**Exhibit 10.54****AMENDMENT NUMBER FIVE****TO****LEASE AGREEMENT FOR A GAMMA KNIFE UNIT**     This AMENDMENT NUMBER FIVE TO LEASE AGREEMENT FOR A GAMMA KNIFE UNIT (this “Amendment”) isdated effective as of the 9th day of May, 2007, and is entered into between GK FINANCING, LLC, aCalifornia limited liability company (“GKF”), and The Regents of the University of California, aCalifornia corporation (“University”), with reference to the following facts:**RECITALS**     A. Reference is made to a certain Lease Agreement for a Gamma Knife Unit (as amended, the“Lease”) which was dated July 6, 1990, but which first became effective on September 17, 1991,between University and American Shared Hospital Services, a California corporation (“ASHS”).     B. The Lease was amended pursuant to a certain Amendment Number One to the Lease Agreement fora Gamma Knife Unit (the “First Amendment”) dated effective August 1, 1995, between University andASHS.     C. The Lease was subsequently assigned (i) by ASHS to its wholly-owned subsidiary, AmericanShared Radiosurgery Services (“ASRS”), and then, by ASRS to GKF, which assignment was consented toby University pursuant to a certain Estoppel Certificate and Consent to Assignment dated December21, 1995; and (ii) by University to UCSF-Stanford Health Care, a California non profit publicbenefit corporation (“UCSF-SHC”).     D. Effective February 6, 1998, UCSF-SHC and GKF entered into a certain Amendment Number Two tothe Lease Agreement for a Gamma Knife Unit (the “Second Amendment”).     E. In 1999, the board of directors of UCSF-SHC resolved to windup the affairs of UCSF-SHC andto dissolve, and as a component of the winding up of the affairs of UCSF-SHC, the Lease wasassigned by UCSF-SHC back to University. This assignment was consented to by the sole formermembers of UCSF-SHC (i.e., the University and the Board of Trustees of Leland Stanford JuniorUniversity), and by GKF.     F. Effective June 22, 2001, University and GKF entered into a certain Amendment Number Threeto Lease Agreement for a Gamma Knife Unit (the “Third Amendment”).     G. Effective April 1, 2002, University and GKF entered into a certain Amendment Number Four toLease Agreement for a Gamma Knife Unit (the “Fourth Amendment”).     H. University desires to replace the existing Leksell Gamma Knife, Model 4C (the “Model 4C”)that is currently being leased by GKF to University pursuant to the Lease, with a Leksell GammaKnife Perfexion unit. In furtherance thereof, (i) University intends to purchase
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the Perfexion unit from GKF; (ii) GKF intends to trade-in the Model 4C to the manufacturer toacquire the Perfexion unit that will be sold to University; (iii) concurrently with the executionof this Amendment, the parties have entered into or intend to enter into a certain Agreement ToPurchase Gamma Knife Perfexion Unit (the “Purchase Agreement”) pertaining to the purchase of thePerfexion unit; and (iv) the parties intend to amend the Lease as set forth below.         NOW THEREFORE, for valuable consideration, the receipt and sufficiency of which areacknowledged, the parties hereby amend the Lease as follows:**1.** **Defined Terms****.**Unless otherwise defined herein, the capitalized terms used hereinshall have the same meanings set forth in the Lease. **2.** **Amendment of the Lease****.**Notwithstanding any provisions to the contrary set forth inthe Lease, the First Amendment, the Second Amendment, the Third Amendment and/or the FourthAmendment, the Lease shall be further amended as follows: a. In consideration for GKF’s agreement to replace the existing Model 4C with thePerfexion to be purchased by University from GKF, commencing from the “EquipmentTransition Date” to and including January 20, 2008 (which is the current expirationdate of the Lease), University shall pay to GKF the “Equipment Transition PeriodPayments” and the “Remaining Period Payments,” which shall be billed and paid as setforth below. As used herein: i. The “Equipment Transition Date” shall mean the date onwhich (1) the Model 4C has been de-installed and is unable to performprocedures; (2) all Permits have been obtained; and (3) the Perfexion has beendelivered to the site and is ready for installation.  ii. The “Equipment Transition Period” shall mean the periodcommencing from the Equipment Transition Date to the date on which thePerfexion has been installed and is ready to perform procedures (which periodis estimated to be approximately 4 to 5 weeks).  iii. The “Equipment Transition Period Payments” shall meanand be equal to: (1) The number of weekdays during the EquipmentTransition Period; multiplied by  (2) * procedure per weekday (the “NegotiatedProcedures”), which Negotiated Procedures were agreed upon between theparties to compensate GKF for the lost number of procedures that wouldhave otherwise been performed during the Equipment Transition Periodbut for the installation of the Perfexion); multiplied by  (3) The lease payment rate that would otherwise beapplicable, assuming (A) the Negotiated Procedures are Proceduresperformed under the Lease; (B) all of the Negotiated Procedures areperformed within the same Rate Year in which the Equipment
