Monte Carlo study on dose distributions from total skin electron irradiation therapy (TSET).

Total skin electron therapy (TSET) has been used to treat mycosis fungoides since the 1950s. Practitioners of TSET rely on relatively crude, phantom-based point measurements for commissioning and treatment plan dosimetry. Using Monte Carlo simulation techniques, this study presents whole-body dosimetry for a patient receiving rotational, dual-field TSET. The Monte Carlo codes, BEAMnrc/DOSXYZnrc, were used to simulate 6 MeV electron beams to calculate skin dose from TSET. Simulations were validated with experimental measurements. The rotational dual-field technique uses extended source-to-surface distance with an acrylic beam degrader between the patient and incident beams. Simulations incorporated patient positioning: standing on a platform that rotates during radiation delivery. Resultant patient doses were analyzed as a function of skin depth-dose coverage and evaluated using dose-volume-histograms. Good agreement was obtained between simulations and measurements. For a cylinder with a 30 cm diameter, the depths that dose fell to 50% of the surface dose was 0.66 cm, 1.15 cm and 1.42 cm for thicknesses of 9 mm, 3 mm and without an acrylic scatter plate, respectively. The results are insensitive to cylinder diameter. Relatively uniform skin surface dose was obtained for skin in the torso area although large dose variations (>25%) were found in other areas resulting from partial beam shielding of the extremities. To achieve 95% mean dose to the first 5 mm of skin depth, the mean dose to skin depth of 5-10 mm and depth of 10-15 mm from the skin surface was 74% (57%) and 50% (25%) of the prescribed dose when using a 3 mm (9 mm) thickness scatter plate, respectively. As a result of this investigation on patient skin dose distributions we changed our patient treatments to use a 3 mm instead of a 9 mm thickness Acrylic scatter plate for clinically preferred skin depth dose coverage.