Development of collimator insert for linac based stereotactic irradiation

The aim of this study is to develop collimator inserts of various sizes whi ch are either not commercially available or are expensive to import. The do simetry parameters such as tissue maximum ratio (TMR), off-axis ratio (OAR) and output factor of the developed collimator insert are compared with tha t of the commercial collimator insert (Radionics). In order to check the su itability of the collimator insert developed locally for clinical use and t o standardize the method of development, a collimator insert of 15 mm ident ical to the one supplied by Radionics is developed with low-melting alloy ( Cerrobend). Moreover for the clinical use of the developed collimator inser t, certain acceptance tests are performed which include a collimator concen tricity test, beam size check and radiation leakage test. The dose verifica tion is carried out with a thermoluminescent dosimeter ((LiF)-Li-7 rods) an d an FBX chemical dosimeter in a human-head-shaped Perspex phantom filled w ith water. The variation between the calculated and measured dose is found to be within +2.4% for (LiF)-Li-7 rods and -2.0% for the FBX chemical dosim eter thus ensuring the suitability of the developed collimator insert for c linical use. This has encouraged us to standardize the method adapted to de velop the collimator insert and to develop collimator inserts of different field sizes.