2313 - Dosimetric Comparison of O-Ring and C-Arm Linac for Volumetric Modulated Arc Therapy with Simultaneous Integrated Boost of Left-Sided Breast Cancer under Deep Inspiration Breath Hold

T. Hui, Y. K. Ho, C. C. Ko, M. K. Wong, Y. T. Ngan, T. W. Mok, L. C. Chan, C. H. Ho, K. H. Lok, S. N. Chan, H. L. Tang, and H. C. Cheng; Union Oncology Centre, Hong Kong, Hong Kong

Purpose/Objective(s): Post-lumpectomy adjuvant radiotherapy for breast cancer reduces local recurrence rate of ten years from 31% to 15.6%. Radiotherapy under deep inspiration breath hold (DIBH) delivered by O-ring linac may improve the time efficiency of treatment delivery. This study compared the quality of treatment plans for hyprofractionated left-sided breast cancer with simultaneous integrated boost (SIB) to lumpectomy cavity without nodal irradiation/involvement under DIBH, planned for O-ring and C-arm linac. Materials/

Methods: Treatment plans with 3 arcs of 150 degree each were generated on O-ring and C-arm linac for 33 patients with left-sided breast cancers with no regional nodal involvement. All patients underwent DIBH planning CT. PTVs were contoured by the same clinical oncologist. Contouring of organs at risk (OARs) and plan acceptance criteria were based on RTOG 1005 and RTOG 1304. Prescription was 40.05 and 48 Gy to PTV and PTV boost in 15 fractions, respectively. Doses to the targets and OARs were compared, including V40.05 Gy and conformity index of PTV; V48 Gy, maximum dose (Dmax), homogeneity index (HI) and conformity index (CI) of PTV boost; V4 Gy, V8 Gy, V16 Gy of left lung, V4 Gy and D10% of right lung; D10% and mean dose (Dmean) of right breast, D5%, D30% and Dmean of the heart and Dmax of the left anterior descending artery (LAD). Total monitor unit (MU) and treatment delivery time were measured and compared. Statistical comparisons were performed using paired t-test and Wilcoxon signed rank test. Statistical significance was defined as p-value < 0.05. All plans underwent plan specific quality assurance and gamma index analysis were done using 3% / 3mm criteria.
Results: All plan acceptance criteria were met and all plans were deliverable with gamma passing rate > 95%. Dosimetric parameters of prescribed dose coverage, HI and CI of PTV and PTV boost showed no statistical significant difference between the two linacs. Lower OAR doses were achieved in the plans of the O-ring linac with statistically significant differences, the mean differences were 3.71% (-7.7% ), 1.88% (-7.5%) and 0.89% (-6.7%) for V4Gy, V8Gy and V16 Gy of left lung, respectively; 14.32 cGy (-3.7%) for D10% of right lung; 35.30 cGy (-15.6%) and 19.91 cGy (-12.9%) for D10% and Dmean of right breast, respectively; 28.90 cGy(-9.2%) and 28.91 cGy (-10.0%) for D30% and Dmean of the heart, respectively and 138.75 cGy (-7.9%) for Dmax of LAD. Despite the higher MU of the plans, the mean treatment delivery time of the O-ring linac was 49.18 sec (-30.8%) shorter. Other parameters showed non-statistically significant differences.
Conclusion: Clinically acceptable and deliverable plans can be generated for O-ring linac, which achieve statistically lower doses to organs at risk and shorter treatment time than C-arm linac.