Monte Carlo determination of radiation induced cancer risks for prostate IMRT patients

Abstract

The application of intensity-modulated radiation therapy (IMRT) has enabled us to deliver high doses to the target volume while sparing the surrounding normal tissues. The drawback of intensity modulation, as implemented using a computer-controlled multi-leaf collimator (MLC), is the larger number of monitor units (MU), and longer beam-on time, as compared to conventional radiotherapy. Additionally, IMRT uses more beam directions, typically 5-9 for prostate treatment, to achieve higly conformal dose and normal tissue sparing. In this work we study radiation induced cancer risks due to IMRT delivery using MLC for prostate patients.

Whole body CT scans were was used in our study in order to calculate the effective dose equivalent from the doses received by individual organs according to NCRP report 116. The dose was computed using EGS4/MCSIM for IMRT and 3D conformal radiotherapy. The effect of the collimator rotation, the distance from the treatment field, and the scatter and leakage contribution to the whole body were investigated.  We calculated the whole body dose equivalent to estimate the increase in the risk of secondary malignancies.

Our results showed an overall two fold increase in the risk of secondary malignancies due to the application of IMRT as compared to conventional radiotherapy. This two fold increase is not necessarily related to a relative two fold increase in MU. The whole body dose equivalent was also, affected by the collimator rotation, the field size and the energy of the photon beam. Smaller field sizes of low energy photon beams (i.e. 6MV) with the MLC axis along the lateral axis of the patient resulted in the lowest whole body dose. Our results can be used for the evaluation of the risk of secondary malignancies for prostate IMRT patients.