Effects on Scattered Radiation Through Flattening Filter Free (FFF) IMRT at 6 and 10 MV Beam Energy

Abstract

Removal of the flattening-filter (FFF) in IMRT decreases matter in the therapeutic beam that contributes to formation of scattered radiation (SR). By this means, it is possible to reduce contamination with ionizing radiation distant from the treated volume. It is yet unclear, what effect various beam energies have on SR in fully modulated FFF-IMRT plans with identical dose distributions. We used a LINAC with 160-leaf MLC to irradiate IMRT plans with almost identical dose distributions and for each beam energy identical therapeutic efficacy for 6 and 10MV with and without flattening filter. Plan a) Emulation of large volume pelvic RT for lymphatic metastasized prostate cancer, single dose 2 Gy. Plan b) small volume plan to prostate/seminal vesicles, single dose 2 Gy. Plan c) Intensely modulated small volume plan to prostate/seminal vesicles with integrated prostate-boost, single doses 2.25/3 Gy. All plans were applied to a RW3-slate phantom. SR was measured with semiflex chamber and a reference- dosimeter in 40 cm distance cranial to the isocenter. With both energies, 6 and 10MV respectively, FFF-IMRT resulted in significantly reduced SR in comparison to conventional IMRT (FF-IMRT). The reduction was more prominent with 10MV FFF-IMRT than with 6MV FFF-IMRT. Comparing energies, FF-IMRT with 10MV led to more SR in comparison to 6MV FF-IMRT. In contrast, with FFF-IMRT less SR was measured with plans A) and B) when 10MV was used in comparison to 6MV, but not with plan C). FFF-IMRT removes effectively a relevant scatter body from the therapeutic beam. Small target volumes (with integrated boost volumes) require more intensive modulation, therefore leading to increased interaction of photons with the MLC. This omits the SR-saving effect of FFF-IMRT. Because of different depth dose curves, more monitor units (MU) are used for comparable efficacy between 6 and 10MV. This leads to more SR with 6MV in comparison to 10MV in lesser modulated FFF-IMRT. Again, this effect is obscured in small target volumes with several doses levels due to more MLC-modulation.