Abstract
Results of the in vitro measurements of cell kill from IMRT fields compared to acute dose delivery are presented. Survival was assayed for three cell lines: Chinese hamster V79 fibroblasts, human cervical carcinoma SiHa and colon adenocarcinoma WiDr. An actual head and neck seven field IMRT plan produced for Varian iX, 120 Millennium MLC linear accelerator was used. The calculated IMRT dose to the point of irradiation in the acrylic phantom was 2.1Gy, verified by measurements with an IC10 ionization chamber. The dose varied within 2% through dimensions of the specimen. Two scenarios were explored: 1. regular IMRT delivery, irradiation time of approximately 5min, and 2. IMRT delivery which required MLC re‐initialization after three fields were delivered, irradiation time of 10min. For comparison the same dose of 2.1Gy was delivered by parallel‐opposed pair (POP) in 75s. Persistent increase in cell survival following irradiation with IMRT fields was observed for all cell lines. This increase varied from small for V79 cells: 0.833±0.018 (95% confidence limits) from POP compared to 0.860±0.040 for IMRT with MLC re‐initialization, to very pronounced for the radiosensitive SiHa cell line: 0.390±0.046 for POP irradiation compared to 0.591±0.080 from IMRT requiring MLC re‐initialization. For WiDr cell line survival was 0.744±0.038 for POP, and 0.809±0.023 for regular IMRT. We observed a persistent, although not always statistically significant increase in cell survival from IMRT fields. While this study does not account for features present in vivo, it provides useful insight into biological consequences of IMRT as assayed by cell survival.
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