Evaluation of tumor motion effects on dose distribution for hypofractionated intensity-modulated radiotherapy of non-small-cell lung cancer.

Respiration-induced tumor motion during intensity-modulated radiotherapy (IMRT) of non-small-cell lung cancer (NSCLC) could cause substantial differences between planned and delivered doses. While it has been shown that, for conventionally fractionated IMRT, motion effects average out over the course of many treatments, this might not be true for hypofractionated IMRT (IMHFRT). Numerical simulations were performed for nine NSCLC patients (11 tumors) to evaluate this problem. Dose distributions to the Clinical Target Volume (CTV) and Internal Target Volume (ITV) were retrospectively calculated using the previously-calculated leaf motion files but with the addition of typical periodic motion (i.e. amplitude 0.36-1.26cm, 3-8sec period). A typical IMHFRT prescription of 20Gy x 3 fractions was assumed. For the largest amplitude (1.26 cm), the average +/- standard deviation of the ratio of simulated to planned mean dose, minimum dose, D95 and V95 were 0.98+/-0.01, 0.88 +/- 0.09, 0.94 +/- 0.05 and 0.94 +/- 0.07 for the CTV, and 0.99 +/-0.01, 0.99 +/- 0.03, 0.98 +/- 0.02 and 1.00 +/- 0.01 for the ITV, respectively. There was minimal dependence on period or initial phase. For typical tumor geometries and respiratory amplitudes, changes in target coverage are minimal but can be significant for larger amplitudes, faster beam delivery, more highly-modulated fields, and smaller field margins.