**Exhibit
      10.3**





**FOURTH
      A** **MENDMENT
      AND FORBEARANCE**


**TO**


**LOAN
      AND SECURITY AGREEMENT**





**THIS
      FOURTH AMENDMENT AND FORBEARANCE** **to
      Loan and Security Agreement** (this **** “Amendment
      and Forbearance”) is entered into on September 14, 2007, by and between




SILICON
      VALLEY BANK (“Bank”)




and
      the
      following (collectively, jointly and severally, the "Borrower") whose address
      is
      20200 Sunburst Street, Chatsworth, California 91311:




NORTH
      AMERICAN SCIENTIFIC ,
      INC., a
      Delaware corporation (“NASI”); and




NORTH
      AMERICAN SCIENTIFIC, INC., a California corporation (“NASI-CA”).






**RECITALS**




**A.**  Bank
      and
      Borrower have entered into that certain Loan and Security Agreement, with an
      Effective Date of October 5, 2005 (as the same has been, and may hereafter
      from
      time to time be amended, modified, supplemented or restated, the “Loan
      Agreement”).




**B.**  Bank
      has
      extended credit to Borrower for the purposes permitted in the Loan Agreement.




**C.**  Borrower
      is currently in default of the Loan Agreement for failing to comply with the
      Minimum Tangible Net Worth Financial Covenant set forth in Section 5 of the
      Amended and Restated Schedule 2 to Loan and Security Agreement for the month
      ending July 31, 2007 and August 31, 2007 (the “Existing Defaults”).




**D.**  Borrower
      has requested that Bank forbear from exercising its rights and remedies against
      Borrower from the date hereof until such time as Bank determines in its
      discretion to cease such forbearance (the “Forbearance Period”). Although Bank
      is under no obligation to do so, Bank is willing to forbear from exercising
      its
      rights and remedies against Borrower through the Forbearance Period on the
      terms
      and conditions set forth in this Amendment and Forbearance, so long as Borrower
      complies with the terms, covenants and conditions set forth in this Amendment
      and Forbearance in a timely manner. Moreover, the parties desire to amend the
      Loan Agreement as herein set forth.




1




**A** **GREEMENT**




**N** **OW,
      THEREFORE,** in
      consideration of the foregoing recitals and other good and valuable
      consideration, the receipt and adequacy of which are hereby acknowledged, and
      intending to be legally bound, the parties hereto agree as follows:




**1.**  **Definitions.** Capitalized terms used but not defined in this Amendment and Forbearance shall
      have the meanings given to them in the Loan Agreement.




**2.**  **Amendments
      and Forbearance to Loan Agreement.** The
      Loan
      Agreement is hereby amended as follows, effective as of the date
      hereof:




**2.1**  **Forbearance
      Period** .
      So long
      as no Event of Default, other than the Existing Defaults, occurs, subject to
      the
      terms and conditions set forth herein, Bank shall forbear from filing any legal
      action or instituting or enforcing any rights and remedies it may have against
      Borrower through the Forbearance Period. Except as expressly provided herein,
      this Amendment and Forbearance does not constitute a waiver or release by Bank
      of any Obligations or of any existing Event of Default (other than the Existing
      Defaults) or of any Event of Default which may arise in the future after the
      date of execution of this Amendment and Forbearance. If Borrower does not comply
      with the terms of this Amendment and Forbearance, Bank shall have no further
      obligations under this Amendment and Forbearance and shall be permitted to
      exercise at such time any rights and remedies against Borrower as it deems
      appropriate in its sole and absolute discretion. Borrower understands that
      Bank
      has made no commitment and is under no obligation whatsoever to grant any
      additional extensions of time at the end of the Forbearance Period.




**2.2**  **Forbearance
      Terms** .
      Repayment and performance of all obligations of Borrower to Bank under the
      Loan
      Agreement and this Amendment and Forbearance shall be secured by the
      Collateral.




