Comparative study between output factors obtained in a linear accelerator used for radiosurgery treatments

Abstract

Purpose: To compare the output factors obtained in a linear accelerator with conical collimators using five models of detectors, through tree different methods: the ratio of detector readings, the “daisy chain” technique (for diodes) and applying the k_Qclin,Qmsr^f_clin,f_msr factors based in the formalism proposed by the IAEA (this one was applied only to tree detectors).

Methods: A linear accelerator Varian-iX was employed with BrainLab conical collimators (30 mm to 7.5 mm), the detectors used were: PTW-PinPoint 31016 (×2), PTW-tipo E 60017 (×2), PTW-microLion 31018 (×2), EDGE (Sun-Nuclear), y PTW-Semiflex 31010. For the first three models were analyzing two detectors with different series. The measurements were carried out in water at depth of 1.5 cm and source to surface distance of 98.5 cm.

Results: With the readings ratio method, all detectors showed differences from 3.5% to more than 15% in the smallest field sizes, for the diodes the “daisy chain” method did not provide significant corrections. Applying the k_Qclin,Qmsr^f_clin,f_msr the detectors  PTW60017, PTW31018 and EDGE showed differences of less than 3%.

Conclusions: In small fields the readings ratio method could introduce significant errors in the output factor determination. Applying the k_Qclin,Qmsr^f_clin,f_msr proved to be a viable option.