Exhibit 10.1(a)

                                VOTING AGREEMENT

      This VOTING AGREEMENT (this "Agreement") is made and entered into as of
October 26, 2003 by and among North American Scientific, Inc., a Delaware
corporation ("Acquiror"), and John A. Friede (the "Company Chairman").

                                    RECITALS

      A. Acquiror, AM Capital I, Inc. ("Merger Sub"), a Delaware corporation and
wholly-owned subsidiary of Acquiror, and NOMOS Corporation, a Delaware
corporation ("Company"), have entered into an Agreement and Plan of Merger (the
"Merger Agreement") as of the date hereof, which provides for the merger (the
"Merger") of Company with and into Merger Sub, which shall continue as surviving
corporation in the Merger.

      B. As of the date hereof, the Company Chairman is the "beneficial owner"
(as defined below) of such number of Company Shares (as defined below) as is set
forth on the signature page of this Agreement.

      C. As an inducement and condition to entering into the Merger Agreement,
Acquiror has required that Company Chairman agree, and Company Chairman has
agreed, to enter into this Agreement.

      D. In consideration of the execution of the Merger Agreement by Acquiror,
Company Chairman is hereby agreeing, from and after the date hereof until the
Company Shares Expiration Date (as defined below) to vote, or cause to be voted
the Company Shares (as defined below) and other such shares of capital stock of
Company over which Company Chairman has voting power in favor of, and otherwise
support, the adoption of the Merger Agreement (as the same may be amended from
time to time), and each of the transactions contemplated by the Merger
Agreement.

      E. In consideration of the execution of the Merger Agreement by Acquiror,
Company Chairman is hereby agreeing, from and after the Closing Date until the
Acquiror Shares Expiration Date: (i) to vote, or cause to be voted, the Acquiror
Shares (as defined below) over which Company Chairman has voting power so as to
elect the individuals nominated by a majority of the Acquiror Board Designees to
the Acquiror Board, (ii) not to acquire beneficial ownership or enter into any
written or unwritten agreement or arrangement to acquire any securities of
Acquiror other than (x) Acquiror Shares acquired in connection with the Merger,
(y) Acquiror Shares acquired as a result of the exercise of stock options (A)
held by Company Chairman as of the Closing Date of the Merger or (B) granted to
the Company Chairman by the Acquiror in connection with the services provided by
Company Chairman as a member of the Board of Directors of the Acquiror and (z)
securities of Acquiror that are subject to the Voting Trust (as defined below),
and (iii) not to vote, or cause not to be voted, all Acquiror Shares over which
Company Chairman has voting power in favor of any Unapproved Transaction (as
defined below).

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      NOW, THEREFORE, the parties hereto, intending to be legally bound, hereby
agree as follows:

      1. Certain Definitions. All capitalized terms used but not defined herein
are used herein as defined in the Merger Agreement. For purposes of this
Agreement, the following terms shall have the following respective meanings:

            (a) "Acquiror Shares" shall mean all securities of Acquiror
(including all shares of Acquiror Common Stock and all additional options,
warrants and other rights to acquire shares of Acquiror Common Stock) of which
Company Chairman acquires beneficial ownership during the period commencing on
the date of this Agreement through the Acquiror Shares Expiration Date,
including, without limitation, through the exchange of Company Shares for
Acquiror Shares pursuant to the terms of the Merger Agreement, exercise of
options, warrants or other rights to acquire such securities of Acquiror, or the
conversion of other securities of Acquiror into such securities of Acquiror. In
the event of a stock dividend or distribution or any change in the Acquiror
Shares by reason of any stock dividend, split-up, recapitalization, combination,
exchange of shares or similar action or transaction, the term "Acquiror Shares"
shall be deemed to refer to and include the Acquiror Shares as well as all
Acquiror Shares issued in such stock dividends and distributions and any shares
or other securities into which or for which any or all of the Acquiror Shares
may be changed or exchanged in such split-up, recapitalization, combination,
exchange of shares or similar action or transaction. The term "Acquiror Shares"
as used in this Agreement shall exclude all Acquiror Shares that are subject to
the Voting Trust.

            (b) "Acquiror Shares Expiration Date" shall mean the earlier to
occur of (i) the date that the Merger Agreement shall have been validly
terminated pursuant to Article IX thereof or (ii) the second anniversary of the
Closing Date.

            (c) "beneficial owner" and "beneficial ownership" shall have the
meaning provided therefore in Rule 13d-3 promulgated under the Securities
Exchange Act of 1934.

