15 days after receipt of such
notice, any Holder requests the inclusion in such Registration Statement of some
or all of the Registrable Securities held by such Holder, the Acquiror shall use
its reasonable best efforts to include such Registrable Securities in such
Registration Statement; provided, however, that during the Initial Period (i)
each Holder shall only be permitted to participate in such registration if the
Acquiror proposes to include the securities of another Person in such
Registration Statement and (ii) each Holder shall only be permitted to request
the inclusion in such Registration Statement of, and the Acquiror

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shall only be required to use its reasonable best efforts to include, up to 25
percent of the Registrable Securities held by each such Holder.

            (b) If the Acquiror acquires another Person pursuant to a merger,
asset sale, stock transfer or other acquisition transaction and, in connection
with such transaction, the Acquiror proposes to file a Shelf Registration
Statement related to the resale of any restricted securities of the Acquiror
received by any Person in connection with such transaction, then the Acquiror
shall provide written notice of such proposed filing to each Holder as soon as
practicable (but in no event less than 20 days before the anticipated filing
date of such Shelf Registration Statement). If within 15 days after receipt of
such notice, any Holder requests the inclusion in such Shelf Registration
Statement of some or all of the Registrable Securities held by such Holder, the
Acquiror shall use its reasonable best efforts to include such Registrable
Securities in such Shelf Registration Statement; provided, however, that during
the Initial Period, each Holder shall only be permitted to request the inclusion
in such Shelf Registration Statement of, and the Acquiror shall only be required
to use its reasonable best efforts to include, up to 25 percent of the
Registrable Securities held by each such Holder.

            3. Reduction of Offering.

            If the managing underwriters of an Underwritten Offering advise the
Acquiror in writing that, in their opinion, the amount of securities of the
Acquiror proposed to be sold in such offering, together with the Registrable
Securities requested to be sold by the Holders under Section 2 hereof and any
securities requested to be sold by any other Person, exceeds the amount of
securities of the Acquiror which can be sold in such offering without adversely
impacting the marketing of the securities being sold by the Acquiror in such
offering or otherwise materially and adversely impacting the success of such
offering, there shall be included in such offering, in the following listed
priority, only the amount of such securities of the Acquiror which, in the
opinion of such managing underwriters, can be sold (the "Maximum Offering Size")
without materially and adversely impacting the marketing of the securities being
sold by the Acquiror in such offering or otherwise materially and adversely
impacting the success of such offering:

            (i) first, all the shares of Common Stock which the Acquiror
      proposes to sell for its own account in such offering;

            (ii) second, all Registrable Securities requested to be included in
      such offering pursuant to Section 2 and any securities requested to be
      included in such offering by any other Person that is entitled to
      piggyback registration rights (allocated, if necessary for such offering
      not to exceed the Maximum Offering Size, pro rata among the Holders and
      such other Persons on the basis of the relative number of shares so
      requested to be included in such Underwritten Offering by each such Holder
      and other Person); and

            (iii) thereafter, any other securities requested to be included in
      such offering.

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            4. Registration Expenses.

            All Registration Fees incident to the performance of or compliance
with this Agreement by the Acquiror shall be borne by the Acquiror. All Selling
Expenses relating to the securities registered on behalf of the Holders shall be
borne by the Holders included in such registration pro rata among each other on
the basis of the number of Registrable Securities so registered.

            5. Registration Procedures.

            In the case of each Shelf Registration Statement effected by the
Company, the Company will keep each Holder advised in writing as to the
initiation of each registration and as to the completion thereof. At its
expense, the Company will do each of the following with respect to such Shelf
Registration Statement:

            (a) Keep such registration effective for a period of ending on the
earlier of the date which is sixty (60) days from the effective date of the
Shelf Registration Statement or such time as the Holder or Holders have
completed the distribution described in the Shelf Registration Statement
relating thereto.

