Total skin electron therapy (TSET): Monte Carlo Simulation and implementation

Introduction: Total skin electron irradiation technique is used in treatment of the mycosis fungoid. The implementation of this technique requires non-standard measurements and complex dosimetry methods. Operating procedures for total skin electron irradiation and its dosimetry vary in different radiation therapy centers in the world. In this article, validation of TSET technique dosimetry data by Monte Carlo simulation is done. Materials and Methods: The electron beam characteristic of RTSEI for the only electron accelerator, located at the radiation center of the Seyed Alshohada Hospital of Isfahan (NEPTUN 10PC) determined by performing Monte Carlo simulations and using EGSnrc-based codes (BEAMnrc and DOSXYZnrc). For the best uniformity of the vertical profile, the optimal angle of gantry was defined at SSD=300 cm. The effect of the degrader plane that is located at a distance of 20 cm from the patient surface, was evaluated on the amount of energy reduction of the beam, the opening of the electron beam field and the homogeneity of the dose distribution. The transversal dose distribution from the whole treatment with Rotational technique was simulated in a CT- based anthropomorphic phantom. Also, the percentage depth dose in the head, neck, thorax, abdomen and legs was obtained for RTSET technique. TSET dosimetry requires measurements in nonreference conditions. Because of the complexity of the required measurements for the commissioning process of this technique, different dosimetric systems were employed such as radiochromic films (EBT3) and an ionization chamber. In particular, for dual-field beams irradiation, the optimal tilt angle was investigated and the dose distribution in the treatment plane was measured by radiochromic films. Dose distributions and percentage depth dose measurements for a total skin electron therapy were measured in an anthropomorphic phantom. Results: The optimal angle of 20o would give the most uniform total profile and the use of a 0.8 cm PMMA degrader in front of the patient leads to a homogeneous distribution of the dose in all directions (the mean relative dose value was 97%±5%, normalizing to 100% at the calibration point level). The percentage depth dose curves in different organs of anthropomorphic phantom for RTSEI indicates that the depth of maximum dose is on the surface of the phantom, Isodose curve of 80% is formed at a depth less than 4 mm and at the depth less than 1.5 mm, the dose decreases to 20% of the maximum dose. Conclusion: The main purpose of this study was to commission and optimize a TSET technique for the treatment of mycosis fungiodes with the NEPTUN 10PC linear accelerator. This was done through an extensive set of measurements and a large number of MC simulations. The results of Monte Carlo calculations were found to be in general agreement with the measurements, providing a promising tool for further studies of dose distribution calculations in TSEI.