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   Transition Date falls; and (C) the actual number of Proceduresperformed prior to the Equipment Transition Date during that sameRate Year are counted. iv. The “Remaining Period” shall mean the period commencingfrom the expiration of the Equipment Transition Period to and including January20, 2008  v. The “Remaining Period Payments” shall mean and be equalto: (1) The number of Procedures that are actuallyperformed from time-to-time during the Remaining Period (the “RemainingPeriod Procedures”), multiplied by  (2) The lease payment rate that would otherwise beapplicable, assuming (A) the Remaining Period Procedures are Proceduresperformed under the Lease; (B) all of the Remaining Period Proceduresare performed within the same Rate Year in which the EquipmentTransition Date falls; and (C) the actual number of proceduresperformed prior to the Equipment Transition Date during that same RateYear and the Negotiated Procedures are counted. vi. “Procedure” shall mean any single fractioncranial/intracranial radiosurgery treatment that is performed by University orits representatives or affiliates, irrespective of whether the Procedure isperformed on the Model 4C, the Perfexion or using any other equipment ordevices. b. University shall be billed on the fifteenth (15th) and last day of each monthfor the Equipment Transition Period Payments and the Remaining Period Paymentspertaining to the Negotiated Procedures and the Remaining Period Procedures performedduring the first and second half of the month respectively. University shall pay GKFwithin forty-five (45) days after being invoiced. University shall promptly inform GKFin writing as to the number of Procedures performed during the Remaining Period inorder to facilitate such billing.  c. Within ten (10) days after University’s receipt of written request by GKF, GKFshall have the right to audit University’s books and records (including, withoutlimitation, the books and records pertaining to any other radiosurgery equipment ordevices) during normal business hours to verify the number of Procedures that have beenperformed by University, and University shall provide GKF with access to such books andrecords; provided that any patient names or identifiers shall not be disclosed.  d. It is understood that no portion of the Equipment Transition Period Paymentsand/or the Remaining Period Payments shall be applied or credited towards the PurchasePrice of the Perfexion.
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 e. Notwithstanding Section 7 of the Third Amendment of the Lease, University willnot (nor have the opportunity to) acquire title to the Model 4C, and title to the Model4C shall remain with GKF, which will be traded-in for the Perfexion as described above.**3.** **Conditions Precedent****.**The amendment of the Lease as set forth in Section 3 above issubject to the prior satisfaction of all of the following conditions: a. The Purchase Agreement shall have been executed by University and GKF, and the“Purchase Price” and all “Taxes” (as defined in the Purchase Agreement) shall have beenpaid in full.  b. University shall not be in breach of the Lease, and no act or omission shallhave occurred which, with the giving of notice and/or the passage of time, wouldconstitute such a breach under the Lease.  c. At all times prior to the Equipment Transition Date, procedures will continueto be scheduled on the Model 4C in the ordinary course of business.**4.** **Captions****.**The captions and paragraph headings used herein are for convenience onlyand shall not be used in construing or interpreting this Amendment. **5.** **Full Force and Effect****.**Except as amended by this Amendment, all of the terms andprovisions of the Lease, including all prior amendments thereto shall remain in full force andeffect.     IN WITNESS WHEREOF, the parties have executed this Amendment effective as of the date firstwritten above.           **GKF:**   **University:**             **GK FINANCING, LLC**   **THE REGENTS OF THE UNIVERSITY OFCALIFORNIA**             By: /s/ Ernest A. Bates, M.D. Ernest A. Bates, M.D.   By: /s/ Ken Jones Ken Jones    Policy Committee Member     Chief Financial Officer  
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