Document:

<PAGE>
                                                                     Exhibit 4.1
             NUMBER        [LOGO OF NOMOS CORPORATION]        SHARES

NMOS
<TABLE>
<CAPTION>
<S>         <C>                                                   <C>
COMMON STOCK                     NOMOS CORPORATION                     COMMON STOCK
            Incorporated under the laws of the State of Delaware    CUSIP 65535P 10 1

PAR VALUE $.0001                                                       SEE REVERSE
                                                                  FOR CERTAIN DEFINITIONS
</TABLE>

This certifies that

is the owner of

      FULLY PAID AND NON-ASSESSABLE SHARES OF THE COMMON STOCK, $.0001 PAR
                                    VALUE, OF

--------------------------------NOMOS CORPORATION-------------------------------

transferable on the books of the Corporation by the holder hereof in person or
by duly authorized attorney upon surrender of this Certificate properly
endorsed. The Certificate and the shares represented hereby are issued under and
shall be subject to the laws of the State of Delaware and all the provisions of
the countersigned and registered by the Transfer Agent and Registrar.
      Witness the facsimile seal of the Corporation and the facsimile signatures
of its duly authorized officers.

Dated:
                           [SEAL OF NOMOS CORPORATION]

        /s/ DAVID J. HAFFNER                       /s/ JOHN W. MANZETTI

             Secretary                     President and Chief Executive Officer

                                                Countersigned and Registered:

                                                By: Mellon Investor Services LLC

                                                    Transfer Agent and Registrar

<PAGE>

                                NOMOS CORPORATION

         This Corporation will furnish without charge to each stockholder who so
requests, a copy of the designations, powers, preferences and relative,
participating, optional or other special rights of each class of stock or series
thereof and the qualifications, limitations or restrictions of such preferences
and/or rights. Any such requests may be addressed to the Sectretary of the
Corporation.

         The following abbreviations when used in the inscription on the face of
this certificate, shall be construed as though they were written out in full
according to applicable laws or regulations:

<TABLE>
<CAPTION>
<S>           <C>                                    <C>
TEN COM       - as tenants in commons                UNIF GIFT MIN ACT  -  ...............Custodian...............
                                                                                (Cust)                 (Minor)
TEN ENT       - as tenants by the entireties                                         under Uniform Gifts to Minors

JT TEN        - as joint tenants with right                                          Act ..............................
                of survivorship and not as                                                       (State)
                tenants in common

                          Additional abbeviations my also be used though not in the above list

For value received, ________________ hereby sell, assign and transfer unto

PLEASE INSERT SOCIAL SECURITY OR OTHER
   IDENTIFYING NUMBER OF ASSIGNEE

--------------------------------------
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|                                    |
|                                    |
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                    (PLEASE PRINT OR TYPEWRITE NAME AND ADDRESS INCLUDING POSTAL ZIP CODE OF ASSIGNEE)
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________________________________________________________________________________________________________________ Shares

of the Stock represented by the within Certificate, and do hereby irrevocably constitute and apoint ___________________

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Attorney to transfer the said Stock on the books of the within-named Corporation with full power of substitution in
the premises.

Dated __________________________________                                  _____________________________________________
                                                                                               Signature

                                                                          __________________________________________THE
                                                                          SIGNATURES TO THIS ASSIGNMENT MUST CORRESPOND
                                                                          WITH THE NAME AS WRITTEN UPON THE FACE OF
                                                                          THE CERTIFICATE IN EVERY PARTICULAR WITHOUT
                                                                          ALTERATION OR ENLRAGEMENT OR ANY CHANGE
                                                                          WHATEVER.

</TABLE><PAGE>

                                                                 Exhibit 10.1(a)

                                NOMOS CORPORATION

                          REGISTRATION RIGHTS AGREEMENT

         This Registration Rights Agreement is entered into as of the day _____
of ________________ 19 _____ , by and among NOMOS CORPORATION, a Delaware
corporation (the "Company") ; and the person identified as Investor on the
signature page hereof (referred to as the "Investor").

         WHEREAS, the Investor is a party to the Common Stock Purchase Agreement
dated as of the date of this Agreement (the "Purchase Agreement"), and such
Investor's rights and obligations under said Purchase Agreement are conditioned
upon the execution and delivery of this Agreement.

         NOW, THEREFORE, in consideration of the mutual promises and covenants
set forth herein, and intending to be legally bound hereby, the parties further
agree as follows:

                   1.     REGISTRATION RIGHTS.

                          1.1      DEFINITIONS. For purposes of this Agreement:

                          (a) The term "Act" means the Securities Act of 1933,
as amended, or any other statute in effect from time to time corresponding to
such Act.

