Section 35.635 - Full calibration measurements on gamma stereotactic radiosurgery units(a) A licensee authorized to use a gamma stereotactic radiosurgery unit for medical use shall perform full calibration measurements on each unit-(1) Before the first medical use of the unit;(2) Before medical use under the following conditions-(i) Whenever spot-check measurements indicate that the output differs by more than 5 percent from the output obtained at the last full calibration corrected mathematically for radioactive decay;(ii) Following replacement of the sources or following reinstallation of the gamma stereotactic radiosurgery unit in a new location; and(iii) Following any repair of the gamma stereotactic radiosurgery unit that includes removal of the sources or major repair of the components associated with the source assembly; and(3) At intervals not exceeding 1 year, with the exception that relative helmet factors need only be determined before the first medical use of a helmet and following any damage to a helmet.(b) To satisfy the requirement of paragraph (a) of this section, full calibration measurements must include determination of-(1) The output within ±3 percent;(2) Relative helmet factors;(3) Isocenter coincidence;(4) Timer accuracy and linearity over the range of use;(7) Treatment table retraction mechanism, using backup battery power or hydraulic backups with the unit off;(8) Helmet microswitches;(9) Emergency timing circuits; and(10) Stereotactic frames and localizing devices (trunnions).(c) A licensee shall use the dosimetry system described in § 35.630(a) to measure the output for one set of exposure conditions. The remaining radiation measurements required in paragraph (b)(1) of this section may be made using a dosimetry system that indicates relative dose rates.(d) A licensee shall make full calibration measurements required by paragraph (a) of this section in accordance with published protocols accepted by nationally recognized bodies.(e) A licensee shall mathematically correct the outputs determined in paragraph (b)(1) of this section at intervals not exceeding 1 month for cobalt-60 and at intervals consistent with 1 percent physical decay for all other radionuclides.(f) Full calibration measurements required by paragraph (a) of this section and physical decay corrections required by paragraph (e) of this section must be performed by the authorized medical physicist.(g) A licensee shall retain a record of each calibration in accordance with § 35.2632 .