Can Breast Magnetic Resonance Imaging Assess Tumor Response to Preoperative Accelerated Partial Breast Irradiation?

Abstract

While Magnetic Resonance Imaging (MR) is commonly performed after neoadjuvant systemic therapy for breast cancer to assess tumor response to therapy, little is known about the tumor response on MR after preoperative radiation. We sought to evaluate the MR changes in patients with Stage IA luminal breast cancer after preoperative accelerated partial breast irradiation (APBI) treated in the prone position and compare the MR changes with tumor diameter on pathology. We hypothesize that following preoperative ABPI there will be a detectable change on MR in maximum contrast enhancement (MCE), time to contrast enhancement (CE) or CE tumor volume that will correspond to reduction in tumor size. Women ≥50 years of age with Stage IA luminal breast cancer enrolled on an IRB approved trial were studied. Using a Q-fix™ therapy positioning device prototype, a CT Simulation in the prone position and then an MR were acquired and fused to contour GTV. Preoperative APBI was 3850 cGy in 10 fractions BID with a minimum of 6-hours between fractions. Another CE-MR was obtained 4 weeks after APBI prior to breast conserving surgery (BCS) on a 3T MR. Gadolinium MR contrast agent was injected at standard dose and rate after completing the first series; 5 series were acquired at 90 second intervals after contrast injection. A visual and quantitative assessment using region of interest was performed to assess the intensity, kinetics and volume of the lesion CE. We used a slice selective segmentation approach to determine the 3D volume and the longest axis diameter. The MCE compared to baseline and the time to MCE was determined. An experienced reader performed a visual classification to assess the MR volume-based changes and kinetic enhancement changes. To estimate the change in invasive tumor size occurring under APBI, we compared the longest axis size on the baseline CE-MR to longest diameter on pathology. Thirteen patients had both MRs and pathology available. Median age was 66 years old (range 51-78). Median days between pre-therapy MR and ABPI was 13 (range 8-19). Median days from ABPI to second MR and BCS was 33 (range 21-40) and 40 (range 34-52), respectively. Eight (62%) patients had a reduction of MCE after APBI, 7 of which had a decrease in pathological size. Ten (77%) patients had an increase of time to MCE. The average reduction of the contrast enhanced tumor volume was 22%. The tumor diameter shrinkage from pre-treatment CE-MR to histology was on average 49%. In 4 patients we found a longest diameter reduction of less than <25%, while in 3 patients 25-50%, and >50% in 6 patients. The treatment effect of preoperative ABPI can be detected on CE-MR. Most irradiated tumors showed either a reduction in maximum contrast enhancement and/or longer time to maximum contrast enhancement. This study demonstrates that preoperative radiation induces changes on MR reflecting treatment effect on tumor vasculature and/or tumor necrosis. Future analysis will include a detailed pathological review.