Determination of correction factors for a 2D diode array device in MV photon beams.

The purpose of this study is to evaluate the dosimetric characteristics of a 2D diode array device (MAPCHECK) and provide correction factors for clinically relevant conditions. Measurements were performed for various collimator settings (5 x 5-40 x 40 cm2), SSDs (80-110 cm), and depths (5-20 cm) for 6 and 15 MV photon beams. The correction factor was determined as the ratio of dose measured by an ionization chamber in a large full scatter water phantom to the dose measured by the MAPCHECK with 30 x 30 cm2 solid water phantom. The total correction factor was separated into three components. The phantom scatter factor was evaluated by taking the ratio of dose measured by MAPCHECK with backscatter material and 30 x 30 cm2 phantom to that by the ionization chamber in an infinite large water phantom renormalized to the reference field size (10 x 10 cm2). This was also compared to the theoretical calculation using ratios of phantom scatter factor between an infinitely large water phantom and 30 x 30 cm2 solid water phantoms. The backscatter effect was determined by taking the ratio of the MAPCHECK measurements with and without 10 cm thick solid water backscatter material also normalized to the reference field size at the depth of measurement. The extra correction factor is defined as the ratio of dose measured by the MAPCHECK (without backscatter) to that measured by the ionization chamber in a large water phantom at depth d for the reference field size (10 x 10 cm2) normalized to the reference depth (10 cm) and reference field size. The total correction factors for 6 and 15 MV photon beams were up to 16% and 11.5%, respectively, due to the difference in phantom scatter in MAPCHECK measurements when the field sizes are larger than 30 cm. The result was consistent with theoretical analysis with phantom scatter alone. The backscatter correction factor varied by up to 2% and increased with increasing field size. The extra correction factor decreases with depths and is < 0.4% for all depths. The authors have verified that correction factors for MAPCHECK are a function of the scaled field size and depth only and are independent of the SSD. Results were verified for off-axis points measured by MAPCHECK.