            (b) Prepare and file with the Commission such amendments and
supplements to such Shelf Registration Statement and the Prospectus used in
connection with such Shelf Registration Statement as may be necessary to comply
with the provisions of the Securities Act with respect to the disposition of all
securities covered by such Shelf Registration Statement for the period set forth
in subsection (a) above;

            (c) Furnish such number of Prospectuses, including any preliminary
prospectuses, and other documents incident thereto, including any amendment of
or supplement to the Prospectus, as a Holder from time to time may reasonably
request;

            (d) Notify each seller of Registrable Securities covered by such
Shelf Registration Statement at any time when a Prospectus relating thereto is
required to be delivered under the Securities Act of the happening of any event
as a result of which the Prospectus included in such Shelf Registration
Statement, as then in effect, includes an untrue statement of a material fact or
omits to state a material fact required to be stated therein or necessary to
make the statements therein not misleading or incomplete in light of the
circumstances then existing, and following such notification promptly prepare
and furnish to such seller a reasonable number of copies of a supplement to or
an amendment of such Prospectus as may be necessary so that, as thereafter
delivered to the purchasers of such shares, such Prospectus shall not include an
untrue statement of a material fact or omit to state a material fact required to
be stated therein or necessary to make the statements therein not misleading or
incomplete in light of the circumstances then existing;

            (e) Provide a transfer agent and registrar for all Registrable
Securities pursuant to such Shelf Registration Statement and a CUSIP number for
all such Registrable Securities, in each case not later than the effective date
of such registration; and

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            (f) Notwithstanding anything contained herein to the contrary, in
connection with any registration contemplated hereunder, the Acquiror may
require the Selling Holders and the underwriters, if any, to furnish the
Acquiror such information regarding the Selling Holders and the underwriters, if
any, and the distribution of the Selling Holders' Registrable Securities as the
Acquiror may from time to time reasonably request.

            6. Holdback Agreement.

            In the event any registration of equity securities of the Acquiror
shall be made in connection with an Underwritten Offering, upon the written
request of the Acquiror and an underwriter of Common Stock or other securities
of the Acquiror, each Holder agrees not to effect any sale or distribution,
including any sale pursuant to Rule 144 under the Securities Act, of any
Registrable Securities, and not to effect any such sale or distribution of any
other equity security of the Acquiror or of any security convertible into or
exchangeable or exercisable for any equity security of the Acquiror (in each
case, other than as part of such Underwritten Offering) during the 15 days prior
to, and during the 90-day period beginning on, the effective date of the
registration statement for such Underwritten Offering; provided that the Holders
have such written request at least 15 days prior to the filing date of the
Registration Statement and provided, further, that all officers and directors of
the Acquiror (other than those participating in the Underwritten Offering) also
agree not to effect any such sale or distribution during such period.

            7. Indemnification.

            (a) Indemnification by the Acquiror. The Acquiror shall,
notwithstanding any termination of this Agreement, indemnify and hold harmless
each Selling Holder and each of its officers, directors, agents and employees,
each Person who controls the Acquiror (within the meaning of Section 15 of the
Securities Act and Section 20 of the Exchange Act), and the officers, directors,
agents and employees of each such controlling Person, to the fullest extent
permitted by applicable law, from and against any and all losses, claims,
damages, liabilities, costs (including, without limitation, reasonable costs of
investigation and reasonable attorneys' fees and expenses) and expenses
(collectively, "Losses"), as incurred, arising out of or relating to (i) any
untrue or alleged untrue statement of a material fact contained in the
Registration Statement, any Prospectus or any form of prospectus or in any
amendment or supplement thereto or in any preliminary prospectus, (ii) any
omission or alleged omission of a material fact required to be stated therein or
necessary to make the statements therein (in the case of any Prospectus or form
of prospectus or supplement thereto, in light of the circumstances under which
they were made) not misleading or (iii) any violation or alleged violation by
the Company of the Securities Act, any state securities laws or any rule or
regulation thereunder applicable to the Company and relating to action or
inaction required of the Company in connection with any offering covered by such
registration, qualification or compliance, except to the extent that any such
untrue statement or omission is based solely upon information regarding such
Selling Holder furnished to the Acquiror in writing by such Selling Holder
stated specifically for use therein.

            (b) Indemnification by the Selling Holders. Each Selling Holder
shall, severally and not jointly, indemnify and hold harmless the Acquiror, its
officers, directors, agents and employees, each Person who controls the Acquiror
(within the meaning of Section 15 of the

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Securities Act and Section 20 of the Exchange Act), and the officers, directors,
agents and employees of each such controlling Person, to the fullest extent
permitted by applicable law, from and against all Losses as incurred, arising
out of or relating to (i) any untrue statement of a material fact contained in
the Registration Statement, any Prospectus, or any form of prospectus, or in any
amendment or supplement thereto or (ii) any omission of a material fact required
to be stated therein or necessary to make the statements therein (in the case of
any Prospectus or form of prospectus or supplement thereto, in light of the
circumstances under which they were made) not misleading to the extent that such
untrue statement or omission is contained in any information so furnished by
such Selling Holder to the Acquiror in writing specifically for inclusion in the
Registration Statement, such Prospectus or such form of prospectus, or in any
amendment or supplement thereto; provided, however that in no event shall any
indemnity under this Section 7(b) exceed the net proceeds from the offering
received by such Holder.

