Flattening filter free volumetric modulated arc therapy for extreme hypofractionation of prostate cancer - Decreasing the treatment time and reducing the impact of prostate motion

Abstract: Purpose: This work examined the feasibility of flattening filter free (FFF) volumetric modulated arc therapy (VMAT) for extreme hypofractionation of prostate cancer. The aim was to investigate the potential gain in treatment time per fraction for extreme hypofractionation of prostate cancer, while preserving or improving the treatment plan quality and radiation delivery quality. The impact of intrafractional prostatic displacement was investigated in a motion study using six different prostate trajectories. Material and methods: Nine patients (CTV volume range: 35.1-82.5 cm3), who received VMAT in Lund between 2013-2015, were selected from the currently on-going HYPO-RT-PC trial. Using single arcs, treatment plans with photon beam qualities 10 MV, 6 MV FFF and 10 MV FFF were created for each patient for the conventional treatment arm, 78 Gy in 2 Gy fractions, and for the hypofractionated treatment arm, 42.7 Gy in 6.1 Gy fractions. Parameters from the dose-volume histograms (DVH) for the different beam qualities were statistically evaluated using a paired sample student’s t-test. Treatment delivery was evaluated through measurements on a Varian TrueBeamTM with a Delta4 pretreatment verification phantom (ScandiDos AB, Uppsala, Sweden). The required beam-on time for each plan was also recorded. A motion study was performed for the patient with the longest treatment times, for one flattened and one unflattened hypofractionated plan using the Hexamotion (ScandiDos AB, Uppsala, Sweden). Trajectory data of six different prostate movement patterns (Ng et al., 2012) were used for studying the dosimetric effects of prostate motion. All results were evaluated relative to a static delivery. Results: The treatment plans were visually reviewed and approved by a senior physician at the department. Evaluating the DVHs of the different treatment plans, no significant differences between beam qualities or between fractionation schedules were observed. All objectives were met for all plans. At the treatment delivery control all plans passed the gamma criterion 3%, 2 mm with a pass rate of 98.8% or higher. The beam-on time for all conventional treatment plans was 1.0 minute. The mean beam-on time was 2.3 minutes for the hypofractionated 10 MV standard plan, 1.3 minutes for the 6 MV FFF and 1.0 minute for the 10 MV FFF, respectively. In the motion study, no or little effect was observed on the pass rate for displacements ≤ 1mm, compared to the static delivery. The shorter treatment delivery was superior for three patterns, while the longer treatment was preferred in the case of a temporal displacement of the prostate. Conclusions: The treatment time for extreme hypofractionation of prostate cancer is reduced to less than half the time by combining FFF-technique with VMAT. The treatment plan quality and radiation delivery quality are equal to those observed for flattened beams. A shorter beam-on time also seems advantageous for the majority of prostate motion patterns investigated. Based on this work, it is feasible to implement FFF VMAT for extreme hypofractionation of prostate cancer (HYPO-RT-PC trial) at the radiotherapy department at Skåne University Hospital, Lund.