**2.3**  **Modified
      Credit Limit (Reduction of Revolving Line Credit Amount and Deletion of NOMOS
      Corporation).** Section
      1 of the Amended and Restated Schedule 2 to Loan and Security Agreement is
      hereby amended to read as follows:






**1.** **CREDIT
      LIMIT**


(Section
      2.1.1):  An
      amount
      not to exceed:





(a)
            the
                lesser of (1)**$3,000,000**at
                any one time outstanding (the “Revolving
                Line Credit Amount”)
                or (2) the sum of the following (the “Borrowing
                Base”):





(i)
            up
                to**80%**(an
                “Advance Rate”) of the amount of NASI Eligible Accounts,plus




(ii)
            up
                to**80%**(an
                “Advance Rate”) of the amount of NASI-CA Eligible Accounts,plus




2




(iii)
            [omitted];plus





(iv)
            the
                lesser of the following (the “Bridge
                Loan Sublimit”):





(A)
            **$1,500,000**or





(B)
            the
                sum of:





(I)
            **20%**(an
                “Advance Rate) of the amount of NASI-CA Eligible Accounts,plus





(II)
            **100%**(an Advance Rate) of the amount of NASI-CA Accounts not deemed to
                be
                Eligible Accounts;





provided , however ,
      in no
      event shall the amount of the Bridge Loan Sublimit exceed **$750,000** until
      such time as Borrower has received a signed term sheet dated after the date
      hereof, satisfactory to Silicon in its sole discretion, for the receipt by
      Borrower of at least $10,000,000 for the issuance of additional equity
      securities of Borrower.


minus




(b)  the
      sum
      of the following:




3




(i)
            the
                amount of all outstanding Letters of Credit (including drawn but
                unreimbursed Letters of Credit);plus





(ii)
            the
                FX Reserve; andplus





(iii)
            the
                aggregate amount of Cash Management Services
                utilizations.





provided , however ,
      that
      Bank shall have the right, in Bank’s discretion, to modify the above Advance
      Rates based upon the results of field audits conducted by Bank.





**Letter
      of Credit Sublimit**


**(Section
                2.1.2):**
            
            
            **$500,000**.





**Foreign
      Exchange Sublimit**


**(Section
                2.1.3):**
            
            
            **$500,000**.





**Cash
      Management Services Sublimit:**


**(Section
                2.1.4):**
            
            
            **$500,000**.





**2.4**  **Consent
      to Agility Capital, LLC Subordinated Debt.** Notwithstanding
      anything to the contrary in the Loan Agreement, Bank hereby consents to the
      Borrower issuing up to $750,000 in subordinated debt to Agility Capital, LLC
      (“Agility”), which debt may be secured by a lien on any or all of Borrower’s
      assets, provided that Agility execute and deliver to Bank, on
      Bank’s
      standard form with such changes thereto as are acceptable to Bank in its good
      faith business judgment, a subordination agreement pursuant
      to which the debt owed by Borrower to Agility will be fully subordinated to
      the
      Obligations and the lien, if any, granted to Agility will be fully subordinated
      to the lien granted in favor of Bank.




**3.**  **Limitation
      of Amendments and Forbearance.**




**3.1**  The
      amendments and forbearance set forth in Section 2, above, are effective for
      the purposes set forth herein and shall be limited precisely as written and
      shall not be deemed to (a) be a consent to any amendment, waiver or
      modification of any other term or condition of any Loan Document, or
      (b) otherwise prejudice any right or remedy which Bank may now have or may
      have in the future under or in connection with any Loan Document.




**3.2**  This
      Amendment and Forbearance shall be construed in connection with and as part
      of
      the Loan Documents and all terms, conditions, representations, warranties,
      covenants and agreements set forth in the Loan Documents, except as herein
      amended, are hereby ratified and confirmed and shall remain in full force and
      effect.