            (c) Conduct of Indemnification Proceedings. If any Proceeding shall
be brought or asserted against any Person entitled to indemnity hereunder (an
"Indemnified Party"), such Indemnified Party shall promptly notify the Person
from which indemnity is sought (the "Indemnifying Party") in writing, and the
Indemnifying Party may assume the defense thereof, including the employment of
counsel reasonably satisfactory to the Indemnified Party and the payment of all
fees and expenses incurred in connection with the defense thereof; provided that
the failure of any Indemnified Party to give such notice shall not relieve the
Indemnifying Party of its obligations or liabilities pursuant to this Agreement,
except to the extent that such failure shall have proximately and adversely
prejudiced the Indemnifying Party.

            An Indemnified Party shall have the right to employ separate counsel
in any such Proceeding and to participate in the defense thereof, but the fees
and expenses of such counsel shall be at the expense of such Indemnified Party
unless (1) the Indemnifying Party has agreed in writing to pay such fees and
expenses, (2) the Indemnifying Party shall have failed promptly to assume the
defense of such Proceeding and to employ counsel reasonably satisfactory to such
Indemnified Party in any such Proceeding or (3) the named parties to any such
Proceeding (including any impleaded parties) include both such Indemnified Party
and the Indemnifying Party and such Indemnified Party shall have been advised by
counsel that a conflict of interest is likely to exist if the same counsel were
to represent such Indemnified Party and the Indemnifying Party (in which case,
if such Indemnified Party notifies the Indemnifying Party in writing that it
elects to employ separate counsel at the expense of the Indemnifying Party, the
Indemnifying Party shall not have the right to assume the defense of the
Indemnified Party and such counsel shall be at the expense of the Indemnifying
Party). The Indemnifying Party shall not be liable for any settlement of any
such Proceeding effected without its written consent. No Indemnifying Party
shall, without the prior written consent of the Indemnified Party, effect any
settlement of any pending Proceeding in respect of which any Indemnified Party
is a party, unless such settlement (i) includes an unconditional written release
of such Indemnified Party, in form and substance reasonably satisfactory to such
Indemnified Party, from all liability on claims that are the subject matter of
such Proceeding and (ii) does not include any statement as to an admission of
fault, culpability or failure to act by or on behalf of such Indemnified Party.

            (d) If the indemnification provided for in this Section 7 is held by
a court of competent jurisdiction to be unavailable to an Indemnified Party with
respect to any loss,

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liability, claim, damage or expense referred to herein, then the Indemnifying
Party, in lieu of indemnifying such Indemnified Party hereunder, shall
contribute to the amount paid or payable by such Indemnified Party as a result
of such loss, liability, claim, damage or expense in such proportion as is
appropriate to reflect the relative fault of the Indemnifying Party on the one
hand and of the Indemnified Party on the other in connection with the statements
or omissions that resulted in such loss, liability, claim, damage or expense as
well as any other relevant equitable considerations. The relative fault of the
Indemnifying Party and of the Indemnified Party shall be determined by reference
to, among other things, whether the untrue or alleged untrue statement of a
material fact or the omission to state a material fact relates to information
supplied by the Indemnifying Party or by the Indemnified Party and the parties'
relative intent, knowledge, access to information and opportunity to correct or
prevent such statement or omission.

            8. Miscellaneous.

            (a) Termination. Except for Section 7 hereof, this Agreement and the
rights and obligations of the Acquiror and the Holders hereunder shall terminate
on the earlier of (a) the first date on which no Registrable Securities remain
outstanding or (b) such time as all of such remaining Registrable Securities may
be sold in any consecutive three-month period in accordance with Rule 144.

            (b) No Third-Party Beneficiaries. This Agreement shall be binding
upon and inure solely to the benefit of each party hereto, and nothing in this
Agreement, express or implied, is intended to or shall confer upon any other
Person any rights or remedies of any nature whatsoever under or by reason of
this Agreement.

