Improved Ipsilateral Breast and Chest Wall Sparing with MR-guided 3-fraction Accelerated Partial Breast Irradiation: A dosimetric study comparing MR-Linac and CT-Linac plans

Abstract: Purpose External beam Accelerated Partial Breast Irradiation (APBI) is subject to treatment uncertainties that must be accounted for through planning target volume (PTV) margin. We hypothesize that MRgRT with reduced PTV margins enabled by real-time cine MRI target monitoring, results in better normal tissue sparing when compared to CT-guided radiotherapy with commonly used clinical PTV margins. In this study, we compare the plan quality of ViewRay MRIdian LinacTM forward planned IMRT with TrueBeam VMAT for a novel 3-fraction APBI schedule. Methods Targets and organs-at-risk (OARs) were segmented for 10 breast cancer patients according to NSABP B39/RTOG 0413 protocol. A 3mm margin was used to generate MR PTV3mm and CT PTV3mm plans, and a 10mm margin for CT PTV10mm. An APBI schedule delivering 24.6Gy to the CTV and 23.4Gy to the PTV in three fractions was used. OAR dose constraints were scaled down from existing 5-fraction APBI protocols. Target and OAR dose-volume metrics for the following datasets were analyzed using Wilcoxon matched-pairs signed rank test: 1) MR PTV3mm vs. CT PTV3mm plans and 2) MR PTV 3mm vs. CT PTV 10mm. Results Average PTV volumes were 84.3±51.9cc and 82.6±55cc (p=0.5), for MR PTV3mm and CT PTV3mm plans, respectively. PTV V23.4Gy, dose homogeneity index (DHI), conformity index (CI) and R50 were similar. There was no meaningful difference in OAR metrics, despite MR PTV3mm being larger than the CT PTV3mm in 70% of the patients. Average PTV volumes for MR PTV3mm and CT PTV10mm plans were 84.3±51.9cc and 131.7±74.4cc, respectively (p=0.002). PTV V23.4Gy was 99% ±0.9 vs. 97.6%±1.4 (p=0.03), for MRPTV3mm and CT PTV10mm; respectively. DHI, CI and R50 were similar. MR PTV3mm plans had better ipsilateral breast (V12.3Gy, 34.8%±12.7 vs. 44.4%±10.9, p=0.002) and chest wall sparing (V24Gy, 8.5cc±5.5 vs. 21.8cc±14.9, p=0.004). Conclusion MR and CT based planning systems produced comparable plans when a 3mm PTV margin was used for both plans. As expected, MR PTV3mm plans produced ipsilateral breast, and chest wall sparing when compared to CT PTV10mm plans due to larger PTV volume in the CT PTV10mm plans. Clinical relevance of these differences in dosimetric parameters is not known.