                          (b) The terms "register,"  "registered," and
"registration" refer to a registration effected by preparing and filing a
registration statement in compliance with the Act and the declaration or
ordering of effectiveness of such registration statement.

                          (c) The term "Common Shares" means shares of the
Company's Common Stock.

                          (d) The  term  "Registrable  Securities"  means
(i) Common Shares, and (ii) any securities of the Company issued as a dividend
or other distribution with respect to, or in exchange or in replacement of, such
Common Shares. Registrable Securities, if transferred pursuant to an exemption
from registration under the Act, will remain Registrable Securities.

                          (e) The term  "Participating  Investor" means an
Investor who has purchased shares pursuant to solicitation by a Placement Agent
(as defined in the Purchase Agreement) as part of the Offering (as defined in
the Purchase Agreement).

                          (f) The term  "Holder"  means any  person  holding
Registrable Securities of the Company as of the date immediately preceding the
date of the Initial Public Offering (as defined below).

                          (g) The  term  "Initial  Public  Offering"  means  the
first underwritten public offering of the Company's securities with an offering
price to public investors of not less than $5,000.000.

<PAGE>

                  1.2 RESTRICTION OF SALE OF SHARES AFTER AN INITIAL PUBLIC
OFFERING. In consideration for the Company agreeing to its obligations under
this Section 1, the Investor agrees that in addition to complying with the
restrictions on the sale, transfer or other disposition of Common Shares by the
Investor pursuant to Section 8 of the Purchase Agreement, the Investor also
agrees not to transfer, sell, make short sales of, loan, grant any option for
the purchase of, or otherwise dispose of any Registrable Securities, except for
private sales to members of Investor's immediate family who agree to be bound by
the provisions of this Registration Rights Agreement (including this Section
1.2) during the period beginning on the effective date of the Initial Public
Offering and ending six months later (the "Lock-up Period") without the prior
written consent of the Company or the managing underwriter or underwriters for
such Initial Public Offering.

                  1.3 RIGHT TO PARTICIPATE IN COMPANY REGISTRATION. Subject to
paragraph 1.9, if at any time after the Lock-Up Period the Company proposes to
register any of its Common Stock under the Act in connection with a primary or
secondary public offering of such securities solely for cash on a form other
than Form S-8, S-4 or comparable Registration Statement, the Company shall, each
such time, promptly give each Holder written notice of such determination. Upon
the written request of any Holder given within twenty days after mailing of any
such notice by the Company, the Company shall use its best efforts to cause to
be registered under the Act all of the Registrable Securities that each such
Holder has requested be requested.

                  1.4 REQUEST FOR REGISTRATION. Subject to paragraph 1.9, if, at
any time after the end of the Lock-up Period, the Company shall receive a
written request (specifying that it is being made pursuant to this paragraph
1.4) from persons holding more than fifty percent (50%) of the Registrable
Securities purchased by the Participating Investors, then the Company shall
promptly notify all other Holders of such request and shall use its best efforts
to cause all Registrable Securities that Holders have requested be so registered
to be registered under the Act.
                  Notwithstanding the foregoing if, at any time after the
two-year anniversary of the last closing date of the Offering (as defined in the
Purchase Agreement) , the Company shall furnish to such Holders a certificate
signed by the President of the Company stating that in the good faith judgment
of the Board of Directors it would be seriously detrimental to the Company or
its shareholders for a registration statement to be filed in the near future,
then the Company's obligation to use its best efforts to file a registration
statement shall be deferred for a period not to exceed four (4) months;
provided, however, that the Company shall not obtain such a deferral more than
once in any 12-month period.

                  The Company shall be obligated to effect only one registration
pursuant to this section 1.4. Any request for registration under this section
1.4 must be for an underwritten public offering to be managed by an underwriter
or underwriters of recognized national standing selected by the Company.

                  1.5 OBLIGATIONS OF THE COMPANY. Whenever required under
paragraphs 1.3 or .1.4 to use its best efforts to effect the registration of any
Registrable Securities, the Company shall, as expeditiously as reasonably
possible:

                                       2
<PAGE>

                          (a) Prepare and file with the Securities and Exchange
Commission ("SEC") a registration statement with respect to such Registrable
Securities and use its best efforts to cause such registration statement to
become and remain effective; provided, however, that in connection with any
proposed registration intended to permit an offering of any securities from time
to time (i.e., a so-called "shelf registration"), the Company shall in no event
be obligated to cause any such registration to remain effective for more than
ninety (90) days.