            (c) Entire Agreement. This Agreement (including the documents and
instruments referred to herein) constitutes the entire agreement and supersedes
all other prior agreements and understandings, both written and oral, among the
parties, or any of them, with respect to the subject matter hereof and shall not
be assigned by operation of law or otherwise.

            (d) Counterparts. This Agreement may be executed in two or more
counterparts, each of which shall be deemed to be an original, but all of which
shall constitute one and the same agreement and, when executed, shall be binding
on the other parties hereto.

            (e) Interpretation. The headings contained in this Agreement are for
reference purposes only and shall not affect in any way the meaning or
interpretation of this Agreement. In this Agreement, unless a contrary intention
appears (i) the words "herein", "hereof" and "hereunder" and other words of
similar import refer to this Agreement as a whole and not to any particular
Article, Section or other subdivision and (ii) reference to any Article or
Section means such Article or Section hereof. No provision of this Agreement
shall be interpreted or construed against any party hereto solely because such
party or its legal representative drafted such provision. Neither this Agreement
nor any uncertainty or ambiguity herein shall be construed or resolved against
any party, whether under any rule of construction or otherwise. No party to this
Agreement shall be considered the draftsman. The parties hereto acknowledge and
agree that this Agreement has been reviewed, negotiated and accepted by all
parties and their attorneys and shall be construed and interpreted according to
the ordinary

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<PAGE>

meaning of the words used so as fairly to accomplish the purposes and intentions
of the parties hereto.

            (f) Notices. All notices and other communications hereunder shall be
in writing and shall be deemed given if delivered personally, mailed by
registered or certified mail (return receipt requested) or sent via facsimile to
the parties at the following addresses (or at such other address for a party as
shall be specified by like notice):

            If to the Acquiror:

            North American Scientific, Inc.
            20200 Sunburst Street
            Chatsworth, California 91311
            Attention: L. Michael Cutrer, President
            Facsimile: (818) 734-5837

            with a copy to:

            McDermott Will & Emery
            2049 Century Park East
            Suite 3400
            Los Angeles, CA 90067-3208
            Attention:  Mark J. Mihanovic
            Fax:  (310) 277-4730

            If to the Holders

            To such address(es) as may be set forth in the books and records of
            NOMOS or the Acquiror.

            with a copy to:

            Cohen & Grigsby
            11 Stanwix Street, 15th Floor
            Pittsburgh, PA 15222
            Attention:  Mark Baseman, Esq.
            Fax:  (412) 209-0672

            (g) Governing Law. THIS AGREEMENT SHALL BE GOVERNED IN ALL RESPECTS,
INCLUDING VALIDITY, INTERPRETATION AND EFFECT, BY THE LAWS OF THE STATE OF
DELAWARE APPLICABLE TO CONTRACTS EXECUTED AND TO BE PERFORMED WHOLLY WITHIN SUCH
STATE.

            (h) Amendments and Waivers. No amendment of any provision of this
Agreement shall be valid unless such amendment shall be in writing and signed by
the Acquiror

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<PAGE>

and the Holders of not less than 66 2/3% of the Registrable Securities then
outstanding. No waiver by any party hereto of any default, misrepresentation or
breach of warranty or covenant hereunder, whether intentional or not, shall be
deemed to extend to any prior or subsequent default, misrepresentation or breach
of warranty or covenant hereunder or affect in any way any rights arising by
virtue of any prior or subsequent such occurrence.

            (i) Severability. Any term or provision of this Agreement which is
invalid or unenforceable in any jurisdiction shall, as to that jurisdiction, be
ineffective to the extent of such invalidity or unenforceability without
rendering invalid or unenforceable the remaining terms and provisions of this
Agreement or affecting the validity or enforceability of any of the terms or
provisions of this Agreement in any other jurisdiction. If any provision of this
Agreement is so broad as to be unenforceable, the provision shall be interpreted
to be only so broad as is enforceable.

            (j) Expenses. Except as provided in Section 4 above, each of the
parties hereto shall bear such party's own costs and expenses (including legal
fees and expenses) incurred in connection with this Agreement.

