Effects of 10 MV and Flattening-Filter-Free Beams on Peripheral Dose in a Cohort of Pediatric Patients

Abstract

To assess the effect of flattening-filter-free (FFF) and 10MV radiation therapy beams on the peripheral dose received by a population of pediatric patients undergoing volumetric modulated arc therapy (VMAT). Twenty-six previously delivered 6MV flattened VMAT pediatric radiation therapy treatments plans were replanned with 6MV flattened, 6MV FFF, and 10MV FFF VMAT. Monte Carlo simulation code EGSnrc was used in conjunction with a measurement-based model to obtain 3-dimensional dose distributions. Peripheral dose delivered by FFF beams was compared with that delivered by 6MV flattened beams. A statistical analysis was performed to determine whether certain clinical factors (eg, target volume, location) were associated with a change in integral relative radiation dose. Neutron dose measurements assessed the neutron contribution from the 6MV flattened and 10MV FFF x-ray beams. Both the 6MV FFF and 10MV FFF beams delivered significantly lowerperipheral radiation doses than 6MV flattened (P<.01). The dose reduction was of 3.9% (95% confidence interval [CI] 2.1-5.7) and 9.8% (95% CI, 8.0-11.6) at 5cm from the PTV and 21.9% (95% CI, 13.7-30.1) and 25.6% (95% CI, 17.6-33.6) at 30cm for 6MV FFF and 10MV FFF beams, respectively. The clinical factorsexamined did not have a significant effect on the relative magnitude of the peripheral dose reduction. The upper limit on the neutron dose was determinedto be 203μSv for the 6MV flattened and 522μSv for the 10MV FFF beam. Both FFF beams significantly (P<.01) reduced the peripheral dose. 10MV FFF was more effective at reducing peripheral dose at distances <5cm from the PTV edge. The neutron doses delivered by all beams were <1% compared with the photon doses. 10MV FFF should be used to minimize peripheral dose.