Tumor-Targeted Dose Escalation For Localized Prostate Cancer Using Mr-Guided Hdr Brachytherapy (Hdr) Or Integrated Vmat (Ib-Vmat). Dosimetry And Early Toxicity Analysis.

To report dosimetry and early toxicity outcomes of tumor-targeted dose-escalation delivered by integrated VMAT (IB-VMAT) or MR-guided HDR brachytherapy (HDR) boost for prostate cancer. Patients diagnosed with localized prostate cancer, with at least 1 identifiable intraprostatic lesion (> 5mm and <33% total prostate volume) on multiparametric MRI (mpMRI) were enrolled in a prospective non-randomized phase II study. All patients received VMAT to the prostate alone (76 Gy in 38 fractions) plus a GTV boost: IB-VMAT (95Gy in 38 fractions) or MR-guided HDR (10Gy single fraction). GTV was delineated on mpMRI and deformably registered to planning CT scans. CTV76 included prostate plus 5mm margin around the GTV avoiding OARs (penile bulb, urethra, rectum/bladder wall). PTV76 was CTV + 5mm AP/ SI and 3mm LR. PTV95 was GTV + 5mm AP/ SI and 3mm LR (IB-VMAT). PTV10 was GTV(s) + 2 mm SI and 1mm AP/RL (HDR). Comparative dosimetry using EQD2 assuming α/β 3 Gy was performed. For HDR, deformable registration to account for anatomical distortion was used to estimate EQD2 and then summed with the external beam dose. Toxicity data was prospectively collected (CTCAE v.4.0). Eighty patients were enrolled. Dosimetric results are shown below.Abstract SU_33_2321; Table 1VMAT-Integrated Boost (n=40)HDR-Boost (n=40)OrganMetricAverage [Gy] (SD)Average [Gy] (SD)GTVD99105.3 (2.8)103.1 (6.7)D50109.4 (1.5)163.0 (33.8)D0.1cc111.7 (1.7)416.7 (100.5)PTV (GTV)D9996.8 (4.3)95.1 (5.4)D50107.5 (1.3)151.3 (23.8)D0.1cc112.2 (1.7)467.9 (174.5)PTV ProstateD9971.6 (1.0)72.7 (2.0)D5081.1 (2.7)81.6 (2.6)D0.1cc112.2 (1.7)485.3 (209.5)Rectum WallD5016.8 (3.6)13.6 (6.5)D0.5cc79.4 (5.0)82.6 (4.8)Bladder WallD5016.8 (3.6)13.6 (6.5)D0.5cc77.2 (5.9)76.2 (4.9)UrethraD5086.4 (7.0)85.4 (5.3)D0.5cc87.4 (7.0)85.7 (6.6)Penile BulbD508.4 (7.7)9.7 (8.9)D0.5cc29.2 (22.6)30.0 (24.7) Open table in a new tab Median follow-up was 24 months (range 6-36). Acute grade 2 GU toxicity was 40% and 42.5% in IB-VMAT and HDR, while GI toxicity was 7.5% and 10%, respectively. One IB-VMAT patient developed grade 3 GU acute toxicity (urinary retention). Of 69 patients with >12 months follow up (33 IB-VMAT and 36 HDR boost), 18.1% (IB-VMAT) and 19.4% (HDR) developed grade 2 GU toxicity, while grade 2 GI toxicity rates were 6% (IB-VMAT) and 2.7% (HDR). Late grade 3 toxicity was observed in 2 patients (IB-VMAT): 1 GU (hematuria attributable to a new bladder cancer) and 1 GI (rectal ulcer in the context of concurrent HIV antiviral therapy). Deterioration in sexual function occurred in 20% IB-VMAT and 12.5% HDR patients. No statistically significant difference in toxicity was found between the two groups. Our early results suggest comparable dosimetry to OARs and rates of Grade 2 toxicity between the two boost techniques. HDR boost achieved higher mean and max GTV doses than IB-VMAT. Further follow-up will determine long-term outcomes.