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                                                                 Exhibit 10.3(a)



                                NOMOS CORPORATION
                             2591 WEXFORD BAYNE ROAD
                               SEWICKLEY, PA 15143

                               September 28 , 1999


Cross-Atlantic Partners
Cross-Atlantic Partners II
Cross-Atlantic Partners III
650 Madison Avenue
New York, New York 10022


     In consideration of the purchase by Cross-Atlantic Partner, Cross-Atlantic
Partners II and Cross-Atlantic Partners III (collectively, "Cross-Atlantic") of
an aggregate of 350,000 shares of Series A Convertible Preferred Stock, par
value $.0001 per share, (the "Preferred Stock") of NOMOS Corporation (the
"Company"), pursuant to a Subscription Agreement dated September 28, 1999 , the
undersigned hereby agree to the terms and obligations of this letter agreement
("Agreement").

1.   DIRECTOR

     1.1 DESIGNATION OF DIRECTOR. At all times during which Cross-Atlantic holds
at least 100,000 shares of Preferred Stock, Cross-Atlantic shall have the right
to nominate one director to the Board of Directors of the Company (such director
being referred to herein as the "Cross-Atlantic Director"). The initial person
nominated to be the Cross-Atlantic Director in John L. Cassis. The Company shall
pay all reasonable travel expenses and other out-of-pocket disbursements of the
Cross-Atlantic Director in connection with his attendance at meetings of the
Company's Board of Directors and committees thereof.

     1.2 BOARD MEETINGS. The Company, at any meeting of the stockholders of the
Company at which members of the Board of Directors are to be elected or at any
time that members of the Board of Directors are to be elected by written
consent, hereby agrees to use its best efforts to cause the Cross-Atlantic
Director to be elected to the Board; PROVIDED that Cross-Atlantic is then
entitled to nominate a director pursuant to Section 1.1 above. John Friede, for
so long as he remains a shareholder of the Company, shall vote or act with
respect to his shares of capital stock of the Company to elect the person
nominated as the Cross-Atlantic Director, who shall initially be John L. Cassis.

     1.3 APPOINTMENT OF NEW DIRECTOR. In the event of the resignation, death,
removal or disqualification of the director elected pursuant to Section 1.2,
Cross-Atlantic shall promptly nominate a new director, and, after written notice
of the nomination has been given by Cross-Atlantic, the Company shall use its
best efforts to cause such nominee to be elected to the Board of Directors;
provided that Cross-Atlantic is then entitled to nominate a director pursuant to
Section 1.1 above. John Friede, for so long as he remains a stockholder of the
Company, shall


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vote or act with respect to his shares of capital stock of the Company to elect
such nominee to the Board of Directors.

     1.4 RESIGNATION OF CROSS-ATLANTIC DIRECTOR. In the event that
Cross-Atlantic is no longer entitled to nominate a director pursuant to Section
1.1 above, Cross-Atlantic agrees to cause the Cross-Atlantic Director then
serving on the Board of Directors to immediately resign from his/her position as
a director of the Company.

2.   MISCELLANEOUS.

     2.1 GOVERNING LAW. This Agreement shall be governed by and construed in
accordance with the laws of the Commonwealth of Pennsylvania.

     2.2 COUNTERPARTS. This Agreement may be executed in counterparts, each of
which shall be deemed an original, but all of which together shall constitute
one and the same instrument.

     2.3 AMENDMENTS AND WAIVERS. This Agreement may not be amended or modified
without the written consent of Cross-Atlantic and the Company, nor shall any
waiver be effective against any party unless in a writing executed on behalf of
such party.

     2.4. SEVERABILITY. If any provision of this Agreement shall be declared
void or unenforceable by any judicial or administrative authority, the validity
of any other provision and of the entire Agreement shall not be affected
thereby.

     2.5 CONFIDENTIALITY OF TERMS. Each party will keep strictly confidential
and not disclose to any third party the terms and conditions of this Agreement
except where required by law.

     2.6 PAYMENT OF FEES. The Company shall pay the reasonable fees and expenses
of Haythe & Curley, special counsel to Cross-Atlantic, for their services in
connection with Cross-Atlantic's purchase of Preferred Stock, up to a maximum of
$7,500.



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     If the foregoing accurately sets forth the agreement that the Company and
Cross-Atlantic have reached with respect to the subject matter hereof, please
indicate your agreement to the terms contained herein by countersigning in the
place indicated below.


                                   Sincerely,



                                   NOMOS CORPORATION

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

                                   Name:   John A. Friede
                                        ---------------------------------------

                                   Title:     Chairman
                                         --------------------------------------


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

                                   John Friede (solely with respect to his
                                   obligations under Section 1.2 and 1.3)



AGREED AND ACCEPTED:


CROSS-ATLANTIC PARTNERS

CROSS-ATLANTIC PARTNERS II

CROSS-ATLANTIC PARTNERS III

By:   /s/  John Cassis
   -----------------------------

Name:   John Cassis
     ---------------------------

Title:     Partner
      --------------------------




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