            (d) "Company Shares" shall mean: (i) all securities of Company
(including all shares of Company Common Stock and all options, warrants and
other rights to acquire shares of Company Common Stock) beneficially owned by
Company Chairman as of the date of this Agreement, as indicated on the signature
page of this Agreement and (ii) all additional securities of Company (including
all additional shares of Company Common Stock and all additional options,
warrants and other rights to acquire shares of Company Common Stock) of which
Company Chairman acquires beneficial ownership during the period from the date
of this Agreement through the Closing Date, including, without limitation,
through the exercise of options, warrants or other rights to acquire such
securities of Company, or the conversion of other securities of Company into
such securities of Company. In the event of a stock dividend or distribution or
any change in the Company Shares by reason of any stock dividend, split-up,
recapitalization, combination, exchange of shares or similar action or
transaction, the term "Company Shares" shall be deemed to refer to and include
the Company Shares as

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well as all Company Shares issued in such stock dividends and distributions and
any shares or other securities into which or for which any or all of the Company
Shares may be changed or exchanged in such split-up, recapitalization,
combination, exchange of shares or similar action or transaction.

            (e) "Company Shares Expiration Date" shall mean the earlier to occur
of (ii) the Closing Date or (ii) the date that the Merger Agreement shall have
been validly terminated pursuant to Article IX thereof.

            (f) "Change of Control Transaction" shall mean any of the following
transactions to which the Acquiror is a party: (i) a merger or consolidation as
a result of which stockholders of the Acquiror immediately prior to such merger
or consolidation hold less than a majority of the voting power of the surviving
corporation of such merger or consolidation; (ii) the sale or transfer of at
least a majority of the voting power of the Acquiror in a single transaction or
a series of related transactions or (iii) the sale, transfer or other
disposition of all or substantially all of the assets of the Acquiror as a going
concern in a single transaction or series of related transactions.

            (g) "Family Member" shall mean (i) Company Chairman; (ii) any
spouse, lineal descendant or antecedent, father, mother, brother or sister of
Company Chairman and (iii) any estate, trust, guardianship, custodianship or
other fiduciary arrangement for the primary benefit of any one or more
individuals named or described in (i) and (ii) above.

            (h) "Person" shall mean any individual, corporation, limited
liability company, partnership, association, trust or any other entity or
organization.

            (i) "Unapproved Transaction" shall mean any Change of Control
Transaction which is not approved by a majority of the Acquiror Board Designees
(or the successors thereto).

            (j) "Voting Trust" shall mean that certain Voting Trust Agreement,
dated as of the Closing Date, between Acquiror and Company Chairman.

      2. Agreement as to Company Shares.

            (a) Vote of Company Shares. Until the Company Shares Expiration
Date, at every meeting of the stockholders of Company called and at every
adjournment or postponement thereof, and on every proposed action or approval by
written consent of the stockholders of Company, Company Chairman shall cause all
Company Shares of which Company Chairman is the beneficial owner to be voted (i)
in favor of, and otherwise support, the adoption of the Merger Agreement (as the
same may be amended from time to time), and each of the transactions
contemplated by the Merger Agreement (as the same may be amended from time to
time), (ii) against any matter that is intended, or could reasonably be expected
to, impede, interfere with, delay, postpone, discourage or adversely affect the
consummation of the Merger and other transactions contemplated by the Merger
Agreement (as the same may be amended from time to time) and (iii) against any
proposals or offers from any Person, other than Acquiror or any affiliate of
Acquiror,

<PAGE>

concerning an Acquisition Proposal involving Company.

            (b) No Other Agreement or Sale of Company Shares. Prior to the
Company Shares Expiration Date, Company Chairman shall not (i) enter into any
agreement or understanding with any Person to vote or give instructions in any
manner inconsistent with the terms of this Section 2 or (ii) sell, transfer,
convey or allow to be sold, transferred or conveyed any Company Shares of which
Company Chairman is the beneficial owner except to a Family Member that has
delivered to the Acquiror a duly executed Voting Agreement in substantially the
same form as this Voting Agreement.

