2979 - Quantifying Changes in Breast Volume Following Ultrahypofractionated Whole vs. Partial Breast RT

03:00pm - 04:00pm PT

Hall F

Screen: 22

POSTER

Presenter(s)

Elizabeth Obi, MD - Cleveland Clinic Foundation, Cleveland, OH

E. E. Obi¹, C. A. Reddy¹, R. D. Tendulkar¹, C. S. Shah², and S. Cherian¹; ¹Department of Radiation Oncology, Cleveland Clinic Foundation, Cleveland, OH, ²Allegheny Health Network Cancer Institute, Department of Radiation Oncology, Pittsburgh, PA

Purpose/Objective(s): Radiation therapy (RT) is utilized following breast conserving surgery to decrease the risk of local recurrence in patients (pts) diagnosed with early stage breast cancer. Five-fraction (fx) regimens to the whole breast (WBRT) and to the partial breast (APBI) have shown to have similar efficacy as more conventional WBRT regimens delivered over 3-5 weeks. However, there is concern that 5 fx WBRT results in increased breast shrinkage compared to conventional WBRT. The purpose of this investigation is to assess change in breast volume following WBRT (26Gy/5 fx) vs. APBI (30Gy/5 fx).

Materials/Methods: All pts with breast cancer treated with partial mastectomy followed by either WBRT (26Gy/5 fx, tangents) or APBI (30Gy/5 fx, IMRT or VMAT) at a single tertiary care institution from 2021-2022 were identified. The pre-surgical (PS), 6 month (mo) and 18 mo mammograms (MAM) following RT were analyzed. We utilized a validated equation, V= (1/3)pR_ccR_mloh_mlo described by Rostas et al. to estimate breast volume via MAM measurements of both the craniocaudal and mediolateral oblique views. The calculated breast volumes from each time point were compared for change in volume. Pts were excluded if they did not have adequate follow up, unavailable MAM, previous breast RT, breast reduction surgery, reconstruction, or additional breast operations. Statistical analysis of unpaired t-test and repeated measures analysis of variance for volume over time were utilized.

Results: A total of 207 potential pts with stage 0-II were identified (136 APBI, 71 WBRT) and 72 (49 APBI, 23 WBRT) met eligibility for analysis. Median age was 68 years old (WBRT: 66 years old, range: 45-85 years, APBI: 69 years old, range 49-84 years). Eighty-six percent of all pts were White (WBRT: n=22; APBI: n=40) and 14% were Black (WBRT: n=1; APBI: n=9). Seventy pts had invasive disease (97%) and 2 had DCIS (3%). Ninety-two percent of pts were ER+ (WBRT: n=17; APBI n=49), 85% PR+ (WBRT: n=16; APBI: n=45), and 11% HER2+ (WBRT: n=4; APBI: n=4). There was a significant decrease in breast volume in the 6 mo and 18 mo post-RT MAM for each cohort compared to the PS MAM (WBRT: PS mean= 1288, 6mo mean=992, 18mo mean= 924 p<0.0001; APBI: PS mean= 1270, 6mo mean=1076, 18mo mean= 1035 p<0.0001). However, there was no significant difference in these mean volumes between the two RT regimens at each of these time points (p=0.096). Additionally, there was no significant change in volume between the 6 mo and 18mo MAM for either cohort (p=0.091). On univariate analysis, there was no significant difference in time between PS MAM to RT, RT to 6mo MAM, or 6mo MAM to 18mo MAM between cohorts (p= 0.092, 0.150, 0.524, respectively).

Conclusion: There was no significant difference in breast volume loss between 5 fx WBRT and APBI at 6 mo and 18 mo following RT; however, longer term follow up with larger patient cohorts are needed. This study adds to the body of literature supporting 5 fx WBRT to be effective without increased breast volume loss.