Treatments of exceptionally large prostate cancer patients with low-energy intensity-modulated photons.

An inverse planning technique was developed for large-sized prostate cancer patients treatments using 6-MV intensity modulated photon beams. Comparisons of the treatment plans between using 6-MV and 18-MV intensity modulated beams were carried out for a cohort of ten patient cases. The dependence of the plan quality on the beam energies was analyzed for these cases. We found that 6-MV produced equivalent plans as 18-MV for both targets and critical structures such as the rectum and the bladder. The differences in the integral dose and the mean dose to the normal tissue surrounding the target between 6- and 18-MV plans were found to be small in contrast to 3D conformal plans. This shows that the low entrance dose of the high energy photon beams is mostly compensated by the high exit dose for even exceptionally large-size patients. In conclusion, 6-MV intensity modulated beam is a feasible choice for treating large-sized prostate cancer patients provided proper inverse planning techniques are adopted.