Behaviour of radiation detectors in various field sizes

Abstract

Firstly, this study investigates the effect of the detector orientation and the detector sensitive volume size on the response of a detector in various radiation beam field sizes. Secondly, to investigates whether results should be reported in geometric field sizes or irradiation field sizes. Finally, to investigates which detector is suitable for small field dosimetry. A PTW 30013 Farmer ionisation chamber, a PTW 31010 semi-flex ionisation chamber and a PTW 60019 synthetic single-crystal diamond detector were irradiated with a 6 MV photon beam, produced by a Siemens Oncor linear accelerator and an Elekta Versa HD linear accelerator, in various field sizes from 0.6 cm x 0.6 cm to 25cm x 25 cm. These detectors were oriented parallel to the beam and perpendicular to the beam. Current was collected to investigate the effect of the detector orientation and the detector sensitive volume size in the central region of a beam profile and the penumbra region of a beam profile. Charges were collected to calculate tissue phantom ratios, ranges of charged particles and output factors. Firstly, the results indicate that the Siemens Oncor linear accelerator and the Elekta Versa HD linear accelerator used in this study have different beam qualities. The results indicate that the ranges of charged particles are dependent on the beam quality of the radiation beam. The results show that the ranges of charged particles obtained using the Siemens Oncor linear accelerator and the Elekta Versa HD linear accelerator were 0.4 cm and 1.3 cm, respectively. Secondly, the results show that the beam profiles and percentage depth doses are dependent on radiation beam field sizes. The output factors are dependent on radiation beam field sizes that are less than 6 cm x 6 cm. Thirdly, beam profiles show that there is strong detector response dependency on detector orientation in beam field sizes that are less than 6 cm x 6 cm obtained using the Siemens Oncor linear accelerator. There is strong detector response dependency on detector orientation in beam field sizes that are less than 4cm x 4cm obtained using the Elekta Versa HD linear accelerator. Finally, the results indicate that there is difference between the calculated irradiation field sizes and geometric field sizes, however there is no discernible difference in reporting results in geometric field sizes or irradiation field sizes. The study concludes that a PTW 30013 Farmer ionisation chamber and a PTW 31010 semi-flex ionisation chamber are not suitable to be used in dosimetry using a Siemens Oncor linear accelerator. The PTW 30013 Farmer ionisation chamber is not suitable to be used in small field dosimetry in an Elekta Versa HD linear accelerator. The study recommends that beam quality of a linear accelerator should be determined to check which detector is suitable to be used in small field dosimetry. This study also recommends that Medical Physicist should use a PTW 60019 synthetic single-crystal diamond detector for dosimetry