Exhibit
      10.1

    


    FOURTH
      AMENDMENT

    TO

    RIGHTS
      AGREEMENT

     

    This
      FOURTH AMENDMENT (this “Amendment”)
      dated
      as of January 14, 2008 to the Rights Agreement, dated as of October 14, 1998
      (the “
      Rights Agreement“),
      as
      previously amended, by and between North American Scientific, Inc., a Delaware
      corporation (the “Company“),
      and
      Computershare Trust Company, N.A., a federally chartered trust company
      (successor Rights Agent to U.S. Stock Transfer Corporation) (the “Rights
      Agent“),
      is
      entered into between such parties. Capitalized terms used but not defined herein
      are used herein as defined in the Rights Agreement.

     

    WHEREAS,
      the
      Board of Directors of the Company has authorized and approved a private
      placement involving the offering, sale and issuance by the Company of shares
      (the “Shares”)
      of the
      Company’s common stock, par value $0.01 per share (“Common
      Stock”),
      and
      warrants to purchase additional shares of Common Stock (the “Warrants,”
and
      collectively with the additional shares of Common Stock purchasable thereby
      and
      the Shares, the “Securities”)
      to a
      limited group of sophisticated investors (the “Investors”)
      pursuant to Securities Purchase Agreements and related transaction agreements
      or
      instruments (the “2008
      Private Placement”).
      The
      Securities Purchase Agreement dated as of December 12, 2007 by and among the
      Company and the Investors in connection with the foregoing and any other related
      transaction agreements or instruments are referred to herein as the
“2008
      Private Placement Documents”;

    


    WHEREAS,
      the
      willingness of the investors to enter into the Securities Purchase Agreements
      is
      conditioned upon, among other things, the supplement or amendment of the Rights
      Agreement on the terms set forth herein;

    


    WHEREAS,
      no
“Distribution Date,” “Share Acquisition Date,” “Redemption Date” or “Final
      Expiration Date” (as such terms are defined in the Rights Agreement) has
      occurred;

    


    WHEREAS,
      pursuant to Section 26 of the Rights Agreement, under circumstances set forth
      therein, (i) the Company may supplement or amend any provision of the Rights
      Agreement without the approval of any holders of certificates representing
      shares of Common Stock of the Company, and (ii) upon the delivery of a
      certificate from an appropriate officer of the Company which states that the
      proposed supplement or amendment is in compliance with the terms of Section
      26
      of the Rights Agreement, the Rights Agent shall execute such supplement or
      amendment;

    


    WHEREAS,
      the Company desires to amend the Rights Agreement as set forth herein and to
      direct the Rights Agent to execute this Amendment;

    


    WHEREAS,
      this Amendment shall become effective immediately; and

    


    
      
         

      

      
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    WHEREAS,
      the Board of Directors of the Company has adopted a resolution determining
      that
      the amendment of the Rights Agreement on the terms set forth herein is advisable
      and in the best interests of the Company and its stockholders;

    


    NOW,
      THEREFORE,
      in consideration of the premises and the mutual agreements herein set forth,
      the
      parties agree as follows:

    


    1. Effectiveness
      of this Amendment.
      This Amendment is executed pursuant to the first sentence of Section 26 of
      the Rights Agreement. The Company, by its execution of this Amendment, hereby
      directs the Rights Agent to execute this Amendment. This Amendment shall take
      effect immediately upon the execution hereof by the Company and the delivery
      of
      the certificate required pursuant to Section 26 of the Rights Agreement;
provided,
      however,
      that if the 2008 Private Placement is not closed for any reason, this Amendment
      shall be deemed rescinded and of no further force and effect.

    


    2. Certification
      of Appropriate Officer.
      The undersigned officer of the Company, being duly authorized on behalf of
      the
      Company, hereby certifies on behalf of the Company to the Rights Agent that
      (a)
      he is an “appropriate officer” as such term is used in Section 26 of the Rights
      Agreement, and (b) this Amendment is in compliance with Section 26 of the Rights
      Agreement.

    


    3. Defined
      Terms.
      All capitalized terms used but not defined in this Amendment shall have the
      meanings assigned to them in the Rights Agreement.

    


    4. No
      Other Provisions Affected.
      Except to the extent expressly amended by this Amendment, all of the provisions
      of the Rights Agreement shall remain in full force and effect.

