Evaluation of surface and build-up region dose for intensity-modulated radiation therapy in head and neck cancer

Despite much development, there remains dosimetric uncertainty in the surface and build-up regions in intensity-modulated radiation therapy treatment plans for head and neck cancers. Experiments were performed to, determine the dosimetric discrepancies in the surface and build-up region between the treatment planning system (TPS) prediction and experimental measurement using radiochromic film. A head and neck compression film phantom was constructed from two semicylindrical solid water slabs. Treatment plans were generated using two commercial TPSS (PINNACLE(3) and CORVUS) for two cases, one with a shallow (similar to 0.5 cm depth) target and another with a deep (similar to 6 cm depth) target. The plans were evaluated for a 54 Gy prescribed dose. For each case, two pieces of radiochromic film were used for dose measurement. A small piece of film strip was placed on the surface and another was inserted within the phantom. Overall, both TPSs showed good agreement with the measurement. For the shallow target case, the dose differences were within +/- 300 cGy (5.6% with respect to the prescribed dose) for PINNACLE(3) and 240 cGy (4.4%) for CORVUS in 90% of the region of interest. For the deep target case, the dose differences were 350 (6.5%) for PINNACLE(3) and +/- 260 cGy (4.8%) for CORVUS in 90% of the region of interest. However, it was found that there were significant discrepancies from the surface to about 0.2 cm in depth for both the shallow and deep target cases. It was concluded that both TPSs overestimated the surface dose for both shallow and deep target cases. The amount of overestimation ranges from 400 to 1000 cGy (similar to 7.4% to 18.5% with respect to the prescribed dose, 5400 cGy). (c) 2005 American Association of Physicists in Medicine.