Exit Dose Measurement for Patient Treatment Dose Verification in Co-60 Teletherapy

Abstract

This work evaluates the possibility of creating an online patient dose verification system through exit dose measurement on a Co-60 teletherapy machine which lacks an electronic portal imaging device (EPID). Exit dose measurements were done by diode and Barracuda and compared to those of an ionization chamber. The diode was first calibrated and the necessary correction factors (depth and field size corrections) determined using a PMMA phantom. The transmission factor of the couch was also determined at gantry angles relevant to the study (0o and 180o) and at various field sizes. The estimated couch transmission factor was 0.96912. The exit dose measurements were done both with PMMA phantom and an anthropomorphic phantom by calculating a dose of 200 cGy to the various depths and corresponding field sizes. The detectors were placed 30 cm underneath the treatment couch for all the measurements. Readings from both detectors compared well with those of the ionization chamber. The percentage deviations were generally lower with the PMMA phantom than for the anthropomorphic phantom though they were all below 2%. The diode deviations range between 0.098 to -1.322 for measurements with PMMA phantom at a depth of 15 cm while those of the Barracuda were between 0.157 and -1.411 at the same conditions. Diode in vivo dosimetry was found to be suitable for the verification of patient dose through exit dose measurements. However the calibration and correction factors must be determined carefully in order to generate more accurate results.