    


    5. Amendment
      to the Rights Agreement.
      The Rights Agreement is hereby amended as follows:

    


    (a) Acquiring
      Person.
      The definition of “Acquiring Person” in Section 1(a) of the Rights
      Agreement is hereby amended by adding to the end of the first paragraph thereof
      the following:

    


    “;
      provided
      further, however,
      that the investors in the 2008 Private Placement (the “2008
      Investors”)
      shall not be deemed to be an “Acquiring Person” due to (a) the execution and
      delivery of the 2008 Private Placement Documents, (b) the announcement or
      commencement thereof, including, without limitation, the acquisition by the
      2008
      Investors of any of the Securities issued or issuable pursuant to the 2008
      Private Placement, and/or (c) the exercise of stock options to purchase shares
      of the Company’s common stock held by the 2008 Investors or their affiliates or
      the acquisition by the 2008 Investors or their affiliates of shares of the
      Company’s common stock as a result of grants made to the 2008 Investors or their
      affiliates or acquisitions of shares of the Company’s common stock by the 2008
      Investors or their affiliates in connection with such 2008 Investors’ or their
      affiliates’ role as a member of the board of directors of the
      Company.”

    


    
      
         

      

      
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    (b) Distribution
      Date.
      Section 1(g) of the Rights Agreement is hereby amended by inserting the
      following sentence at the end of such Section 1(g):

    


    “In
      addition, notwithstanding anything to the contrary contained herein, no
      Distribution Date shall occur solely as a result of the execution and delivery
      of the 2008 Private Placement Documents (or any amendments thereto in accordance
      with the terms thereof), or any actions taken by the 2008 Investors pursuant
      to
      the terms of any of the 2008 Private Placement Documents or the consummation
      of
      the transactions contemplated by the 2008 Private Placement Documents or the
      announcement or commencement thereof, including, without limitation, the
      acquisition by the 2008 Investors of any of the Securities issued or issuable
      pursuant to the 2008 Private Placement Documents.”

    


    (c) Share
      Acquisition Date.
      Section 1(v) of the Rights Agreement is hereby amended by inserting the
      following sentence at the end of such Section 1(v):

    


    “In
      addition, notwithstanding anything to the contrary contained herein, no Share
      Acquisition Date shall occur solely as a result of the execution and delivery
      of
      the 2008 Private Placement Documents (or any amendments thereto in accordance
      with the terms thereof), or any actions taken by the 2008 Investors pursuant
      to
      the terms of any of the 2008 Private Placement Documents or the consummation
      of
      the transactions contemplated by the 2008 Private Placement Documents or the
      announcement or commencement thereof, including, without limitation, the
      acquisition by the 2008 Investors of any of the Securities issued or issuable
      pursuant to the 2008 Private Placement Documents.”

     

    (d) Adjustment
      of Purchase Price, Number and Kind of Securities or Number of
      Rights.
      Section 11(a)(ii)(C) of the Rights Agreement is hereby amended by adding to
      the
      end of the first paragraph thereof the following:

     

    “;
      provided
      further, however,
      that a Person shall not be deemed to have become the Beneficial Owner of 20%
      or
      more of the Common Shares then outstanding for the purposes of this Section
      11(a)(ii)(C) as a result of the exercise of stock options to purchase shares
      of
      the Company’s common stock held by the 2008 Investors or their affiliates or the
      acquisition by the 2008 Investors or their affiliates of shares of the Company’s
      common stock as a result of grants made to the 2008 Investors or their
      affiliates or acquisitions of shares of the Company’s common stock by the 2008
      Investors or their affiliates in connection with such 2008 Investors’ or their
      affiliates’ role as a member of the board of directors of the
      Company.”

     

    (e) Adjustment
      of Purchase Price, Number and Kind of Securities or Number of
      Rights.
      Section 11 of the Rights Agreement is hereby amended by adding to the end
      thereof the following:

     

    “(o) Notwithstanding
      anything to the contrary contained herein, the provisions of this Section 11
      shall not apply to or be triggered by the execution and delivery of the of
      the
      2008 Private Placement Documents (or any amendments thereto in accordance with
      the terms thereof), or any actions taken by the 2008 Investors pursuant to
      the
      terms of any of the 2008 Private Placement Documents, or the consummation of
      the
      transactions contemplated by the 2008 Private Placement Documents, or the
      announcement or commencement thereof, including, without limitation, the
      acquisition by the 2008 Investors of any of the Securities issued or issuable
      pursuant to the 2008 Private Placement Documents.”