                          (b) Prepare  and file  with  the  SEC,  in a timely
manner, such amendments and supplements to such registration statement and the
prospectus used in connection with such registration statement as may be
necessary to comply with the provisions of the Act with respect to the
disposition of all securities covered by such registration statement.

                          (c) Furnish to the  Holders and  deliver as  directed
such numbers of copies of a prospectus, including a preliminary prospectus, in
conformity with the requirements of the Act, and such other documents as they
may reasonably request in order to facilitate the disposition of Registrable
Securities owned by them.

                          (d) Use its best  efforts to register  and qualify the
securities covered by such registration statement under such other securities or
Blue Sky laws of such jurisdictions as shall be reasonably appropriate for the
distribution of the securities covered by the registration statement, provided
that the Company shall not be required in connection therewith or as a condition
thereto to qualify to do business or to file a general consent to service of
process in any such states or jurisdictions, and further provided that (anything
in this Agreement to the contrary notwithstanding with respect to the bearing of
expenses) if any jurisdiction in which the securities shall be qualified shall
require that expenses incurred in connection with the qualification of the
securities in that jurisdiction be borne by selling shareholders, then such
expenses shall be payable by selling shareholder pro rata, to the extent
required by such jurisdiction.

                  1.6 FURNISH INFORMATION. It shall be a condition precedent to
the obligations of the Company to take any action pursuant to Section I that the
Holders shall furnish to the Company such information regarding them, the
Registrable Securities held by them, and the intended method of disposition of
such securities as the Company shall reasonably request and as shall be required
in connection with the action to be taken by the Company.

                  1.7 COMPANY REGISTRATION EXPENSES. In the case of any
registration effected pursuant to paragraph 1.3, the Company shall .bear all
registration and qualification fees and expenses (excluding prorated SEC
registration fees and underwriters, discounts and commissions) , including any
other additional costs and disbursements of counsel for the Company that result
from the inclusion of securities held by the Holders in such registration;
provided, however, that each selling Holder shall bear the fees and costs of its
own counsel.

                  1.8 EXPENSES OF DEMAND REGISTRATION. The Company shall bear
all expenses incurred in connection with a registration pursuant to paragraph
1.4 (excluding Investors' attorneys, fees and underwriters' discounts and
commissions), including without limitation all

                                       3
<PAGE>

registration and qualification fees, printers, and accounting fees and Company's
attorneys, fees; provided, however, that the Company shall not be required to
pay for any expenses of any registration proceeding begun pursuant to paragraph
1.4 if the registration request is subsequently withdrawn, unless the Holders
agree to forfeit their right to one demand registration pursuant to paragraph
1.4.

                  1.9 UNDERWRITING REQUIREMENTS. In connection with any offering
involving an underwriting of shares pursuant to Sections 1.3 and 1.4 hereof the
Company shall not be required to include any of the Holders' Registrable
Securities in such underwriting unless they accept the terms of the underwriting
as agreed upon between the Company and the underwriters selected by it. If the
managing underwriter or underwriters of such public offering advise the Company
that, in their opinion, the amount of Registrable Securities to be included in
any such offering pursuant to the request of the Holders would adversely affect
the success of such offering, the Company will include in such offering on
behalf of such Holders, the amount of Registrable Securities equal to the total
amount which, in the opinion of such managing underwriter or underwriters, can
be sold without such adverse effect, and such Registrable Securities shall be
allocated on a pro-rata basis among the Holders of the Registrable Securities
requested to be included in such offering.

                 1.10 DELAY OF REGISTRATION. No Holder shall have any right to
take any action to restrain, enjoin, or otherwise delay any registration as the
result of any controversy that might arise with respect to the interpretation or
implementation of this Section 1.

                 1.11 INDEMNIFICATION AND CONTRIBUTION. Subject to paragraph 1.
8, in the event any Registrable Securities are included in a registration
statement under Section 2:

