Part II | Chapter 9 186 Table 1 – Dose prescription and organ-at-risk (OAR) constraints for MRI-guided high-dose-rate brachytherapy treatment. to 90% and 95% of the target, respectively), V100%, V150%, and V200% (relative target volume receiving 100%, 150%, and 200% of the prescribed dose, respectively). For the bladder and rectum, D1cm3 and D2cm3 (highest dose to 1.0 cm3 and 2.0 cm3 of the OAR) were obtained. For the urethra, D10% (highest dose to 10%of the urethra) was collected. Themedian values (and range) of target volume coverage, high-dose volumes and OAR doses were obtained. The paired data was compared using the nonparametric Wilcoxon signed-rank test. To correct for multiple testing, p-values < 0.01 were considered statistically significant. All analyses were performed using IBMSPSS Statistics 25 (Chicago, IL, USA). Results The delineated tumour volume (GTV) ranged from 0.7 cm3 to 17.8 cm3 and CTV volume ranged from 2.9 cm3 to 24.7 cm3 (Table S1, Supplementary A). Figure 2 shows an exemplary dose distribution with the 50%, 100%, 150%, and 200% isodose-lines for a FS-HDR-BT and an MR-Linac plan. Table 2 presents the dosimetric results for the FS-HDR-BT and MR-Linac plans. Out of the 30 FS-HDRBT plans, 16 (53%) did not reach the prescribed dose of 19.0 Gy to ³ 95% of the target, compared to 13 (43%) for the MR-Linac plans. Of the 16 plans that failed to meet the 19.0 Gy dose prescription for FS-HDR-BT, five succeeded with the MR-Linac. Contrarily, two out of the 13 that failed with the MRLinac system, succeeded with FS-HDR-BT. Additionally, for both FS-HDR-BT and MR-Linac, two plans (7%) failed to meet the secondary dose prescription of at least 17.0 Gy to 90% of the target volume. All these plans did, however, reach a dose of at least 17.0 Gy with the other treatment modality. The two plans that failed with FS-HDR-BT had a large CTV (33% and 60% of prostate volume) surrounding the urethra. The two plans that failed with MR-Linac had a tumour located in the base and seminal vesicle, closely related to the bladder and rectum. In addition, no target volume received 200% (38.0 Gy) of the prescribed dose with MR-Linac treatment planning, whereas the median target volume receiving 38.0 Gy was 25% for FS-HDR-BT (p < 0.001). OAR dose constraints were met in all MR-Linac plans. In 5 (17%) FS-HDR-BT plans, OAR constraints were violated; in 3 (10%) cases the urethra D10% Target/OAR Dosimetric parameter Prescribed dose or constraint CTV/PTV* D95% ³ 19.0 Gy CTV/PTV* D90% ³ 17.0 Gy Urethra D10% < 17.7 Gy Bladder D1cm3 < 12.0 Gy Rectum D1cm3 < 12.0 Gy *CTV for FS-HDR-BT plan and PTV for MR-Linac plan. Legend: CTV = Clinical Target Volume. PTV = Planning Target Volume. DXX% = Highest dose to XX% of the volume of interest. DXcm3 = Highest dose to X cm3 of the volume of interest.
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