    


    
      
         

      

      
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    (f) Consolidation,
      Merger or Sale or Transfer of Assets or Earning Power.
      Section 13 of the Rights Agreement is hereby amended by adding to the end
      thereof the following:

    


    “(f) Notwithstanding
      anything to the contrary contained herein, the provisions of this Section 13
      shall not apply to or be triggered by the execution or delivery of the of the
      2008 Private Placement Documents (or any amendments thereto in accordance with
      the terms thereof), or any actions taken by the 2008 Investors pursuant to
      the
      terms of any of the 2008 Private Placement Documents, or the consummation of
      the
      transactions contemplated by the 2008 Private Placement Documents, or the
      announcement or commencement thereof, including, without limitation, the
      acquisition by the 2008 Investors of any of the Securities issued or issuable
      pursuant to the 2008 Private Placement Documents.”

    


    (g) References
      to Rights Agent.
      All references in the Rights Agreement to “U.S. Stock Transfer Corporation”
shall for all purposes be deemed to refer to “Computershare Trust Company,
      N.A.”

    


    (h)
      Amendment
      to Section 1(c) of the Rights Agreement.
      Section 1(c) of the Rights Agreement is hereby amended by deleting the phrase
      “State of California” and replacing it with the phrase “Commonwealth of
      Massachusetts”.

    


    (i) Amendment
      to Section 1(d) of the Agreement.
      Section 1(d) of the Rights Agreement is hereby amended by deleting the term
      “Pacific” in both instances and replacing each with the term “Eastern”.


    


    (j) Amendment
      to Section 2 of the Agreement.
      Section 2 of the Agreement is hereby amended by (i) adding the following
      language to the end of the second sentence: “; upon ten (10) days’ prior written
      notice to the Rights Agent.”; and (ii) adding the following language after the
      last sentence: “The Rights Agent shall have no duty to supervise, and shall in
      no event be liable for, the acts or omissions of any such Co-Rights Agent.”


    


    
      
         

      

      
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    (k) Amendment
      to Section 21 of the Agreement. Section 21 of the Agreement is hereby amended
      and replaced in its entirety as follows: 

    


    “Change
      of Rights Agent. The Rights Agent or any successor Rights Agent may resign
      and
      be discharged from its duties under this Agreement upon 30 calendar days’ notice
      in writing mailed to the Company and to each transfer agent of the Common Shares
      by registered or certified mail, and to the holders of the Right Certificates
      by
      first-class mail. In the event the transfer agency relationship in effect
      between the Company and the Rights Agent terminates, the Rights Agent will
      be
      deemed to have resigned automatically and be discharged from its duties under
      this Agreement as of the effective date of such termination, and the Company
      shall be responsible for sending any required notice. The Company may remove
      the
      Rights Agent or any successor Rights Agent upon 30 calendar days’ notice in
      writing, mailed to the Rights Agent or successor Rights Agent, as the case
      may
      be, and to each transfer agent of the Common Shares by registered or certified
      mail, and to the holders of the Right Certificates by first-class mail. If
      the
      Rights Agent shall resign or be removed or shall otherwise become incapable
      of
      acting, the Company shall appoint a successor to the Rights Agent. If the
      Company shall fail to make such appointment within a period of 30 calendar
      days
      after giving notice of such removal or after it has been notified in writing
      of
      such resignation or incapacity by the resigning or incapacitated Rights Agent
      or
      by the holder of a Right Certificate (who shall, with such notice, submit his
      Right Certificate for inspection by the Company), then the registered holder
      of
      any Right Certificate may apply to any court of competent jurisdiction for
      the
      appointment of a new Rights Agent. Any successor Rights Agent, whether appointed
      by the Company or by such a court, shall be a corporation organized and doing
      business under the laws of the United States or of any state of the United
      States, in good standing, which is authorized under such laws to exercise
      corporate trust or stockholder services powers and is subject to supervision
      or
      examination by federal or state authority and which has at the time of its
      appointment as Rights Agent, along with its Affiliates, a combined capital
      and
      surplus of at least $50 million. After appointment, the successor Rights Agent
      shall be vested with the same powers, rights, duties and responsibilities as
      if
      it had been originally named as Rights Agent without further act or deed; but
      the predecessor Rights Agent shall deliver and transfer to the successor Rights
      Agent any property at the time held by it hereunder, and execute and deliver
      any
      further assurance, conveyance, act or deed necessary for the purpose. Not later
      than the effective date of any such appointment, the Company shall file notice
      thereof in writing with the predecessor Rights Agent and each transfer agent
      of
      the Common Shares, and mail a notice thereof in writing to the registered
      holders of the Right Certificates. Failure to give any notice provided for
      in
      this Section 21, however, or any defect therein, shall not affect the legality
      or validity of the resignation or removal of the Rights Agent or the appointment
      of the successor Rights Agent, as the case may be.” 