            (k) Construction. (i) Any reference to any federal, state, local or
foreign statute or law shall be deemed also to refer to all rules and
regulations promulgated thereunder, unless the context requires otherwise. The
parties hereto intend that each representation, warranty and covenant contained
herein shall have independent significance. If any party hereto has breached any
representation, warranty or covenant relating to the same subject matter as any
other representation, warranty or covenant (regardless of the relative levels of
specificity) which the party has not breached, it shall not detract from or
mitigate the fact that the party is in breach of the first representation,
warranty or covenant.

            (ii) Any of the defined terms in this Agreement, unless the context
otherwise requires, may be used in the singular or the plural, depending on the
reference. In this Agreement, unless clearly indicated otherwise, references to
dollar amounts are to the lawful money of the United States of America.

            (l) Equitable Relief. The parties hereto agree that the remedies at
law for any breach of the terms of this Agreement are inadequate. Accordingly,
the parties hereto consent and agree that an injunction may be issued to
restrain any breach or alleged breach of such provisions. The parties hereto
agree that terms of this Agreement shall be enforceable by a decree of specific
performance. Such remedies shall be cumulative and not exclusive, and shall be
in addition to any other remedies which the parties may have at law or in
equity.

            (m) Recapitalizations, Etc. In the event that any capital stock or
other securities are issued in respect of, in exchange for, or in substitution
of, any Registrable Securities by reason of any reorganization,
recapitalization, reclassification, merger, consolidation, spin-off, partial or
complete liquidation, stock dividend, split-up, sale of assets, distribution to
stockholders or combination of the Registrable Securities or any other change in
the Acquiror's capital structure, appropriate adjustments shall be made in this
Agreement so as to

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<PAGE>

fairly and equitably preserve, to the extent practicable, the original rights
and obligations of the parties hereto under this Agreement.

            (n) Waiver of Jury Trial. EACH PARTY HERETO HEREBY IRREVOCABLY
WAIVES ALL RIGHT TO TRIAL BY JURY IN ANY ACTION, PROCEEDING OR COUNTERCLAIM
(WHETHER BASED ON CONTRACT, TORT OR OTHERWISE) ARISING OUT OF OR RELATING TO
THIS AGREEMENT OR THE ACTIONS OF THE PARTIES HERETO IN THE NEGOTIATION,
ADMINISTRATION, PERFORMANCE AND ENFORCEMENT HEREOF.

                               [Signatures follow]

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<PAGE>

            IN WITNESS WHEREOF, the parties hereto have executed this Agreement
as of the date first above written.

                                NORTH AMERICAN SCIENTIFIC, INC.

                                By: /s/ L. Michael Cutrer
                                    --------------------------------------------
                                    Name:  L. Michael Cutrer
                                    Title: President and Chief Executive Officer

                                Holders:

                                /s/ John A. Friede
                                ------------------------------------------------
                                John A. Friede

                                CORPORATE OPPORTUNITIES FUND, L.P.

                                By: SMM Corporate Management, LLC,
                                    General Partner

                                By: /s/ James C. Gale
                                    --------------------------------------------
                                Name:  James C. Gale
                                Title: Chief Investment Officer

                                CORPORATE OPPORTUNITIES FUND
                                (INSTITUTIONAL), L.P.

                                By: SMM Corporate Management, LLC,
                                    General Partner

                                By: /s/ James C. Gale
                                    --------------------------------------------
                                Name:  James C. Gale
                                Title: Chief Investment Officer

                                CROSS ATLANTIC PARTNERS, INC.

                                By: /s/ John L. Cassis
                                    --------------------------------------------
                                Name:  John L. Cassis
                                Title: Partner

                                CROSS ATLANTIC PARTNERS I, K/S

                                By: /s/ John L. Cassis
                                    --------------------------------------------
                                Name:  John L. Cassis
                                Title: Partner

                                CROSS ATLANTIC PARTNERS II

                                By: /s/ John L. Cassis
                                    --------------------------------------------
                                Name:  John L. Cassis
                                Title: Partner

                                CROSS ATLANTIC PARTNERS III

                                By: /s/ John L. Cassis
                                    --------------------------------------------
                                Name:  John L. Cassis
                                Title: Partner

<PAGE>

                                CROSS ATLANTIC PARTNERS IV, K/S

                                By: /s/ John L. Cassis
                                    --------------------------------------------
                                Name:  John L. Cassis
                                Title: Partner

                                SMH NOMOS LLC

                                By: /s/ Bruce R. McMaken
                                    --------------------------------------------
                                Name:  Bruce R. McMaken
                                Title: President

