Measurements of peripheral dose for multileaf collimator based linear accelerator

Background In radiation therapy, peripheral dose (PD), or the dose outside the geometrical boundaries of the radiation field, is of clinical importance when anatomical structures such as foetus in pregnant women, gonads, and lenses of the eye, with low dose tolerances are involved. Even a small percentage of the total treatment dose might cause injury in such cases. The sources of peripheral dose are leakage from the treatment unit, scatter from the secondary collimators and beam modifiers such as wedges and blocks, and internal scatter originating in the patient Aim To determine the peripheral dose (PD) for multileaf collimator (MLC) based linear accelerator in water equivalent slab phantom for open and wedged fields. Materials/Methods PD measurements were carried out for 6 and 15MV photons using a 0.4cc parallel plate chamber in the slab phantom. Measurements were performed for different field sizes at different depths (D-max, 5cm and 10cm) and up to a maximum distance of 30cm beyond the field edges. PD was measured using wedge filters also. PD was further computed using a three-dimensional treatment planning system (3D TPS). Results For 6MV photon beams, the maximum PD for open beams at 5cm distance from the field edge was 3.42% and the minimum PD at 20cm distance was 0.11%. For 15MV, the maximum PD for open beam at 5cm distance was 3.07% and the minimum PD was 0.14%. For wedge filters, the maximum PD measured at 5cm distance for 6 and 15MV photons were 5.56% (60 degrees Wedge) and 5.03% (45 degrees wedge). The TPS PD values showed minimal variation from the measured values. Conclusions The PD due to MLC and beam modifiers would definitely be helpful to assess the doses received by the relevant critical structures outside the treatment field.