Monitor unit optimization in RapidArc plans for prostate cancer

Intensity-modulated radiation therapy (IMRT) has become a standard treatment for prostate cancer based on the superior sparing of the bladder, rectum, and other surrounding normal tissues compared to three-dimensional conformal radiotherapy, despite the longer delivery time and the increased number of monitor units (MU). The novel RapidArc technique represents a further step forward because of the lower number of MUs per fraction and the shorter delivery time, compared to IMRT. This paper refers to MU optimization in RA plans for prostate cancer, using a tool incorporated in Varian TPS Eclipse. The goal was to get the lowest MU RA plan for each patient, keeping a well-defined level of PTV coverage and OAR sparing. Seven prostate RA plans (RA MU-Optimized) were retrospectively generated using the MU optimization tool in Varian Eclipse TPS. Dosimetric outcome and nontarget tissue sparing were, compared to those of RA clinical plans (RA Clinical) used to treat patients. Compared to RA Clinical, RA MU-Optimized plans resulted in an about 28% (p = 0.018) reduction in MU. The total integral dose (ID) to each nontarget tissue (but not the penile bulb) showed a consistent average relative reduction, statistically significant only for the femoral heads. Within the intermediate dose region (40-60 Gy), ID reductions (4%-17% p < 0.05) were found for the rectum, while a slight but significant (0.4%-0.9%, p < 0.05) higher ID was found for the whole body. Among the remaining data, the mean dose to the bladder was also reduced (-12%, p = 0.028). Plans using MU optimization are clinically applicable and more MU efficient, ameliorating the exposure of the rectum and the bladder to intermediate doses.