<PAGE>

                                CITY OF MILFORD PENSION & RETIREMENT PLAN
                                NORWALK EMPLOYEE PENSION PLAN
                                NFIB EMPLOYEE PENSION TRUST
                                NFIB CORPORATE ACCOUNT
                                STATE OF OREGON PERS/ZCG
                                BREARLEY SCHOOL GENERAL INVESTMENT FUND
                                ASPHALT GREEN
                                ESTATE OF ANDREW HEISKELL
                                E.H. MANDELL TRUST F/B/O OLIVIA MANDELL
                                E.H. MANDELL TRUST F/B/O PETER MANDELL
                                JEANNE MORENCY
                                WILLIAM B. LAZAR
                                HBL CHARITABLE UNITRUST
                                ESTATE OF GRACE WILLENS C/O RONALD WILLENS,
                                    ADMINISTRATOR
                                MURRAY CAPITAL LLC
                                ALBERT J. ZESIGER
                                BARRIE RAMSAY ZESIGER
                                ALEXA ZESIGER CARVER
                                WOLFSON INVESTMENT PARTNERS LP
                                PUBLIC EMPLOYEE RETIREMENT SYSTEM OF IDAHO
                                SUSAN URIS HALPERN
                                WELLS FAMILY LLC
                                A. CAREY ZESIGER
                                CITY OF STAMFORD FIREMEN'S PENSION FUND
                                NICOLA ZESIGER MULLEN
                                ROANOKE COLLEGE
                                THE FERRIS F. HAMILTON FAMILY TRUST
                                MARY ANN S. HAMILTON TRUST FOR SELF
                                THE LAZAR FOUNDATION
                                MORGAN TRUST CO. OF THE BAHAMAS LTD. AS TRUSTEE
                                   U/A/D 11/30/93
                                PLANNED PARENTHOOD OF NYC
                                THE JENIFER ALTMAN FOUNDATION

                                By:  ZESIGER CAPITAL GROUP LLC
                                      as agent and attorney in fact

                                     By:  /s/ James Cleary
                                        ----------------------------------------
                                     Name:  James Cleary
                                     Title: Managing DirectorExhibit 10.7

                AMENDED AND RESTATED STOCK PURCHASE AGREEMENT

      THIS AMENDED AND RESTATED STOCK PURCHASE AGREEMENT, dated as of May 12,
2004 (this "Agreement"), is by and between John A. Friede ("Friede"), Karen F.
Holden ("Holden" and together with Friede, the "Sellers") and the Trust U/W
Moses L. Annenberg (the "Purchaser").

                                    RECITALS

      WHEREAS, Friede and Holden acquired the Shares (as defined below) pursuant
to an acquisition of NOMOS Corporation, a Delaware corporation ("NOMOS"), by
North American Scientific, Inc., a Delaware corporation ("NASI" and such
acquisition, the "Acquisition"); and

      WHEREAS, Friede is a party to a Lock-Up Agreement, between NASI and Friede
entered into in connection with the Acquisition (the "Lock-Up Agreement") and
Voting Agreement, dated as of October 26, 2003, between NASI and Friede (the
"Voting Agreement");

      WHEREAS, Friede owns and desires to sell 1,100,000 shares of common stock
of NASI (the "Friede Shares"), under the conditions of, and as permitted by, the
Lock-Up Agreement;

      WHEREAS, Holden owns and desires to sell 81,529 shares of common stock of
NASI (the "Holden Shares" and together with Friede Shares, the "Shares"); and

      WHEREAS, the Purchaser desires to purchase the Shares on the terms and
subject to the conditions set forth in this Agreement.

      NOW, THEREFORE, in consideration of the mutual covenants and agreements
contained herein, the parties hereto agree as follows:

                              I. PURCHASE; PAYMENT

      1.1 Purchase. Subject to the terms and conditions hereof, the Purchaser
hereby agrees to purchase from (i) Friede, the Friede Shares and (ii) Holden,
the Holden Shares, for the purchase price per Share equal to the closing market
price on the NASDAQ National Market on the trading day immediately preceding the
Delivery (as defined below) (the "Closing Market Price" and the aggregate
purchase price, the "Purchase Price").

