Room: AAPM ePoster Library
Hypofractionated radiotherapy for prostate cancer reduces the inconvenience of an extended treatment course but the appropriate treatment margin to ensure tumor control while minimizing toxicity is not standardized. Margins can be determined mathematically but clinical validation is the gold standard. Using a novel dose accumulation workflow with iterative CBCT (iCBCT) images, we were able to validate margins for hypofractionated radiotherapy.
Ten patients were treated to the prostate on an IRB approved hypofractionated clinical trial (NCI 20110491). Prescription dose was 3625 cGy to >95% of the PTV in 5 fractions with a boost to 4000 cGy to the high risk GTV (if applicable). PTV margin expansion was 5 mm isotropic except 3 mm posterior, no margin for the GTV. Daily iCBCT images were obtained while practicing strict bladder and rectal filling protocols. Using a novel adaptive dose accumulation workflow, synthetic CTs were created for each fraction by deforming the daily iCBCT to the planning CT. These synthetic CTs were sent to the planning system and using the daily shifts, the treatment plan was recalculated. These daily dose distributions were accumulated and target coverage and organ dose were assessed.
Although the PTV coverage dropped from 96.1±0.9% to 86.5±3.5% (p<0.001) for the accumulated dose, the prostate coverage was not compromised (99.4±0.8% vs. 98.9±1.3%; p=0.112). The differences in bladder V37Gy (3.2±2.7 cc vs. 2.8±4.0 cc; p=0.504) and rectum V36Gy (0.4±0.4 cc vs. 0.5±0.8 cc; p=0.780) were not significantly different. Four patients received a boost to the GTV and a significant decrease in coverage was noted (95.4±0.5% vs. 71.6±13.7%; p=0.037) in the accumulated dose.
The novel dose accumulation workflow demonstrated acceptable margins for hypofractionated prostate radiotherapy to achieve tumor control while minimizing toxicity. Results showed that a margin in the GTV may be appropriate.