                                (a) To the extent permitted by law, the
Company will indemnify and hold harmless each Holder requesting or joining in a
registration, any underwriter (as defined in the Act) for it, and each person,
if any, who controls such Holder or underwriter within the meaning of the Act or
the Securities Exchange Act of 1934 (the "1934 Act") against any losses, claims,
damages or liabilities, joint or several, to which they may become subject under
the Act, the 1934 Act or other-wise, insofar as such losses, claims, damages or
liabilities (or actions in respect thereof) arise out of or are based on any
untrue or alleged untrue statement of any material fact contained in such
registration statement, including any preliminary prospectus or final
prospectus, or any amendments or supplements thereto, or arise out of or are
based upon the omission or alleged omission to state therein a material fact
required to be stated therein, or necessary to make the statements therein not
misleading or arise out of any violation by the Company of any rule or
regulation promulgated under the Act or the 1934 Act applicable to the Company
and relating to action or inaction required of the Company in connection with
any such registration; and will reimburse each such Holder, such underwriter, or
such controlling person for any legal or other expenses reasonably incurred by
them in connection with investigating or defending any such loss, claim, damage,
liability or action; provided, however, that the indemnity agreement contained
in this paragraph 1.11(a) shall not apply to amounts paid in settlement of any
such loss, claim, damage, liability or action if such settlement is effected
without the consent of the Company (which consent shall not be unreasonably
withheld) nor shall the Company be liable in any such case for any such loss,
claim, damage, liability or action to the extent that it arises out of or is
based upon an untrue statement or alleged untrue statement

                                       4
<PAGE>

or omission or alleged omission made in connection with such registration
statement, preliminary prospectus, final prospectus, or amendments or
supplements thereto, in reliance upon and in conformity with written information
furnished expressly for use in connection with such registration by any such
Holder, underwriter or controlling person.

                                (b) To the extent permitted by law, each
Holder requesting or joining in a registration will severally and not jointly
indemnify and hold harmless the Company, each of its directors, each of its
officers who have signed the registration statement, each person, if any, who
controls the Company within the meaning of the Act or the 1934 Act, and each
agent and any under-writer for the Company (within the meaning of the Act or the
1934 Act) against any losses, claims, damages or liabilities, joint or several,
to which the Company or any such director, officer, controlling person, agent or
underwriter may become subject, under the Act or otherwise, insofar as such
losses, claims, damages or liabilities (or actions in respect thereto) arise out
of or are based upon any untrue statement or alleged untrue statement of any
material fact contained in such registration statement, including any
preliminary prospectus or final prospectus, or any amendments or supplements
thereof, or arise out of or are based upon the omission or alleged omission to
state therein a material fact required to be stated therein or necessary to make
the statements therein not misleading, in each case to the extent, but only to
the extent, that such untrue statement or alleged untrue statement or omission
or alleged omission was made in such registration statement, preliminary or
final prospectus, or amendments or supplements thereto, in reliance upon and in
conformity with written information furnished by such Holder expressly for use
in connection with such registration; and each such Holder will reimburse any
legal or other expenses reasonably incurred by the Company or any such director,
officer, controlling person, agent or underwriter in connection with
investigating or defending any such loss, claim, damage, liability or action;
provided, however, that the indemnity agreement contained in this paragraph
1.11(b) shall not apply to amounts paid in settlement of any such loss, claim,
damage, liability or action if such settlement is effected without the consent
of such Holder (which consent shall not be unreasonably withheld).

                                (c) Promptly after receipt by an indemnified
party under this paragraph of notice of the commencement of any action, such
indemnified party will, if a claim in respect thereof is to be made against any
indemnifying party under this paragraph, notify the indemnifying party in
writing of the commencement thereof and (unless the interest of the indemnifying
party conflicts with that of the indemnified party) the indemnifying party shall
have the right to participate in, and, to the extent the indemnifying party so
desires, jointly with any other indemnifying party similarly noticed, to assume
the defense thereof with counsel mutually satisfactory to the parties. The
failure to notify an indemnifying party promptly of the commencement of any such
action, if prejudicial to his ability to defend such action, shall relieve such
indemnifying party, to the extent that he is prejudiced thereby, of any
liability to the indemnified party under this paragraph, but the Omission so to
notify the indemnifying party will not relieve him of any liability that he may
have to any indemnified party otherwise than under this paragraph.

                                (d) In order to provide for just and equitable
contribution to joint liability under the Act in any case in which either (i)
any Holder exercising rights under this Agreement, or any controlling person of
any such Holder, makes a claim for indemnification

                                       5
<PAGE>

pursuant to this paragraph 1.11 but it is judicially determined (by the entry of
a final judgment or decree by a court of competent jurisdiction and the
expiration of time to appeal or the denial of the last right of appeal) that
such indemnification may not be enforced in such case notwithstanding the fact
that this paragraph 1.11 provides for indemnification in such case, or (ii)
contribution under the Act may be required on the part of any such selling
Holder or any such controlling person in circumstances for which indemnification
is provided under this paragraph 1.11; then, and in each such case, the Company
and such Holder will contribute to the aggregate losses, claims, damages or
liabilities to which they may be subject (after contribution from others) in
such proportion so that such Holder is responsible for the portion represented
by the percentage that the public offering price of its Registrable Securities
offered by the registration statement bears to the public offering price of all
securities offered by such registration statement, and the Company is
responsible for the remaining portion; provided, however, that, in any such
case, (A) no such Holder will be required to contribute any amount in excess of
the public offering price of all such Registrable Securities offered, by it
pursuant to such registration statement; and (B) no person or entity guilty of
fraudulent misrepresentation (within the meaning of Section 11(f) of the Act)
will be entitled to contribution from any person or entity who was not guilty of
such fraudulent misrepresentation.