      1.2 Delivery. As soon as practicable after the closing date of the
Acquisition (the "Delivery Date"), Friede and Holden shall deliver to the
Purchaser at 153 East 53rd Street, 23rd Floor, New York, NY 10022, Attention:
Marjorie Nesbitt, the Friede Shares and the Holden Shares, respectively, each
duly endorsed in blank or accompanied by executed stock power in blank (such
delivery of the Shares, the "Delivery"), and the Purchaser shall pay the
Purchase Price in accordance with Section 1.3 against the Delivery.

      1.3 Payment. The Purchase Price shall be paid by the Purchaser by wire
transfer of immediately available funds to such accounts as shall be designated
in writing by the Sellers, as follows:

<PAGE>

            (a) to Friede in respect of the Friede Shares, in an amount equal to
the product of 1,100,000 and the Closing Market Price; and

            (b) to Holden in respect of the Holden Shares, in an amount equal to
the product of 81,529 and the Closing Market Price.

If the Sellers have not provided the account information in writing by the
Delivery, payments under this Section 1.3 shall be made by check.

      1.4 Further Actions.

            (a) The Purchaser hereby agrees to be bound by the terms and
provisions of the (i) Lock-Up Agreement in respect of the Friede Shares and (ii)
the Voting Agreement, provided, however, the Trust shall be permitted to acquire
beneficial ownership of any Acquiror Shares (as defined therein) from Friede,
which Friede acquired in connection with the Acquisition, free of the
requirements of Section 3(b) thereof, to the extent that the Trust agrees to be
bound by the other terms and provisions of the Voting Agreement in respect of
such shares, and to execute and deliver any document or other instrument to
effectuate the same reasonably requested by Friede.

            (b) The Sellers agree to take all actions requested by the Purchaser
in connection with effectuating the transfer or registration of the Shares in
the name of the Purchaser or such other nominee as the Purchaser may designate,
including, if applicable, payment of any transfer taxes or fees of the transfer
agent or the registrar.

            (c) Upon receipt of written notice from NASI of a proposed filing of
a registration statement with respect to an offering of its common stock as set
forth in the Registration Rights Agreement to be entered into between the
Holders thereunder and NASI, Friede agrees to use his best efforts to extend
such registration rights in respect of the Friede Shares.

                       II. REPRESENTATIONS AND WARRANTIES

      2.1 Representations and Warranties of the Sellers. Each Seller, jointly
and severally, represents and warrants as of the date hereof and as of the
Delivery:

            (a) This Agreement has been duly executed and delivered by such
Seller, and constitutes a legal, valid and binding obligation of such Seller
enforceable in accordance with its terms.

            (b) As of the closing date of the Acquisition, such Seller shall own
of record and beneficially the Friede Shares or Holden Shares, as the case may
be. At the Delivery, the Shares shall be free and clear of all Encumbrances,
including, without limitation, any agreement, understanding or restriction
affecting the voting rights, transfer or other incidents of record or beneficial
ownership pertaining to the Shares, except with respect to the Friede Shares,
the Lock-Up Agreement and the Voting Agreement, to the extent as set forth in
Section 1.4(a)(ii) above. As used herein, "Encumbrances" shall mean any claim,
lien, pledge, option, charge, easement,

                                      -2-
<PAGE>

security interest, encumbrance, restriction (other than in respect of federal
and state securities laws) or other right of a third party.

            (c) The execution and delivery of this Agreement and such Seller's
performance of its obligations hereunder will not (i) conflict with or result in
a breach of the terms and conditions of, or constitute any default under, any
contract, agreement or instrument to which such Seller is a party, (ii) violate
any statute, rule, regulation, order, judgment, writ, injunction decree or
award, or (iii) require the consent, approval, authorization or other action by
any governmental or regulatory authority or other third party.

            (d) To the best knowledge of such Seller, the disclosure with
respect to NASI and NOMOS in the Joint Proxy Statement/Prospectus prepared in
connection with the Acquisition does not contain an untrue statement of a
material fact or omit to state a material fact necessary in order to make the
statements therein, in the light of the circumstances under which they were
made, not misleading.

The Sellers agree to indemnify and hold harmless the Purchaser in respect of any
losses, costs, damages and expenses incurred as a consequence of any breach or
alleged breach by the Sellers or either of them of any representation or
warranty contained this Section 2.1.

