A Study of the Optimal Ptv Margin for Patients Receiving Cbct-Guided Prostate Imrt Based on Cbct Dose Calculation

PURPOSE Variation in daily setup and rectum/bladder filling leads to uncertainties in the delivery of prostate IMRT. To determine the optimal PTV margin for CBCT-guided prostate IMRT, daily CBCTs were used to estimate prostate and OAR doses, compared to those expected from planning. METHODS Patients diagnosed with prostate cancer were treated with prostate IMRT to 70Gy in 28 fractions. Daily CBCT was utilized for image guidance. The prostate CTV and OARs were contoured on all CBCTs. IMRT plans were created using 1 mm, 3 mm, 5 mm and 7 mm CTV to PTV margins. Dosimetric analysis of the plans following daily CBCT translational shifts was performed. RESULTS 28 CBCT sessions were evaluated for preliminary results. Planned prostate CTV V100% was 100% for all PTV margins. Based on CBCT analysis, the actual cumulative V100% was 96.65±4.73%, 98.82±2.72%, 99.86±0.53% and 99.96±0.11% for 1-, 3-, 5- and 7 mm margins, respectively. The planned rectum V70Gy was 0.88%, 2.67%, 6.02% and 13.42%, for 1-mm, 3-mm, 5-mm and 7-mm PTV margins. The actual cumulative V70Gy for the rectum was 5.47±2.7%, 9.64±3.12%, 13.68±3.39% and 20.82±3% for 1-, 3-, 5- and 7 mm margins, respectively. The planned vs. delivered rectum V60 Gy was 6.99 vs. 14.73±3.63% (1 mm), 9.83 vs. 18.43±3.73% (3), 13.61 vs. 22.5±3.86% (5) and 23.20 vs. 30.82±4 % (7). The planned vs. delivered bladder V70Gy was 0.01%, 0.24%, 0.98% and 1.75% vs. 0.24±0.25%, 0.93±0.69%, 2.46±1.36% and 4.07±2% for 1-, 3-, 5- and 7 mm margins, respectively. CONCLUSION Daily setup variation during prostate IMRT yields differences in the actual vs. expected doses received by the prostate CTV, rectum and bladder. The magnitude of these differences is significantly affected by the PTV margin utilized. A 3-5 mm PTV margin appears to be optimal for prostate IMRT when daily CBCT is used for image guidance.