                  1.12 REPORTS UNDER SECURITIES EXCHANGE ACT OF 1934. With a
view to making available to the Holders the benefits of Rule 144 promulgated
under the Act and any other rule or regulation of the SEC that may at any time
permit a Holder to sell securities of the Company to the public without
registration, the Company agrees to use its best efforts to:

                                (a) Make and keep public information available,
as those terms are understood and defined in Rule 144, at all times subsequent
to the later of W two years after the last closing date of the offering (as
defined in the Purchase Agreement) and (ii) the effective date of the IPO; and

                                (b) File with the SEC in a timely manner all
reports and other documents required of the Company under the 1934 Act.

                  1.13 TRANSFER OF REGISTRATION RIGHTS. The rights of the
Investor hereunder may only be transferred to persons who acquire at least
twenty percent (20%) of the Registrable Securities originally issued to such
Investor and who agree to. be bound by the provisions of this Registration
Rights Agreement. The Investor shall give written notice of any such transfer at
such time of transfer, stating the name and address of the transferee and
identifying the securities with respect to which the rights are ,being assigned.

                  2.   RESTRICTION ON TRANSFER OF SHARES. All certificates for
Common Shares now or hereafter issued by the Company shall he marked with the
following legend:

                   THE SHARES REPRESENTED BY THIS CERTIFICATE HAVE BEEN ACQUIRED
                   FOR INVESTMENT AND HAVE NOT BEEN .REGISTERED UNDER THE
                   SECURITIES ACT OF 1933 OR THE SECURITIES LAWS OF ANY STATE
                   (THE "ACTS") AND MAY NOT 13E TRANSFERRED OR

                                       6
<PAGE>

                   OTHERWISE DISPOSED OF UNLESS THEY HAVE BEEN REGISTERED UNDER
                   THE ACTS OR AN EXEMPTION FROM REGISTRATION IS AVAILABLE.
                   TRANSFER OF SUCH SHARES IS ALSO RESTRICTED BY THE PROVISIONS
                   OF A COMMON STOCK PURCHASE AGREEMENT DATED ________________
                   19 ______.

                   3.  TERMINATION.  This Agreement  shall  terminate and be
without further force or effect on the sale of all of the Registrable Securities
held by the Investor.

                  4.   NOTICES. All notices required or permitted to be given
to the Company or the Investor pursuant to any of the terms hereof shall be sent
by certified mail, return receipt requested, postage prepaid, addressed if to
the Investor at the address set forth on the signature pages or if to the
Company, to the address set forth on the cover page of the Purchase Agreement.

                  5.   ENTIRE AGREEMENT. This Agreement contains the entire
agreement of the parties with respect to the subject matter hereof.

                  6.   AMENDMENTS AND WAIVERS. Any term of this Agreement may be
amended, and the observance of any term of this Agreement may be waived (either
generally or in a particular instance and either retroactively or prospectively)
, with the written consent of persons holding fifty percent (50t) of the Common
Shares which were purchased by the Participating Investors. Any amendment or
waiver effected in accordance with this Section shall be binding upon each party
to this Agreement, any person who may become a party, and the Company. Each
party hereto acknowledges that by operation of this Section persons holding
fifty percent (50k) of the Common Shares which have been purchased by the
Participating Investors pursuant to the offering will have the right and power
to amend this Agreement.

                  7.   SUCCESSORS AND ASSIGNS.  This  Agreement  shall be
binding upon and inure to the benefit of the parties hereto, their heirs, legal
representatives, successors and assigns.

                  8.   Counterparts.  This  Agreement may be executed in
counterparts, each of which shall be an original, but all of which together
shall constitute one and the same agreement.

                  9.   APPLICABLE  LAW. This  Agreement  shall be governed by
the laws of the Commonwealth of Pennsylvania.

                                       7
<PAGE>

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

                                NOMOS CORPORATION

                                By:
                                -----------------------------------------------

                                INVESTOR:

                                [By:                                          ]
                                -----------------------------------------------

                                       8

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