Focal salvage radiotherapy for recurrent prostate cancer 189 Discussion We performed a comparative planning study to evaluate the feasibility of delivering a single 19.0 Gy dose to a local recurrent prostate cancer lesion using a 1.5 T MR-Linac system. The simulated MRLinac plans were compared to clinically delivered FS-HDR-BT plans. Both treatment planning techniques showed comparable target coverage (see Table 2), though with both techniques the target dose of 19.0 Gy was not reached for some cases. The SBRT technique combined with the additional 1 mm PTV margin resulted in a higher median dose to both the urethra and rectum for the MR-Linac plans. However, no OAR constraints were violated. Delivering a single 19.0 Gy dose to a recurrent prostate cancer lesion with acceptable target dose coverage, while respecting OAR constraints, thus seems feasible on an MR-Linac. To our knowledge, no studies have investigated the feasibility of focal salvage SBRT to deliver a single fraction 19.0 Gy dose to an intraprostatic lesion in the recurrent setting with corresponding OAR constraints. Henderson et al.16 reported a planning study on single session treatment using SBRT and a 3 mm PTV margin. They prescribed 15.0 Gy and 19.0 Gy to ³ 95% and 65-75% of the PTV, respectively, with a boost of 21.0 Gy to ³ 95% of the MRI-visible intraprostatic lesion plus a 3 mm margin. The median rectal D2cm3 was 14.4 Gy and median urethral V20.8Gy was 20.8%. However, the different treatment setting with different OAR constraints makes it hard to extrapolate these results to our study. Currently, Zilli et al.17,18 are investigating the delivery of a single 19.0 Gy dose to the whole prostate gland and two-thirds of the seminal vesicles with urethra sparing (down to 17.0 Gy) in a primary setting. For bladder and rectum constraints, they use a V20Gy < 1.0 cm3. Again, because of the different setting and constraints, the results of this study are not directly comparable to our results. OAR constraints were met for all MR-Linac plans. We found a higher median urethral and rectal dose with the MR-Linac plans compared to FS-HDR-BT. This can be explained by the more gradual dose fall-off with SBRT and the larger PTV. Besides a higher rectum D1cm3, the mean dose to the rectum is also likely to increase. However, limited data is available on toxicity in patients treated with focal salvage radiotherapy. In our FS-HDR-BT group, grade 2 and higher GI toxicity was extremely low, with no grade 3 GI toxicity so far and only 4% new-onset grade 2 GI toxicity, as reported by Van Son et al.19 Although rectal dose is slightly higher with SBRT, it is still below the constraint and therefore low GI-toxicity is expected. Bladder D1cm3 and D2cm3 were comparable for FS-HDR-BT and MR-Linac. This is probably caused by the more cranial position of the bladder with respect to the prostate and therefore this organ is easier to avoid with SBRT than the rectum. Again, the differences were quite small and probably clinically irrelevant. Nevertheless, prospective studies should assess this. As expected, larger volumes receiving 150% or 200% of the prescribed dose were reached with FSHDR-BT. A means to increase the volume receiving high dose levels using an MR-Linac might be to include an additional – higher – dose prescription to the GTV. A higher dose to the tumour potentially leads to longer biochemical progression-free survival (BPFS). However, the exact relationship between these high-dose volumes and BPFS in this patient group has to be established. 9
RkJQdWJsaXNoZXIy MTk4NDMw