Exhibit 10.4

                                Lock-Up Agreement

North American Scientific, Inc.
20200 Sunburst Avenue
Chatworth, California

Re:   NOMOS Corporation

Ladies and Gentlemen:

      The undersigned is an owner of record of certain shares of capital stock
of NOMOS Corporation, a Delaware corporation (the "Company"), or securities
convertible into or exercisable for shares of capital stock of the Company
("Company Capital Stock"). North American Scientific, Inc., a Delaware
corporation (the "Acquiror"), and the Company are parties to an Agreement and
Plan of Merger, dated as of October 26, 2003 (as amended from time to time, the
"Merger Agreement"), pursuant to which the shares of Company Capital Stock held
by the undersigned are to be converted into the right to receive, as
consideration or a portion of the consideration for such shares of Company
Capital Stock, shares of common stock, par value $0.01 per share, of the
Acquiror (the "Acquiror Common Stock"), in accordance with the terms of the
Merger Agreement. Capitalized terms that are used but not defined in this
Agreement are used herein as defined in the Merger Agreement.

      In connection with and in consideration of the foregoing, the undersigned
hereby agrees that the undersigned will not offer to sell, contract to sell, or
sell, dispose of, loan, pledge or grant any rights with respect to any shares of
Acquiror Common Stock (any such transaction or action being referred to herein
as a "Disposition") acquired in connection with the transactions contemplated by
the Merger Agreement or any options or warrants to purchase any shares of
Acquiror Common Stock acquired in connection with the transactions contemplated
by the Merger Agreement (collectively, "Securities") without the prior written
consent of Acquiror, for a period commencing on the Closing Date and continuing
until the date that is 545 days after the Closing Date (the "Lock-Up Period");
provided, that, the undersigned may engage in any Disposition with respect to
any Securities held by the undersigned which have been registered under the
Securities Act of 1933, as amended, pursuant to that certain Registration Rights
Agreement by and among the Acquiror and the other parties thereto.

      In addition, the undersigned expressly agrees that the undersigned may
not, during the Lock-Up Period, engage in any hedging or other transaction which
is designed to or could reasonably be expected to lead to or result in a
Disposition of Securities, irrespective of whether such Securities would be
disposed of by someone other than the undersigned. Such prohibited hedging or
other transaction would include, without limitation, any short sale (whether or
not against the box) or any purchase, sale or grant of any right (including,
without limitation, any put or call option) with respect to any Securities or
with respect to any security (other than a broad-based market basket or index)
that included, relates to or derives any significant part of its value from
Securities.

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      Notwithstanding anything contain in the foregoing to the contrary, the
undersigned may sell, contract to sell, or sell, dispose of, or otherwise
transfer for value or otherwise, the Securities (i) as a bona fide gift or
gifts, provided that the donee or donees thereof agree to be bound by the
restrictions set forth herein or (ii) to members of the undersigned's immediate
family or to any trust for the direct or indirect benefit of the undersigned or
the immediate family of the undersigned, provided that the transferees thereof
agree to be bound by the restrictions set forth herein. For purposes of this
Lock-Up Agreement, "immediate family" shall mean (i) the undersigned; (ii) any
spouse, lineal descendant or antecedent, father, mother, brother or sister of
the undersigned 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.

      The undersigned also agrees and consents to the entry of stop transfer
instructions with the Acquiror's transfer agent and registrar against the
transfer of shares of Acquiror Common Stock or other Securities acquired in
connection with the transactions contemplated by the Merger Agreement held by
the undersigned except in compliance with the foregoing restrictions. Acquiror,
acting alone and in its sole discretion, may waive any provisions of this
Lock-Up Agreement without notice to any third party.

      This Lock-Up Agreement is irrevocable and will be binding on the
undersigned and the respective successors, heirs, personal representatives and
assigns of the undersigned.

      This Lock-Up Agreement shall automatically terminate on the earlier of (i)
the expiration of the Lock-Up Period or (ii) such time as the Merger Agreement
is terminated in accordance with its terms.

                                        Very truly yours,


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