4




**4.**  **Representations
      and Warranties.** To
      induce Bank to enter into this Amendment and Forbearance, Borrower hereby
      represents and warrants to Bank as follows:




**4.1**  Immediately
      after giving effect to this Amendment and Forbearance (a) the
      representations and warranties contained in the Loan Documents are true,
      accurate and complete in all material respects as of the date hereof (except
      to
      the extent such representations and warranties relate to an earlier date, in
      which case they are true and correct as of such date), and (b) no Event of
      Default has occurred and is continuing;




**4.2**  Borrower
      has the corporate power and authority to execute and deliver this Amendment
      and
      Forbearance and to perform its obligations under the Loan Agreement, as amended
      by this Amendment and Forbearance;




**4.3**  The
      organizational documents of Borrower delivered to Bank on the Effective Date
      remain accurate and complete and have not been amended, supplemented or restated
      since the Effective Date and are, and continue to be, in full force and
      effect;




**4.4**  The
      execution and delivery by Borrower of this Amendment and Forbearance and the
      performance by Borrower of its obligations under the Loan Agreement, as amended
      by this Amendment and Forbearance, have been duly authorized;




**4.5**  The
      execution and delivery by Borrower of this Amendment and Forbearance and the
      performance by Borrower of its obligations under the Loan Agreement, as amended
      by this Amendment and Forbearance, do not and will not contravene (a) any
      law or regulation binding on or affecting Borrower, (b) any material
      agreement by which Borrower or its property is bound, (c) any order,
      judgment or decree of any court or other governmental or public body or
      authority, or subdivision thereof, binding on Borrower, or (d) the
      organizational documents of Borrower;




**4.6**  The
      execution and delivery by Borrower of this Amendment and Forbearance and the
      performance by Borrower of its obligations under the Loan Agreement, as amended
      by this Amendment and Forbearance, do not require any order, consent, approval,
      license, authorization or validation of, or filing, recording or registration
      with, or exemption by any governmental or public body or authority, or
      subdivision thereof, binding on either Borrower, except as already has been
      obtained or made; and




**4.7**  This
      Amendment and Forbearance has been duly executed and delivered by Borrower
      and
      is the binding obligation of Borrower, enforceable against Borrower in
      accordance with its terms, except as such enforceability may be limited by
      bankruptcy, insolvency, reorganization, liquidation, moratorium or other similar
      laws of general application and equitable principles relating to or affecting
      creditors’ rights.




**5.**  **Prior
      Agreement** .
      The
      Loan Documents are hereby ratified and reaffirmed and shall remain in full
      force
      and effect. This Amendment and Forbearance is not a novation and the terms
      and
      conditions of this Amendment and Forbearance shall be in addition to and
      supplemental to all terms and conditions set forth in the Loan Documents. In
      the
      event of any conflict or inconsistency between this Amendment and Forbearance
      and the terms of such documents, the terms of this Amendment and Forbearance
      shall be controlling, but such document shall not otherwise be affected or
      the
      rights therein impaired.




5




**6.**  **Release
      by Borrower** .




**6.1**  **FOR
      GOOD AND VALUABLE CONSIDERATION** ,
      Borrower hereby forever relieves, releases, and discharges Bank and its present
      or former employees, officers, directors, agents, representatives, attorneys,
      and each of them, from any and all claims, debts, liabilities, demands,
      obligations, promises, acts, agreements, costs and expenses, actions and causes
      of action, of every type, kind, nature, description or character whatsoever,
      whether known or unknown, suspected or unsuspected, absolute or contingent,
      arising out of or in any manner whatsoever connected with or related to facts,
      circumstances, issues, controversies or claims existing or arising from the
      beginning of time through and including the date of execution of this Amendment
      and Forbearance (collectively “ **Released
      Claims** ”).
      Without limiting the foregoing, the Released Claims shall include any and all
      liabilities or claims arising out of or in any manner whatsoever connected
      with
      or related to the Loan Documents, the Recitals hereto, any instruments,
      agreements or documents executed in connection with any of the foregoing or
      the
      origination, negotiation, administration, servicing and/or enforcement of any
      of
      the foregoing.




**6.2**  In
      furtherance of this release, Borrower expressly acknowledges and waives any
      and
      all rights under Section 1542 of the California Civil Code, which provides
      as
      follows:




“ **A
      general release** does not
      extend to claims which the creditor does not know or expect to exist in his
      favor at the time of executing the release, which if known by him must have
      materially affected his settlement with the debtor.” (Emphasis
      added.)




