CONTACT RADIOTHERAPY MACHINE CALIBRATION BY MEANS OF TLDS

X ray tubes for contact and cavity radiotherapy although not built any more, are still operating in several centres essentially for the trea tment of small superficial skin tumours. With field sizes ranging from 0.5 cm to 2 cm in diameter and voltages from 15 to 60 kV. these machi nes are characterised by an extremely low inherent filter, a short foc us-skin distance and a very high dose rate. Nowadays, the main reason for their use lies in the ease of beam positioning on very small targe ts that would be difficult to irradiate with current radiotherapy mach ines. On the other hand, given the low energies involved and the small dimensions of the beams, the output calibration and the depth-dose me asurements are somewhat difficult to carry out. The usual ion chambers do not allow performance of correct dose measurements for radiation f ields smaller than 1.5 cm in diameter. It is shown that it is possible to overcome the above inconveniences by means of thermoluminescence d osemeters (TLD). The TLDs used were LiFTLD-100, routinely employed in patient in vivo dosimetry. Their size (3 x 3 x 0.5 mm3) suited the pur pose of this experiment, allowing for complete machine dosimetric char acterisation whereas ionometric methods are difficult.