Investigation of Effect of Filter on the Total Skin Irradiation Using Monte Carlo Simulation

Abstract

The standard total skin irradiation (TSI) procedure for patients with Mycosis fungoides at our clinic is the Stanford technique where dual overlapping electron beams are directed at patient standing at an extended source to skin distance (SSD) of 300 cm. Patients rotate along the cranial-caudal axis in 6 positions to get full coverage to skin. Recently we developed and implemented a technique of laying down position for frail patients. One key requirement for this technique is the use of customized Cu filter to broaden the beam. The purpose of this study is to investigate the effect of the filter designs on the standard standing technique with a single beam. The filter is placed at the interface mount of the linac. The material studied include Cu, Fe, Au, Zn, and Ag. These materials were chosen based on their availability and stability. The thickness ranges from 0.05 mm to 0.55 mm, depending on the material. The extended SSD ranges from 250 cm to 350 cm. Our recent MC simulation study using the EGSnrc system on the laydown technique with 0.25 mm Cu filter has produced results that agree well with the commissioning measurement. For each material, we varies the thickness and SSD, to study the following quantities: percent depth dose (PDD), profile at dmax, and output at dmax, and compared them with the standard dual beam setup at 300 cm SSD that has been used in the clinic. Higher atomic number material and/or thicker filter has higher scattering power to broaden beam, but also attenuates beam more with lower output. No material can produce acceptable profile flatness (±10% within the central 160 cm region) at the SSD of 250 cm, suitable for a small linac room. At 300 SSD, Au (0.1 mm), Ag (0.25 mm) and Cu (0.45 mm) are acceptable. Zn (0.45 mm) requires SSD of 325 cm to meet the requirement. For all 4 configurations, the dmax is 0.87 - 0.99 cm, similar to dual beam with no filter at 0.97 cm. The R50 is 1.85 – 1.91 cm, compared to 2.06 cm for dual open beams. The output ranges from 0.025 – 0.029, slightly lower than the open beams at 0.040. The Monte Carlo simulation is a very useful tool to investigate the effect of the different filters and provides valuable suggestion in the technique optimizations. The result suggests that the standard standing position TSI technique can be implemented using a single beam instead of dual overlapping beam if the customized filters are used. Actual manufacturing and measurement will be performed to validate these simulation results.