Peripheral dose from a dual energy linear accelerator equipped with tertiary multileaf collimators and enhanced...

Peripheral dose (PD) or the dose outside the geometrical boundaries of the radiation field is of clinical importance when anatomical structures with low dose tolerances might be involved(1). It is the aim of this study is to estimate the PD on linear accelerators on different wedge systems without multileaf collimator (MLC). Measurements were performed on a dual energy linear accelerator equipped with tertiary MLC and enhanced dynamic wedge (EDW). Measurements were made using an ionization chamber embedded in a Radiation Field Analyser (RFA-300) with the secondary collimator and MLC setting of 5x5, 10x10, 15x15, and 20x20 cm2, and with the MLC fully retracted. The effects of SSD on PD were measured at three SSDs of 90, 100, and 110 cm for the irradiation fields of 5x5, 10x10, 15x15, and 20x20 cm2 and the effects of the three different wedges (Upper wedge, Lower Wedge and Enhanced Dynamic Wedge) on PD were measured for 45° wedges with field size of 15x15 cm2. Data were taken from 3 cm to 24 cm away from the field edge. Results show that due to tertiary MLC, PD can be reduced by means of a factor of two to three at certain distance from the edge of the field compared with TG-36 data. In between the wedges, the PD was less for the EDW when compared with the upper and lower physical wedges. We conclude that the reduction in PD is significant in reducing or eliminating the need for external peripheral shielding to reduce the dose on affected critical organs.