      3. Agreement as to Acquiror Shares.

            (a) Vote of Acquiror Shares. Until the Acquiror Shares Expiration
Date, at every meeting of the stockholders of Acquiror called and at every
adjournment or postponement thereof, and on every proposed action or approval by
written consent of the stockholders of Acquiror, Company Chairman shall cause
all Acquiror Shares of which Company Chairman is the beneficial owner to be
voted in favor of, and otherwise support, the election of the individuals
nominated by a majority of the Acquiror Board Designees (or the successors
thereto) to the Acquiror Board so long as (i) Company Chairman or, if the
Company Chairman is no longer serving on the Acquiror Board, an individual
nominated by the Company Chairman (the "Chairman Nominee"), is among the
individuals nominated by a majority of the Acquiror Board Designees (provided,
however, that the condition set forth in this clause (i) shall no longer be
required to be satisfied, and Company Chairman shall be required to vote, or
cause to be voted, the Acquiror Shares held hereby in accordance with this
Section 3(a), with respect to such election if a majority of the Acquiror Board
Designees has reasonably determined that each of the individuals that have been
timely nominated by the Company Chairman to be the Chairman Nominee for such
election (which shall not number more than three) are not qualified or suitable
to be a director of the Acquiror) and (ii) John Manzetti or, if John Manzetti is
no longer serving on the Acquiror Board, an individual nominated by Company
Chairman (the "Manzetti Nominee"), is among the individuals nominated by a
majority of the Acquiror Board Designees (provided, however, that the condition
set forth in this clause (ii) shall no longer be required to be satisfied,
Company Chairman shall be required to vote, or cause to be voted, the Acquiror
Shares held hereby in accordance with this Section 3(a), with respect to such
election if a majority of the Acquiror Board Designees has reasonably determined
that each of the individuals that have been timely nominated by Company Chairman
to be the Manzetti Nominee for such election (which shall not number more than
three) are not qualified or suitable to be a director of the Acquiror).

            (b) Acquisition of Acquiror Shares. Until the Acquiror Shares
Expiration Date, Company Chairman shall not acquire beneficial ownership of any
Acquiror Shares, other than (x) Acquiror Shares acquired in connection with the
Merger, (y) Acquiror Shares acquired as a result of the exercise of stock
options (A) held by Company Chairman as of the Closing Date of the Merger or (B)
granted to the Company Chairman by the Acquiror in connection with the services
provided by Company Chairman as a member of the Board of Directors of the
Acquiror and (z) securities of

<PAGE>

Acquiror that are subject to the Voting Trust.

            (c) Prohibition Against Voting for Unapproved Transaction. Company
Chairman shall not vote, and shall prevent any Acquiror Shares (as defined
below) over which Company Chairman has voting power from otherwise being voted,
in favor of any Unapproved Transaction.

            (d) No Other Agreement. Company Chairman shall not enter into any
agreement or understanding with any Person to vote, or give instructions to
vote, any Acquiror Shares in any manner inconsistent with the terms of this
Section 3.

      4. Irrevocable Proxy. As security for the agreements of Company Chairman
provided for herein, Company Chairman hereby grants and delivers to Acquiror
concurrently with the execution and delivery of this Agreement:

            (a) a proxy in the form attached hereto as Exhibit A ( the "Company
Shares Proxy"), which shall be irrevocable to the fullest extent permitted by
applicable law, with respect to all Company Shares of which Company Chairman is
the beneficial owner as of and subsequent to the date hereof until the Company
Shares Expiration Date; and

            (b) a proxy in the form attached hereto as Exhibit B ( the "Acquiror
Shares Proxy"), which shall be irrevocable to the fullest extent permitted by
applicable law, with respect to all Acquiror Shares of which Company Chairman is
the beneficial owner as of and subsequent to the date hereof until the Acquiror
Shares Expiration Date.

      5. Representations and Warranties of Company Chairman. Company Chairman
hereby represents and warrants to Acquiror that:

            (a) as of the date hereof and at all times until the Company Shares
Expiration Date (unless indicated otherwise), with respect to the Company
Shares, Company Chairman (i) is the beneficial owner of, and has good and valid
title to, the Company Shares at such time owned by him as of and subsequent to
the date hereof, free and clear of any liens, claims, options, rights of first
refusal, co-sale rights, charges or other encumbrances, except as set forth on
Schedule 5.2(a) attached hereto or as provided in Section 2(b) above, (ii) as of
the date hereof, does not beneficially own or have any written or unwritten
agreement or arrangement to acquire any securities of Company other than the
shares of Company Common Stock and options and warrants to purchase shares of
Company Common Stock indicated on the signature page of this Agreement, except
as set forth on Schedule 5.2(a) and (iii) has sole voting power, sole power of
disposition, sole power to issue instructions with respect to the matters set
forth in Section 2 hereof, sole power to demand appraisal rights and sole power
to agree to all of the matters set forth in this Agreement, in each case with
respect to all of the Company Shares at such time owned by him as of and
subsequent to the date hereof, with no limitations, qualifications or
restrictions on such rights, subject to applicable federal securities laws and
the terms of this Agreement; and

            (b) as of the date hereof, Company Chairman does not beneficially

<PAGE>

own, or have any written or unwritten agreement or arrangement to acquire, any
securities of Acquiror other than the agreement to acquire Acquiror Shares in
connection with the Merger or except as set forth on Schedule 5.2(b).

