Intrafraction Variability and Deformation Quantification for Accelerated Partial-Breast Irradiation (APBI)

Abstract

While IGRT reduces setup error for linac-based APBI, IGRT cannot fully account for deformations arising from intrafraction motion. This study sought to evaluate intra-fraction variability and local deformations of the lumpectomy cavity (LC), breast, and nearby organs at risk. Sixteen early-stage (T0-T2N0M0) left-sided post-lumpectomy breast cancer cases and 1 bilateral case with discernible LCs were retrospectively evaluated on an IRB-approved study. During simulation, each patient underwent a free-breathing CT and 10-phase 4DCT. For each patient, treated breast, heart, lungs, and LC were delineated on the end-inhale (EI) 4DCT phase. B-spline multi-resolution deformable image registration was employed to propagate contours to all phases. For the entire population, respiration-induced motion was calculated via centroid analysis. To quantify deformation, displacement vector fields (DVFs) were analyzed between EI and end-exhale (EE) datasets for the breast and LC volumes by classifying the vector magnitudes as < 2 mm, 2-4 mm, and > 4 mm. For the patient population, LC intra-fraction motion was 0.7 ± 0.7 mm, 1.1 ± 0.7 mm, and 1.8 ± 1.2 mm in lateral (LAT), anterior-posterior (A-P), and superior-inferior (S-I) directions, respectively. Similar intra-fraction motion results were obtained for the treated breast: 0.4 ± 0.3 mm, 0.9 ± 0.5 mm, and 1.6 ± 0.9 mm LAT, A-P, and S-I directions, respectively. In the most extreme case, LC and breast excursions of ∼2 mm and ∼3 mm were observed in the A-P and S-I directions, respectively. Respiration-induced motion was dominant S-I for the heart (3.8 ± 1.7 mm) and lungs (3.6 ± 2.0 mm). Over all patients, DVF analysis revealed that 51.2 ± 20.3% (range, 9.6 - 82.6%) of the treated breast volume deformed between 2-4 mm, whereas 11.3 ± 12.7% deformed > 4 mm (range, 0 to 36.7%). The majority of LC volumes (60.7 ± 37.6%) deformed 2-4 mm. However, a small subset demonstrated > 4 mm deformation (6.8 ± 14.6%). The patient with the largest deformation (∼45% of the LC volume deformed > 4 mm) also demonstrated the largest intra-fraction cavity and breast excursions. Overall, LC and breast demonstrated < 3 mm intra-fraction centroid motion, whereas heart and lung were slightly more mobile. Deformation characterization of the LC and breast revealed large variability among patients, suggesting value in patient-specific margins for APBI. A subset of volumes showed substantial deformation (> 4 mm), which, if unaccounted for, could potentially impact dose distribution. Further work is needed to resolve dominant regions of deformation. Coupling these results with inter-fraction setup analysis will likely yield better margin design, and potentially support the need for image-guided adaptive radiation therapy for APBI.