Monte Carlo determination of radiation-induced cancer risks for prostate patients undergoing intensity- modulated...

The application of intensity-modulated radiation therapy (IMRT) has enabled the delivery of high doses to the target volume while sparing the surrounding normal tissues. The drawbacks of intensity modulation, as implemented using a computer controlled multileaf collimator (MLC), are the larger number of monitor units (MUs) and longer beam-on time as compared with conventional radiotherapy. Additionally, IMRT uses more beam directions--typically 5-- 9 for prostate treatment--to achieve highly conformal dose and normal-tissue sparing. In the present work, we study radiation-induced cancer risks attributable to IMRT delivery using MLC for prostate patients. Whole-body computed tomography scans were used in our study to calculate (according to report no. 116 from the National Council on Radiation Protection and Measurements) the effective dose equivalent received by individual organs. We used EGS4 and MCSIM to compute the dose for IMRT and three-dimensional conformal radiotherapy. The effects of collimator rotation, distance from the treatment field, and scatter and leakage contribution to the whole-body dose were investigated. We calculated the whole-body dose equivalent to estimate the increase in the risk of secondary malignancies. Our results showed an overall doubling in the risk of secondary malignancies from the application of IMRT as compared with conventional radiotherapy. This increase in the risk of secondary malignancies is not necessarily related to a relative increase in MUs. The whole-body dose equivalent was also affected by collimator rotation, field size, and the energy of the photon beam. Smaller field sizes of low energy photon beams (that is, 6 MV) with the MLC axis along the lateral axis of the patient resulted in the lowest whole-body dose. Our results can be used to evaluate the risk of secondary malignancies for prostate IMRT patients.