    


    (l) Amendment
      to Section 26 of the Agreement. Section 26 of the Agreement is hereby amended
      to
      add the following phrase at the end of the next to last sentence thereof: “so
      long as such supplement or amendment shall not adversely affect the rights,
      duties or obligations of the Rights Agent under this Agreement.”

     

    
      
         

      

      
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    (m) Amendment
      to the Agreement.
      The
      Rights Agreement is hereby amended to add the following new
      section:

    


    “Section
      35. Force
      Majeure.
      Notwithstanding anything to the contrary contained herein, the Rights Agent
      shall not be liable for any delays or failures in performance resulting from
      acts beyond its reasonable control including, without limitation, acts of God,
      terrorist acts, shortage of supply, breakdowns or malfunctions, interruptions
      or
      malfunction of computer facilities, or loss of data due to power failures or
      mechanical difficulties with information storage or retrieval systems, labor
      difficulties, war, or civil unrest.”

    


    6. References
      to the Rights Agreement.
      All references in the Rights Agreement, and the exhibits thereto, to the Rights
      Agreement or any specific provision thereof (including references that use
      the
      terms “hereto” and “hereof”), as well as in the legends affixed to certificates
issued
      for Common Stock pursuant to Section 3(c) of the Rights Agreement, shall,
      without any specific references expressly and individually to any of the
      foregoing amendments, automatically be deemed references to the Rights Agreement
      or the applicable specific provisions thereof (as the case may be) as amended
      by
      this Amendment, with the same force and effect as if expressly and individually
      amended in that respect by this Amendment.

    


    7. Governing
      Law.
      This Amendment shall be governed by and construed in accordance with the laws
      of
      the State of Delaware without reference to the conflicts or choice of law
      principles thereof.

    


    8. Counterparts.
      This Amendment may be executed in any number of counterparts, each of which
      shall be deemed an original, and all of which together shall constitute one
      and
      the same instrument.

    


    9. Fax
      Transmission.
      A
      facsimile, telecopy or other reproduction of this Amendment may be executed
      by
      one or more parties hereto, and an executed copy of this Amendment may be
      delivered by one or more parties hereto by facsimile or similar instantaneous
      electronic transmission device pursuant to which the signature of or on behalf
      of such party can be seen, and such execution and delivery shall be considered
      valid, binding and effective for all purposes. At the request of any party
      hereto, all parties hereto agree to execute an original of the Amendment as
      well
      as any facsimile, telecopy or other reproduction thereof.

    


    
      
         

      

      
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    IN
      WITNESS WHEREOF,
      the
      Parties have caused this Amendment to be duly executed and attested as of the
      date and year first above written. 

     

    
      	
               

            	
               

            	
               

            
	
               

            	
              NORTH
                AMERICAN SCIENTIFIC, INC.

            
	
               

               

            	
               

               

            	
               

               

            
	
               

            	
              By:  

            	
              /s/
                James W. Klingler

            
	
               

            	
              Name:
                James W. Klingler

            
	
               

            	
              Its:
                Sr.V.P. & Chief Financial
                Officer

            


    

     

    
      	
               

            	
               

            	
               

            
	
               

            	
              COMPUTERSHARE
                TRUST COMPANY, N.A.

            
	
               

               

            	
               

               

            	
               

               

            
	
               

            	
              By:  

            	
              /s/
                Kellie Gwinn

            
	
               

            	
              Name:
                Kellie Gwinn

            
	
               

            	
              Its:Vice
                President

            


    

     

    
      
         

      

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