**6.3**  By
      entering into this release, Borrower recognizes that no facts or representations
      are ever absolutely certain and it may hereafter discover facts in addition
      to
      or different from those which it presently knows or believes to be true, but
      that it is the intention of Borrower hereby to fully, finally and forever settle
      and release all matters, disputes and differences, known or unknown, suspected
      or unsuspected; accordingly, if Borrower should subsequently discover that
      any
      fact that it relied upon in entering into this release was untrue, or that
      any
      understanding of the facts was incorrect, Borrower shall not be entitled to
      set
      aside this release by reason thereof, regardless of any claim of mistake of
      fact
      or law or any other circumstances whatsoever. Borrower acknowledges that it
      is
      not relying upon and has not relied upon any representation or statement made
      by
      Bank with respect to the facts underlying this release or with regard to any
      of
      such party’s rights or asserted rights.




**6.4**  This
      release may be pleaded as a full and complete defense and/or as a
      cross-complaint or counterclaim against any action, suit, or other proceeding
      that may be instituted, prosecuted or attempted in breach of this release.
      Borrower acknowledges that the release contained herein constitutes a material
      inducement to Bank to enter into this Amendment and Forbearance, and that Bank
      would not have done so but for Bank’s expectation that such release is valid and
      enforceable in all events.




6




**6.5**  Borrower
      hereby represents and warrants to Bank, and Bank is relying thereon, as
      follows:




**(a)**  Except
      as
      expressly stated in this Amendment and Forbearance, neither Bank nor any agent,
      employee or representative of Bank has made any statement or representation
      to
      Borrower regarding any fact relied upon by Borrower in entering into this
      Amendment and Forbearance.




**(b)**  Borrower
      has made such investigation of the facts pertaining to this Amendment and
      Forbearance and all of the matters appertaining thereto, as it deems
      necessary.




**(c)**  The
      terms
      of this Amendment and Forbearance are contractual and not a mere recital.




**(d)**  This
      Amendment and Forbearance has been carefully read by Borrower, the contents
      hereof are known and understood by Borrower, and this Amendment and Forbearance
      is signed freely, and without duress, by Borrower.




**(e)**  Borrower
      represents and warrants that it is the sole and lawful owner of all right,
      title
      and interest in and to every claim and every other matter which it releases
      herein, and that it has not heretofore assigned or transferred, or purported
      to
      assign or transfer, to any person, firm or entity any claims or other matters
      herein released. Borrower shall indemnify Bank, defend and hold it harmless
      from
      and against all claims based upon or arising in connection with prior
      assignments or purported assignments or transfers of any claims or matters
      released herein.




**7.**  **Counterparts.** This
      Amendment and Forbearance may be executed in any number of counterparts and
      all
      of such counterparts taken together shall be deemed to constitute one and the
      same instrument.




**8.**  **Effectiveness** .
      This
      Amendment and Forbearance shall be deemed effective upon (a) the due execution
      and delivery to Bank of this Amendment and Forbearance by each party hereto
      and
      (b) Borrower’s payment of an amendment fee in an amount equal to $7,500.




**9.**  **Governing
      Law.** This
      Amendment and Forbearance and the rights and obligations of the parties hereto
      shall be governed by and construed in accordance with the laws of the State
      of
      California.




[Signature
      page follows.]




7




**IN
      WITNESS
      WHEREOF,** the
      parties hereto have caused this Amendment and Forbearance to be duly executed
      and delivered as of the date first written above.





“Borrower”:
              
              **NORTH
                AMERICAN SCIENTIFIC, INC.**
              A
                Delaware corporation
              
              
              By/s/John
                B. Rush
              
              President
                or Vice President
            “Bank”:
              
              **SILICON
                VALLEY BANK**
              
              
              
              By/s/Derek
                Brunelle
              
              TitleVice
                President“Borrower”:
              
              **NORTH
                AMERICAN SCIENTIFIC, INC.**
              A
                California corporation
              
              
              By/s/John
                B. Rush
              
              President or Vice President




8