Investigation of effect of filter on the stand-up technique for total skin irradiation by Monte Carlo simulation.
PURPOSE: The aim of this study was to investigate dosimetric effects of scattering filter on the stand-up technique for total skin irradiation (TSI) with a single electron field by Monte Carlo (MC) simulation. METHODS: MC simulations were performed with BEAMnrc and DOSXYZnrc packages under EGSnrc environment. Scattering filter of a metal disc was mounted in the accessory slot. The filter materials (Cu, Fe, Au, Zn, Ag) were investigated, with thickness ranging from 0.05 to 0.55 mm, depending on material. The extended source to skin distance (SSD) ranging from 250 to 350 cm was studied. The following dosimetric quantities were evaluated: percent depth dose (PDD), profiles and output factor at depth of maximum, and composite dose distribution on a 30-cm diameter cylindrical phantom. They were compared with the standard dual beam technique used at our clinic. The effects on different patient sizes were also studied. RESULTS: No filter produced acceptable profile flatness (±10% within the central 160 cm) at 250 cm SSD. At 300 cm SSD, Au (0.1 mm), Ag (0.25 mm), Cu (0.5 mm) produced acceptable flatness while Zn (0.45 mm) required 325 cm SSD. For these four configurations, the dmax was 0.90-0.99 cm, similar to dual beam (0.97 cm); R50 was 1.85-1.91 cm, compared with dual beam of 2.06 cm; the output factor ranged from 0.025 to 0.029, lower than the dual beam (0.080). With the composite fields for four configurations, the dmax was 0.10 cm, compared with dual beam (0.16 cm). The surface dose was 97%, similar to dual beam (96%). B-factor was 3.3-3.4, compared with dual beam of 3.1. The maximum X-ray contamination was 3%, higher than dual beam (1%). CONCLUSIONS: The investigation suggests the TSI stand-up technique can be implemented using a single electron beam if a customized filter is used. More dosimetric measurements are needed to validate the MC results and clinical implementation.
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