Measurement of Dose Distribution in Small Beams of Philips 6 and 8 MVX Linear Accelerator

Suh, Tae Suk; Yoon, Sei Chul; Shinn, Kyung Sub; Bank, Yong Whee

Abstract

The work suggested in this paper addresses a method for collecting beam data for small circular fields. Beam data were obtained from Philips 6 and 8 MV LINAC at Dept. Radiation Therapy at Gainesville Incorporated and Shands Teaching Hospital. Specific quantities measured include tissue maximum ratio (TMR), off-axis ration (OAR) and relative output factor (ROF). In small field irradiation, special collimators were used to produce circular fields of 1 cm to 3 cm diameter in 2 mm steps, measured at SAD (soura axis distance) of 100 cm. Diode detector was chosen for primary beam measurement and compared with measurements made with photographic film and TLD dosimeters. The measured TMRs and OARs were formulated from limited measurements to generate basic beam data for reference set-up. The empirical formula were later, extended and generalized for any possible set-up using the trends of fitting parameters. The measured TMRs and OARs were well represented by the fitting formula developed.