Commissioning and optimization of a total skin electron therapy technique using a high rate electron facility

English: Total skin electron therapy (TSET) is the treatment of choice for several malignant diseases of the skin (Kaposi sarcoma, mycosis fungoides). Several different techniques have been developed in various centers, in order to achieve homogeneous dose distribution over a large irradiation field (200 x 80 ern"). However, to implement a TSET technique one has to account for a variety of parameters, from geometric (room design, space constrains) to physical (number, angle and energy of the beams). To obtain the most acceptable dose distribution an extensive set of measurements and a large number of calculations have to be performed. Therefore Monte Carlo simulation of TSET can facilitate optimization of this technique. In this study we implemented and optimized a TSET technique using 4 and 6 MeVelectron beams. The dosimetrie procedure intended to obtain adequate dose uniformity over the entire surface of the patient, and to reduce the patient treatment time using a high dose rate facility on the Elekta Precise accelerator. The EGS4/BEAM code package running on a Windows based platform was used for the MC simulation. Percentage depth-dose curves and beam profiles were calculated and measured experimentally for the 40x40 cm2 nominal field at both 100 cm SSD and at the patient surface at the treatment plane (SSD 350 cm) for a single beam. The accuracy of the simulated beam was validated by the good correspondence (within less than 2%) between measured beam characteristic parameters (Rso, dmax, Rp) and Monte Carlo calculated results. To obtain a uniform profile vertically, two vertical angles of incidence were used. The angle between the two beams that gave best uniformity was considered the optimum angle. The patient is to be placed on a rotating platform perpendicular to the beam and rotated through 60 degree increments to obtain six horizontal directions of beam incidence. The doses expected in the patient were measured with Kodak EDR2 films positioned at different levels between slices of a Rando phantom. TLDs were placed on the surface to relate the film measurements to dose. The delivered doses in the treatment plane were compared to simulated data that was obtained from the MC simulation. The penetration depth of the dose distribution varied over various scanning directions between 2-3 mm and 3- 4 mm for 4 and 6 MeV respectively. This information is useful when treatment of lesions of different thickness are being considered. The composite percentage depth dose of all six dual fields for both 4 and 6 MeV yielded an 80 % dose at - 7 mm and - 9 mm depth, respectively. Good dose uniformity was achieved for both energies and it was about ± 5% for 4 MeVand about ± 3% for 6 MeVover a range of - 100 to +100 cm. The bremsstrahlung contamination was 0.9 and 1.3 %. Generally there was good agreement between the dose distribution calculated with MC and measured with films, thus validating our MC calculations. The dose distributions in phantom were found to comply with the guidelines described in the AAPM TG-23 protocol, showing the suitability of this technique for treatments of the skin diseases. The HDRE is a useful operational mode providing reasonable output, field size, and Xray contamination. Use of a dual field technique produces reasonable beam uniformity over an area large enough to allow total skin electron therapy in a conventional treatment room. Monte Carlo techniques provided a guiding principle to assist the verification of the beam characterization of a TSET technique. The absolute calibration of dose to the patient required the measurement of the ratio "skin dose to calibration point dose"; thiswas achieved by measurements with a parallel plate ionization chamber and TLDs.