QUALITY ASSURANCE IN RADIOTHERAPY BY IN VIVO DOSIMETRY; ENTRANCE DOSE MEASUREMENT

Abstract

Quality assurance in radiotherapy by in vivo dosimetry was performed at the Division of Radiation Oncology, Department of Radiology, Siriraj Hospital during Au- gust 1996 to January 1997. The entrance doses of a total number of 467 treatment set- ups (182 cancer patients) undergoing radiation therapy with Cobalt-60 Teletherapy unit were measured with semiconductor detectors. From the study, the global results of the percentage ratios of the measured dose and calculated dose showed a Gaussian fre- quency distribution which a mean and one standard deviation value were 99.2+3.34 %. This meaned that the uncertainty caused by a systematic and random errors in the treat- ment delivery were 0.8% and 3.34% respectively. Eighty-seven percents of all treat- ment set-ups are reliable due to the dose delivered fitted in ±5% of the prescribed dose while the treatments with a large error (2SD) were found in 2.99%. Source of the uncer- tainties in this study arised from incorrect dose calculation, contour irregularities, hu- man mistakes in treatment setting-up, insufficient immobilization and erroneous in the entrance dose measurements themselves.