      2.2 Representations and Warranties of the Purchaser. The Purchaser
represents and warrants as of the date hereof and as of the Delivery:

            (a) The Purchaser is a trust validly exiting under the laws of the
jurisdiction of its formation. Each person executing this Agreement, on behalf
of the Purchaser, in a representative or fiduciary capacity has full power and
authority to execute and deliver this Agreement in such capacity and on behalf
of the Purchaser. The Purchaser has all necessary right and power to perform its
obligations pursuant to this Agreement.

            (b) The Purchaser is an "accredited investor" as defined in
Regulation D and understands that its acquisition of the Shares have not been
registered under the Securities Act of 1933, as amended (the "Securities Act")
or the securities laws of any state. The Purchaser is acquiring the Shares for
its own account and not with a view to the distribution or resale of the Shares
except pursuant to a registration statement declared effective under, or an
exemption from the registration requirements of, the Securities Act.

            (c) The Purchaser acknowledges that the certificates representing
the Shares may bear a legend to the effect that the Shares are subject to
restrictions on transfer, sale or assignment under the Securities Act or any
state securities laws and, with respect to the Friede Shares, under the Lock-Up
Agreement and Voting Agreement.

                           III. CONDITIONS PRECEDENT

      3.1 Conditions Precedent to Delivery. The Purchaser's obligation to
purchase the Shares is subject to the satisfaction of the following conditions
at the Delivery:

                                      -3-
<PAGE>

            (a) The representations and warranties of the Sellers contained in
this Agreement shall be correct and complete on and as of the Delivery Date with
the same effect as though made on and as of the Delivery Date (after giving
effect to the transactions contemplated by this Agreement).

            (b) The purchase of and payment for the Shares shall not be
prohibited by any law or governmental order, rule, ruling, regulation, release,
interpretation or opinion applicable to the Purchaser and shall not subject the
Purchaser to any penalty, tax, liability, or other onerous condition.

                               IV. MISCELLANEOUS

      4.1 Expenses. Each Seller and Purchaser shall each be liable for its own
costs and expenses incurred in connection with the negotiation, preparation,
execution or performance of this Agreement.

      4.2 Entire Agreement; Amendment and Waiver. This Agreement constitutes the
entire agreement among the parties pertaining to the subject matter hereof and
supersedes all prior agreements, understandings, negotiations and discussions,
whether oral or written, of the parties. No supplement, modification or waiver
of this Agreement shall be binding unless executed in writing by all parties. No
waiver of any of the provisions of this Agreement shall be deemed or shall
constitute a waiver of any other provision hereof (whether or not similar), nor
shall such waiver constitute a continuing waiver unless otherwise expressly
provided.

      4.3 Severability. In the event that any one or more of the provisions
contained in this Agreement or in any other instrument referred to herein,
shall, for any reason, be held to be invalid, illegal or unenforceable in any
respect, such invalidity, illegality or unenforceability shall not affect any
other provision of this Agreement or any other such instrument.

      4.4 Governing Law. THIS AGREEMENT SHALL BE GOVERNED BY, AND CONSTRUED IN
ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK (OTHER THAN ANY CONFLICT OF
LAWS RULE WHICH MAY RESULT IN THE APPLICATION OF THE LAWS OF ANY OTHER
JURISDICTION).

      4.5 Counterparts. This Agreement may be executed in two or more
counterparts, each of which shall be deemed an original but all of which shall
together constitute one and the same document.

      4.6 Existing Agreement. This Agreement amends and restates the Stock
Purchase Agreement, dated as of May 4, 2004, between the parties hereto, in its
entirety.

                            [SIGNATURE PAGE FOLLOWS]

                                      -4-
<PAGE>

      IN WITNESS WHEREOF, the parties have each executed this Agreement as of
the day first above written.

                                   PURCHASER:

                                   TRUST U/W MOSES L. ANNENBERG

                                       By:  /s/ John A. Friede
                                          --------------------------------------
                                          Name: John A. Friede, as Co-Trustee

                                       CITICORP TRUST BANK, FSB, as Co-Trustee

                                          By:   /s/ Marjorie E. Nesbitt
                                             -----------------------------------
                                             Name:  Marjorie E. Nesbitt
                                             Title: Vice President, Citibank

                                    SELLERS:

                                    /s/ John A. Friede
                                    --------------------------------------------
                                    Name: John A. Friede

                                    /s/ Karen F. Holden
                                    --------------------------------------------
                                    Name: Karen F. Holden

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