      6. Covenants of Company Chairman. Company Chairman hereby covenants to
Acquiror, that, with respect to the Acquiror Shares, Company Chairman will have
sole voting power, sole power of disposition, sole power to issue instructions
with respect to the matters set forth in Section 3 hereof, sole power to demand
appraisal rights and sole power to agree to all of the matters set forth in this
Agreement, in each case with respect to all of the Acquiror Shares at such time
owned by him as of and subsequent to the date hereof, with no limitations,
qualifications or restrictions on such rights, subject to applicable federal
securities laws and the terms of this Agreement.

      7. No Solicitation of Proxies. Prior to the Company Shares Expiration
Date, Company Chairman agrees (solely in his capacity as a stockholder and/or
option holder of Company) that neither he nor any of his representatives
(including any investment banker, attorney or accountant retained by him) shall,
and that he shall use his reasonable best efforts to cause his representatives
not to (and shall not authorize any of them to) directly or indirectly, engage
in any solicitation (as defined in Regulation 14A of the Rules and Regulations
of the Exchange Act) of other stockholders of Company to vote or take any other
action against the Merger Agreement (as the same may be amended from time to
time) or the Merger or any of the other transactions contemplated by the Merger
Agreement (as the same may be amended from time to time).

      8. Further Assurances. From time to time, at Acquiror's request and
without further consideration, Company Chairman shall execute and deliver such
additional documents and take all such further action as may be reasonably
deemed necessary or appropriate by Acquiror to consummate the transactions
contemplated by this Agreement.

      9. Termination. This Agreement shall terminate and shall have no further
force or effect as of the Acquiror Shares Expiration Date.

      10. Miscellaneous.

            (a) Waiver. No waiver by an party hereto of any condition or any
breach of any term or provision set forth in this Agreement shall be effective
unless in writing and signed by each party hereto. The waiver of a condition or
any breach of any term or provision of this Agreement shall not operate as or be
construed to be a waiver of any other previous or subsequent breach of any term
or provision of this Agreement.

            (b) Severability. In the event that any term, provision, covenant or
restriction set forth in this Agreement, or the application of any such term,
provision, covenant or restriction to any person or entity or set of
circumstances, shall be determined by a court of competent jurisdiction to be
invalid, unlawful, void or unenforceable to any extent, the remainder of the
terms, provisions, covenants and restrictions set forth in this Agreement, and
the application of such terms, provisions,

<PAGE>

covenants and restrictions to persons or entities or circumstances other than
those as to which it is determined to be invalid, unlawful, void or
unenforceable, shall remain in full force and effect, shall not be impaired,
invalidated or otherwise affected and shall continue to be valid and enforceable
to the fullest extent permitted by applicable law. If any provision of this
Agreement is so broad as to be unenforceable, such provision shall be
interpreted to be only so broad as is enforceable.

            (c) Binding Effect; Assignment. This Agreement and all of the terms
and provisions hereof shall be binding upon, and inure to the benefit of, the
parties hereto and their respective successors and permitted assigns, but
neither his Agreement nor any of the rights, interests or obligations of Company
Chairman hereunder may be assigned to any other Person without the prior written
consent of Acquiror; provided, however, that the Company Chairman may assign his
rights, interests or obligations to another Person without the prior written
consent of Acquiror with respect to any Company Shares sold, transferred or
conveyed to a Family Member in accordance with Section 2(b) of this Agreement.

            (d) Amendments. This Agreement may not be modified, amended, altered
or supplemented, except upon the execution and delivery of a written agreement
executed by each of the parties hereto.

