Evaluation of effective source position for local linear accelerator and inserts

All types of treatment planning systems need some input measured beam data. Such data differ in type and number depending on the model of electron bea m algorithm used inside. In addition to the number of percentage depth dose (%DD) and cross beam profiles, the effective source surface distance param eter SSDeff was also recommended to be measured and transferred to the plan ning system; Output measurements were carried out using 0.6cc cylindrical F armer type ion chamber placed in water phantom at depth of dose maximum, in Radiat. Phys. Dep., Malmo Hospital, Lund University, Sweden. Results were collected for all available energies combined with both applicators and ins erts on a Varian Clinac 2100C. SSDeff,ff was determined from the ionization measurements using equation given by Khan et al.(1991). Comparing the resu lts with those obtained by Roback et al. (1995) on a similar Varian Clinac 2100C, it was found that the deviation of SSDeff was < 7.5 % for combinatio n of energies, field sizes, and inserts except at 6 MeV. The variations of SSDeff than the nominal SSD reflect the importance of corrections against a ir gap present in irregular clinical situations. SSDeff should be measured during commissioning of both accelerator or/and local treatment-planning co mputer. Inserts used in measurements should be from the same material and t hickness as that an used in clinical work.