Small fields characterization of teletherapy cobalt-60 photon beam: An experimental and Monte-Carlo study

Abstract

This research work aims to study small field characteristics of the 60Co source-based teletherapy machine by varying the detector size and source size. Further, the feasibility of clinical use of small fields on 60Co source-based teletherapy unit has been studied. In this study, square fields size varying from 1 × 1 cm2 to 10 × 10 cm2 were used. Two different ionization chambers (SNC125c and Pinpoint) having different sensitive volumes were used for experimental measurements, and the tool for particle simulation (TOPAS) version 3.5 toolkit was used for Monte Carlo (MC) simulations. All the data was measured as per the recommendation of the International Atomic Energy Agency (IAEA) technical report series-483 (TRS-483). Firstly, MC simulated beam data (beam profiles and percentage depth dose) was validated against the experimentally measured beam data and found to be agreed within 2%/2 mm gamma criteria. The SNC125c underestimate the output factors (OFs) by 7% and 5% when compared to MC calculated OFs for small beams of 1 × 1 cm2 and 1.5 × 1.5 cm2, respectively. The full-width half-maxima (FWHM) of all field sizes agreed with the set geometric field size other than 1 × 1 cm2 and 1.5 × 1.5 cm2, for which it was found most deviated, and it shows significant dependency on the size of the source. The present study showed that chamber size significantly impacts the measurement of small fields characteristics, mainly OFs. Further, source size is another parameter that significantly affects the beam profiles that limits the use of 60Co teletherapy machine for field sizes smaller than 3 × 3 cm2.