            (e) Specific Performance; Injunctive Relief. Each of the parties
hereto hereby acknowledges that (i) the representations, warranties, covenants
and restrictions set forth in this Agreement are necessary, fundamental and
required for the protection of Acquiror and to preserve for Acquiror the
benefits of the Merger, (ii) the representations, warranties, covenants and
restrictions set forth in this Agreement relate to matters which are of a
special, unique and extraordinary character that gives each such representation,
warranty, covenant and restriction a special, unique and extraordinary value and
(iii) a breach of any representation, warranty, covenant or restriction set
forth in this Agreement, or any other term or provision of this Agreement, will
result in irreparable harm and damage to Acquiror which cannot be adequately
compensated by a monetary award. Accordingly, Acquiror and Company Chairman
hereby expressly agree that in addition to any and all other remedies available
at law or in equity, Acquiror shall be entitled to the immediate remedy of
specific performance, a temporary or permanent restraining order or preliminary
injunction or such other form of injunctive or equitable relief as may be used
by any court of competent jurisdiction to restrain or enjoin any of the parties
hereto from breaching any representations, warranties, covenants or restrictions
set forth in this Agreement or to specifically enforce the terms and provisions
hereof.

            (f) Governing Law. This Agreement shall be governed by and
construed, interpreted and enforced in accordance with the internal laws of the
State of Delaware without giving effect to any choice or conflict of law
provision, rule or principle (whether of the State of Delaware or any other
jurisdiction) that would cause the application of the laws of any jurisdiction
other than the State of Delaware.

            (g) Notices. All notices and other communications hereunder shall be

<PAGE>

in writing and shall be deemed duly given (i) on the date of delivery if
delivered personally, (ii) on the date of confirmation of receipt (or, the first
business day following such receipt if the date is not a business day) of
transmission by telecopy or facsimile or (iii) on the date of confirmation of
receipt (or, the first business day following such receipt if the date is not a
business day) if delivered by a nationally recognized courier service. All
notices hereunder shall be delivered as follows, or pursuant to such other
instructions as may be designated in writing by the party to receive such
notice:

                  (i)   If to Acquiror:

                        North American Scientific, Inc.
                        20200 Sunburst Street
                        Chatsworth, California 91311
                        Attention: L. Michael Cutrer, President
                        Fax: (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, Esq.
                        Fax: (310) 277-4730

                  (ii)  If to Company Chairman: To the address for notice set
                        forth on the signature page hereof.

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

            (h) Enforcement; Consent to Jurisdiction; Waiver of Jury Trial.

                  (i) EACH PARTY HERETO IRREVOCABLY AND UNCONDITIONALLY WAIVES
TRIAL BY JURY IN ANY LEGAL ACTION OR PROCEEDING IN RELATION TO THIS AGREEMENT
AND FOR ANY COUNTERCLAIM THEREON.

                  (ii) All rights, powers and remedies provided under this
Agreement or otherwise available in respect hereof at law or in equity shall be
cumulative and not alternative, and the exercise of any such rights, powers or
remedies by any party hereto shall not preclude the simultaneous or later
exercise of any other right, power or

<PAGE>

remedy by such party.

            (i) Entire Agreement. This Agreement, the Company Shares Proxy, the
Acquiror Shares Proxy, the Merger Agreement contain the entire understanding of
the parties in respect of the subject matter hereof supersede all prior
negotiations and understandings between the parties with respect to such subject
matter.

            (j) Headings. The section headings set forth in this Agreement are
for convenience of reference only and shall not affect the construction or
interpretation of this Agreement in any manner.

            (k) Counterparts. This Agreement may be executed in counterparts,
each of which shall be deemed an original, and both of which together shall
constitute one and the same instrument.

<PAGE>

      IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be
duly executed on the day and year first above written.

NORTH AMERICAN SCIENTIFIC, INC.                 COMPANY CHAIRMAN


By: /s/ L. Michael Cutrer                       /s/ John A. Friede
    ----------------------------------------    --------------------------------
    Signature of Authorized Signatory           Signature

Name: L. Michael Cutrer                         Print Address:
      --------------------------------------

Title: President and Chief Executive Officer    ________________________________
       -------------------------------------    ________________________________
                                                ________________________________

                                                Telephone Number
                                                ________________________________

                                                Facsimile Number

                                                ________________________________

                                                Shares Beneficially Owned:

                                                1,510,574.61   shares of Company
                                                               Common Stock

                                                164,975        shares of Company
                                                               Common Stock
                                                               issuable upon
                                                               exercise of
                                                               outstanding
                                                               options or
                                                               warrants

                                                40,101.52      shares of Company
                                                               Series A
                                                               Preferred Stock

                                                1,481,035.40   shares of Company
                                                               Series A
                                                               Preferred Stock

                      [SIGNATURE PAGE TO VOTING AGREEMENT]