DOSIMETRIC EVALUATION OF A WIDELY USED KILOVOLTAGE X-RAY UNIT FOR ENDOCAVITARY RADIOTHERAPY

In this paper we present the dosimetric data of a Therapax DTX300 kilo voltage x-ray unit for endocavitary rectal irradiation. The unit if op erated at tube voltage of 40-60 kVp (30 mA) with an added filtration o f 0.2-0.4 mm Al generates acceptable beam qualities comparable to thos e of the original Papillon technique. Relative dosimetric measurements were performed at the cone end (37.2 cm SSD) of a 3 cm diameter recta l cone using various detectors to ensure the accuracy. A Monte Carlo m ethod was used to calculate correction factors for the diode used in t he percentage depth-dose (PDD) measurement, and to study the effect of the detector size on the beam profile. The PDD data were determined u sing the diode measurement corrected for its energy and angular respon se. It was found that the PTW N23342 and Markus parallel-plate chamber can be used directly to measure the PDD for this beam quality with 2% uncertainty. Measurement and Monte Carlo results have shown that the detector size has a significant effect on the penumbral profile. Film and diode detectors have a better spatial resolution compared to ioniz ation chambers, but they may,give an incorrect profile tail due to eit her nonlinear response at low energy or angular dependence. This can b e corrected using the ionization-chamber measurement, based on the Mon te Carlo analysis. The isodose distributions for this x-ray unit are p resented. (C) 1998 American Association of Physicists in Medicine.