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

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 magnetic resonance–guided radiation therapy with reduced PTV margins enabled by real-time cine magnetic resonance image (MRI) target monitoring results in better normal tissue sparing compared with computed tomography (CT)-guided radiation therapy with commonly used clinical PTV margins. In this study, we compare the plan quality of ViewRay MRIdian Linac forward planned intensity modulated radiation therapy and TrueBeam volumetric modulated arc therapy for a novel 3-fraction APBI schedule. Methods and Materials Targets and organs at risk (OARs) were segmented for 10 patients with breast cancer according to NSABP B39/RTOG 0413 protocol. A 3 mm margin was used to generate MR PTV 3mm and CT PTV 3mm plans, and a 10 mm margin was used for CT PTV 10mm . An APBI schedule delivering 24.6 Gy to the clinical target volume and 23.4 Gy to the PTV in 3 fractions was used. OAR dose constraints were scaled down from existing 5-fraction APBI protocols. Target and OAR dose-volume metrics for the following data sets were analyzed using Wilcoxon matched-pairs signed-rank test: (1) MR PTV 3mm versus CT PTV 3mm plans and (2) MR PTV 3mm versus CT PTV 10mm . Results Average PTVs were 84.3 ± 51.9 cm 3 and 82.6 ± 55 cm 3 ( P = .5) for MR PTV 3mm and CT PTV 3mm plans, respectively. PTV V23.4Gy, dose homogeneity index, conformity index (CI), and R50 were similar. There was no meaningful difference in OAR metrics, despite MR PTV 3mm being larger than the CT PTV 3mm in 70% of the patients. Average PTVs for MR PTV 3mm and CT PTV 10mm plans were 84.3 ± 51.9 cm 3 and 131.7 ± 74.4 cm 3 , respectively ( P = .002). PTV V23.4Gy was 99% ± 0.9% versus 97.6% ± 1.4% ( P = .03) for MR PTV 3mm and CT PTV 10mm , respectively. Dose homogeneity index, CI, and R50 were similar. MR PTV 3mm plans had better ipsilateral breast (V12.3Gy, 34.8% ± 12.7% vs 44.4% ± 10.9%, P = .002) and chest wall sparing (V24Gy, 8.5 ± 5.5 cm 3 vs 21.8 ± 14.9 cm 3 , P = .004). Conclusions MR- and CT-based planning systems produced comparable plans when a 3 mm PTV margin was used for both plans. As expected, MR PTV 3mm plans produced better ipsilateral breast and chest wall sparing compared with CT PTV 10mm . The clinical relevance of these differences in dosimetric parameters is not known.