Skin Dose Distributions from Total Skin Electron Irradiation Therapy (TSET).

Purpose/Objective(s) 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 resultant whole-body dosimetry for a patient receiving rotational, dual-field TSET. Materials/Methods 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 a 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 (DVH). Results 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 3mm (9 mm) thickness scatter plate, respectively. Conclusion The Monte Carlo simulations provide details of skin dose coverage as a function of depth that experimental methods are unable to obtain. For the first time, the calculated 3D skin dose distributions can be analyzed by using dose-volume histograms (DVHs) for quantitative dose coverage at specified skin depths for the whole body skin. These results shed light on the patient entire skin dose coverage for treated with rotational technique and can be useful for clinicians in choosing the appropriate thickness of beam scatter(degrader) plate in order to achieve the desired skin depth dose coverage. Based on the result of this investigation we now use 3 mm instead of 9 mm thick beam scatter (degrader) plate for optimum skin depth dose coverage in our institution. We plan to follow the results with the 3 mm degrader as an expansion cohort on our ongoing prospective study for clinical effectiveness.