RESULTS Both plans resulted in technically demanding mask preparation. The IMRT plan required prone treatment, which combined with the weight of the bolus, would be uncomfortable. The HDR mask was less restrictive and did not require securement to a head tray. The HDR plan achieved target coverage of 50-160%, whereas the IMRT plan had target coverage 50-120%. The IMRT plan resulted in a maximum brain dose which was lower by 20%. However, the planning process was 5 times longer and a high number of segments were required, which resulted in a much longer treatment time. CONCLUSIONS There are advantages and disadvantages to each technique. The dose distribution of the HDR plan may be improved better catheter placement. The choice may depend upon the availability of personnel and equipment. This study will be extended to evaluate how closely the calculated dose matches the predicted dose. A comparison will then be made with conventional approaches of combined lateral opposing electron and photon fields with brain blocking and electron arc therapy. E14eE16. RADIOTHERAPY SESSION 3 E14 Monte Carlo (Mc) Optimization of a Total Skin Electron Therapy Technique Y.A. MOHAMMED a and C.A. WILLEMSE b a Medical Physics Department, Faculty of Science & Technology, Al-Neelain University, Khartoum, Sudan, b Medical Physics Department, Faculty of Health Sciences, University of the Free State, P.O. Box 339 Bloemfontein, South Africa INTRODUCTION Total skin electron therapy (TSET) is used for the treatment of malignant diseases of the skin, notably, mycosis fungoides. Several techniques have been developed in various centers, in order to achieve a homogeneous dose distribution over the complete body surface. To implement a TSET technique one has to optimize a variety of parameters, e.g. number, angle and energy of the beams. Monte Carlo simulation of TSET can facilitate this optimization. AIM In this study we aim to implement and optimize a TSET technique using the 4 and 6 MeV electron beams and the high dose rate facility on the Elekta Precise accelerator. METHODS The EGS4/BEAM MC code was used. Percentage depth-dose curves and beam profiles were calculated and measured experimentally for the 40x40 cm 2 nominal field at both 100 cm SSD and at the pa- tient treatment plane (SSD 350 cm). The accuracy of the simulation was evaluated by comparison of measured beam parameters (R 50, dmax, Rp) with Monte Carlo calculated results. To obtain a uniform dose profile over the length of a standing patient, two vertical an- gles of incidence were used. The angle between the central axes of the two beams was optimized for best uniformity of dose. The patient will be standing on a rotating platform perpendicular to the beam and rotated through 60 degree increments to obtain six directions of incidence for the dual fields (total of twelve fields). The doses delivered in a Rando phantom by the complete treat- ment technique were measured with Kodak EDR2 films positioned between different slices of the phantom. TLDs were placed on the surface to relate the film measurements to dose. The measured doses were compared to data obtained from the MC simulation. RESULTS The d max depth from different angles of incidence varied between 2-3 mm and 3- 4 mm for 4 and 6 MeV respectively. This information is useful when the lesions of different thickness are being treated. The composite percentage depth dose of all six dual fields for the 4 and 6 MeV yielded an R 80 of w 7 mm and w 8 mm, respectively. Dose uniformity was within Æ 5% for 4 MeV and Æ 3% for 6 MeV. The bremsstrahlung contamination was 0.9% and 1.3 % respectively. CONCLUSION Good agreement was found between the dose distribution calcu- lated with MC and measured with films. 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 treatment of Mycosis fungoides. E15 Determination of Electron Beam Output Factors For A 14-Mev Linear Accelerator (Phillips Sl75/14n) Vuyisile JONAS, F.C.P. DU PLESSIS and C.A. WILLEMSE Department of Medical Physics, University of the Free State, P.O. Box 339, Bloemfontein AIM The purpose of this study was to determine and to characterize electron beam output factors as a function of energy and field size for a Phillips SL75/14N linear accelerator. METHODS AND MATERIALS A farmer type ionization chamber associated with PTW UNIDOS electrometer was used to take readings for larger field sizes (lateral dimensions >3cm) and KODAK X-Omat V films were used for smaller SAAPMB Congress 2007 137