The purpose of this action is to perform reference dosimetry measurements using alanine dosimeters and cylindrical ionization chambers of different cavity lengths. Comparison of the alanine and ion chambers’ reference dosimetry results will be performed to validate the use of the Farmer chamber for reference dosimetry measurments in the 60 mm reference field of the CyberKnife system and to establish the correction factors for the used chambers according to the dosimetric protocol suggested by Alfonso et al (Med Phys 35, 5179-5186 (2008)).

 

The following detectors were used :

  • PTW – 30013 Farmer ion chamber   (23 mm cavity length, 0.6 cc cavity volume)
  • PTW – 31013 Semiflex ion chamber (16.25 mm cavity length, 0.3 cc cavity volume)
  • PTW – 31010 Semiflex ion chamber (6.5 mm cavity length, 0.125 cc cavity volume)
  • Scdx - Wellhofer CC13 ion chamber (5.8 mm cavity length, 0.130 cc cavity volume)
  • PTW – 31014 PinPoint ion chamber (5 mm cavity length, 0.015 cc cavity volume)
  • 10 Alanine cylindrical pellets (5 mm diameter, 2.5 mm height)

 

All detectors were positioned with their geometrical center at 50 mm depth within water phantom (PTW-MP3 motorized water phantom) and irradiated from an SAD of 800 mm using the 60 mm diameter IRIS field. The ion chambers were situated with their stem perpendicular to beam axis. Alanine pellets were positioned using the holder presented in the presentd figure. A dose of 10 Gy was delivered to each alanine pellet to reduce uncertainty. The same dose was also delivered to the rest of detectors to exclude systematic uncertainties associated with the linac output linearity. The formalism suggested by Alfonso et al  was used to measure the reference dose with the ion chambers and results were compared with the corresponding values of alanine dosimeters to experimentally establish the correction factors of the employed chambers