Room: Track 6
Purpose: In this study we assess accuracy and efficiency of a novel extremely hypo-fractionated prostate cancer treatment, which reported 5-years bRFS >90% with <3% late GU and GI grade 2 toxicities in over 200 patients.
Methods: Since 2013, low and intermediate risk prostate cancer patients in our institution are being treated with 5 times 9Gy to the PTV (=CTV+2mm). Rectum-, bladder- and urethra-sparing is prioritized over PTV-coverage during planning. Patient reproducibility is improved using an air-filled (150cc) rectal balloon and three re-usable, intra-urethral electromagnetic beacons. Reference treatment position for both prostate and OARs is reached after repeated CBCT-imaging. Beam delivery was interrupted for beacon motions beyond 2mm for 5sec. We determined workload and systematic (S) and random (s) errors for all geometrical uncertainties during treatment in the patient group and calculated the corresponding PTV margin (M=2S+0.7s).
Results: Workload and beacon motions were determined for 189 cases with a mean CTV of 50±20cc (1SD). Using on average 3 CBCTs to reach the reference position, mean total treatment time was 20min. Strict OAR planning constraints were generally met at the cost of reduced PTV coverage to on average D99%=31.6Gy, while PTV Dmean remained 44.9Gy. Fifty-three motion-related treatment interruptions (6%) were performed. Regarding intra-fraction beacon motion, sub-millimetre SDs (S and s) were observed in all directions. Prostate deformations due to variations in rectal balloon and bladder yielded SDs of ca 1mm, and prostate position errors in the reference CBCT amounted to 1-1.5mm SDs. This resulted in traditional PTV margins of ca. 3-4.5mm, with average Dmean=43.3Gy and D99%=26.4Gy.
Conclusion: Our OAR sparing, 5x9Gy prostate treatment uses smaller margins than prescribed using traditional recipes, resulting in reduced PTV coverage. Clinical results so far are excellent however, justifying our conformal avoidance approach and questioning applicability of traditional recipes for these hypo-fractionated treatments.