Room: Exhibit Hall | Forum 9
Purpose: To evaluate the dosimetric impact of daily positioning variations on whole-breast radiotherapy (WBRT) patients treated in the prone position.
Methods: Daily CBCT scans were acquired for 30 patients positioned using a CDR Systems prone breast board (CDR Systems Inc.). Treatment for early stage (â‰¤II) breast cancer was prescribed with standard dose (50Gy/25 fractions) or hypo-fractionation (42.56Gy/16 fractions) for 13 and 17 patients, respectively. Systematic and random errors were calculated from the translational CBCT shifts and used to calculate population-based setup margins; only fractions that did not require re-positioning of patients were included. The dosimetric impact of translational shifts of Â±0.5cm in each dimension was simulated in the TPS (Pinnacle v9.8) for PTV, heart, and lung. Paired Studentâ€™s t-tests at Î±=0.01 were used to compare dose metrics after correction for multiple testing (p<0.002).
Results: Of 588 total fractions, 19.2% required patient rotation and were excluded from analysis. Mean translations in 477 fractions were 0.28Â±0.27cm, 0.42Â±0.37cm, 0.47Â±0.3cm in the anterior-posterior, superior-inferior and lateral directions leading to calculated setup margins of 0.61cm, 0.92cm and 1.10cm, respectively. Simulated shifts showed significant, but minor, changes in all dose metrics: V95% for PTV decreased by 2.7Â±1.6%, and lung V20Gy increased by 0.5Â±0.6%. For left-sided treatments (n = 18), mean heart dose increased from 106Â±67 to 155Â±120cGy for anteriorly simulated shifts. Shifts from the original plan caused PTV hotspots (V105%) to increase by 6.1%Â±5.5%, which highly correlated (r = 0.74) with the total MU of wedged fields. Simulated plans exceeded institutional dose constraints in only 2 plans (V105%) in which all fields were wedged.
Conclusion: Despite the large setup variations that occur when positioning prone breast patients, dose delivery is robust for 3DCRT plans. To maintain planned dose metrics, anterior shifts towards the heart and the contribution of wedged fields should be minimized.