Orthovoltage computer planning with a new type of variable collimator

Full text: The Liverpool Hospital Radiation Oncology Department has recently installed and commissioned a Pantak Therapax DXT-300 orthovoltage therapy treatment unit. Purchased with this was a variable collimator capable of producing rectangular fields up to 20 x 20 cm2. Occasionally patients who are treated with, for example, mediastinal deep therapy portals go on to have further megavoltage therapy (eg. total body irradiation (TBI)). The orthovoltage dose distribution in lung tissue can be the limiting factor in terms of proceeding with TBI due to the inherent risk of radiation induced pneumonitis. The present work assesses the ability of the Department's Varian Cadplan treatment planning system (TPS) to calculate dosage distribution for orthovoltage radiation fields delineated by the variable collimator. Data was collected using a Scanditronix RFA-300 beam data acquisition system (BDAS). For each of four qualities (0.85, 1.7, 2.6, and 3.8 mm Cu) profile data at five depths and central axis depth dose data were collected for square field sizes of 3, 4, 6, 8, 10, 12, 15 and 20 cm2, as input to the TPS. Isodose distributions produced by the TPS were then checked against the BDAS collected isodoses. Good agreement was found under these conditions of irradiation of a homogeneous medium. Work is ongoing to assess the accuracy of the TPS for simple heterogeneities which are introduced to these orthovoltage beams. It is expected that the heterogeneity correction method used by the TPS will be in error when higher atomic number (Z) media are introduced (eg. bone analogues) due to the fact that the algorithm assumes that tissue attenuation is related solely to electron density, which is generally valid for megavoltage photon beams where Compton interactions are predominant. With the increasing contribution from photo-electric interaction processes as the orthovoltage beam quality decreases, preliminary results have shown increasing TPS deviation under these circumstances of higher Z heterogeneities