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

[摘要] English: Total skin electron therapy (TSET) is the treatment of choice for several malignantdiseases of the skin (Kaposi sarcoma, mycosis fungoides). Several different techniqueshave been developed in various centers, in order to achieve homogeneous dosedistribution over a large irradiation field (200 x 80 ern). However, to implement a TSETtechnique 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 themost acceptable dose distribution an extensive set of measurements and a large numberof calculations have to be performed. Therefore Monte Carlo simulation of TSET canfacilitate optimization of this technique. In this study we implemented and optimized aTSET technique using 4 and 6 MeVelectron beams. The dosimetrie procedure intendedto obtain adequate dose uniformity over the entire surface of the patient, and to reduce thepatient 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 theMC simulation. Percentage depth-dose curves and beam profiles were calculated andmeasured experimentally for the 40x40 cm2 nominal field at both 100 cm SSD and at thepatient surface at the treatment plane (SSD 350 cm) for a single beam. The accuracy ofthe simulated beam was validated by the good correspondence (within less than 2%)between measured beam characteristic parameters (Rso, dmax, Rp) and Monte Carlocalculated results. To obtain a uniform profile vertically, two vertical angles of incidencewere used. The angle between the two beams that gave best uniformity was consideredthe optimum angle. The patient is to be placed on a rotating platform perpendicular to thebeam and rotated through 60 degree increments to obtain six horizontal directions ofbeam incidence. The doses expected in the patient were measured with Kodak EDR2films positioned at different levels between slices of a Rando phantom. TLDs wereplaced on the surface to relate the film measurements to dose. The delivered doses in thetreatment plane were compared to simulated data that was obtained from the MCsimulation. The penetration depth of the dose distribution varied over various scanning directionsbetween 2-3 mm and 3- 4 mm for 4 and 6 MeV respectively. This information is usefulwhen treatment of lesions of different thickness are being considered. The compositepercentage 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 bothenergies 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 MCand measured with films, thus validating our MC calculations. The dose distributions inphantom were found to comply with the guidelines described in the AAPM TG-23protocol, 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 Xraycontamination. Use of a dual field technique produces reasonable beam uniformityover an area large enough to allow total skin electron therapy in a conventional treatmentroom. Monte Carlo techniques provided a guiding principle to assist the verification ofthe beam characterization of a TSET technique. The absolute calibration of dose to thepatient required the measurement of the ratio skin dose to calibration point dose; thiswas achieved by measurements with a parallel plate ionization chamber and TLDs.

[发布机构] University of the Free State