Purpose: To investigate the dosimetric differences in treatment plans for prostate with and without seminal vesicle using volumetric modulated arc therapy (VMAT) plans with flattened and flattening-filter-free (FFF) beams.
Methods: Nine patients treated with 1.8-2.5Gy/fx and 6MV-flattened beam (6x) were identified. Each patient’s VMAT plans were recalculated for 10MV FFF beams (10FFF). All of the optimization parameters were set to be identical for flattened and FFF beam plans. The maximum dose rate for 6x (600 MU/min) and for 10FFF (2400 MU/min) were allowed during optimization; however, the treatment planning system automatically optimized at a lower dose rate for FFF plans. Plans optimized with 10FFF beams were compared with the plans optimized and calculated with 6x beams. Various metrics of these two plans were evaluated in this study. Studied metrics included mean dose to rectum, bladder, bowel, body, target; maximum dose to femoral heads, and penile bulb; conformity index; target dose inhomogeneity; and Total MU.
Results: All generated plans, regardless of the beam modality, met clinically accepted objectives. Both 6x and 10FFF beams provided similar target coverage and dose conformity. In comparison, 10FFF beam provided better solution in sparing rectum, bladder, bowel, femoral heads and penile bulb than 6x beam. Mean dose to the body decreased with 10FFF by 5.2%. Average maximum dose to the femoral heads also decreased with 10FFF by 4.2%. 10FFFF increased target dose inhomogeneity. In most cases, failure after external beam radiation show that the area responsible for the local recurrence is the intraprostatic dominant nodule. Finally, total MU was lower in plans using 10FFF.
Conclusion: Both treatment modalities provided clinically acceptable treatment plans. The results of this study indicated that 10FFF plans resulted in better plan quality metrics, with relatively shorter delivery times. FFF beam’s target dose inhomogeneity may be used for a focal dose escalation.