The purpose of this task was to measure the output factors of the small fields and experimentally calculate the correspondoing correction factors prosposed in the Alfonso et al (Med Phys 35, 5179-5186 (2008)) dosimetric protocol for small and non-standards fields. 

The following detectors were used :  

  • PTW – 31014 PinPoint ion chamber 
  • PTW – 60008 shielded diode and its new version 60016
  • PTW – 60012 unshielded diode and its new version 60017
  • SunNuclear EDGE detector
  • TLD100, LiF microcubes (1mm x 1mm x 1mm)
  • Alanine cylindrical pellets (5 mm diameter and 2.5 mm height)
  • EBT Gafchromic films
  • Polymer gel dosimeters

The output factors of the 5 mm, 7.5 mm, 10 mm, 12.5 mm and 15 mm diameter small fields were measured using the detectors given in the above list. All measurements were performed at 15 mm depth using SADs of 650 mm, 800 mm and 1000 mm. Alignment of all detectors along beam axis (except for EBT films and polymer gels) was performed using the symmetry of the beam. For the EBT films and polymer gels the delivered dose distribution on the detectors was exploited to find the beam axis as described by Pantelis et al  Med Phys 35, 2312-2320 (2008). 

For the irradiation of the microchambers and the diodes a number of 100 MUs was used. For the microchambers measurements were performed using positive and negative polarities and results were averaged. For the irradiation of the TLD, alanine, EBT and gel dosimeters the same dose of 100 cGy, 1000 cGy, 500 cGy and 3000 cGy, respectively was delivered using each small field adjusting accordingly the number of MUs.

For the 5 mm and 7.5 mm smaller fields each measurement was repeated 10 times to account for the repositioning uncertainty of the IRIS collimator. For the larger fields each measurement was repeated 5 times, since the repositioning uncertainty of the IRIS collimator in the corresponding output factor measurements is minimal.

The dose distribution of each small field measured by the gel dosimeters was used to calculate the volume average correction factor of the different dosimeters, as described in our previous work (Pantelis et al, Med Phys 37, 2369-2379 (2010)).

The measured output factors using the PTW 60016 and 60017 diodes were compared with the corresponding results of their predecessors (i..e, 60008 and 60012) to find any differences between thee detectors.