2660 - Post-Lumpectomy Radiation Therapy and Patient-Reported Shoulder Function: A Dosimetric Analyses of the Shoulder Skeletal Muscles

R. A. Braudy¹, L. Koehler², P. Ludewig², S. Edlund³, E. Ehler³, D. C. Mathew^3,4, S. R. Alcorn⁵, and J. Yuan³; ¹Twin Cities Orthopedics, Edina, MN, ²Department of Family Medicine and Community Health, University of Minnesota Medical School, Minneapolis, MN, ³Department of Radiation Oncology, University of Minnesota Medical School, Minneapolis, MN, ⁴Minneapolis Radiation Oncology PA, Edina, MN, ⁵University of Minnesota: Department of Radiation Oncology, Minneapolis, MN

Purpose/Objective(s): Chronic shoulder and upper extremity dysfunction are common in breast cancer survivors following surgery and adjuvant radiation therapy (RT) and can significantly affect quality of life. We hypothesize that radiation dose to key skeletal muscles located within the radiation field may impact patient-reported shoulder function in breast cancer survivors at least 1 year following the completion of post-lumpectomy RT. Materials/

Methods: A total of 24 women in a single institution participated in this study. All received whole breast RT +/- lumpectomy cavity boost. Key skeletal muscles were retrospectively contoured on each individual’s simulation computed tomography scan. Dose volume histograms were constructed to determine mean dose and V10-V40 for each muscle, with prescription dose converted to dose equivalent of 2 Gy (EQD2), assuming an alpha/beta ratio of 2.5 Gy. The Penn Shoulder Score (PSS) was calculated to capture patient-reported shoulder function. Spearman correlation analyses were used to examine associations between dose parameters and PSS. One-way ANOVAs were utilized to determine differences in radiation dose received in skeletal muscles and PSS by cancer quadrant, boost treatment, and boost location.
Results: The pectoralis minor (PMin) had significantly greater mean radiation dose and V10-V40 values than both the pectoralis major (PMaj) and the serratus anterior (SA) (p < 0.001), and the PMaj had a significantly greater mean radiation dose and V10 – V40 values than the SA (p < 0.001). Those who received boosts in upper quadrants had significantly greater mean radiation dose to the PMin than when boosts were in the central or lower quadrants (p < 0.001). Those who had a boost in the outer breast quadrants had higher V40 PMin values than those who had a boost in the inner breast quadrants (p=0.011). Patients with central breast tumors reported lower PSS function scores than those with upper or lower quadrant tumors (p = 0.045), but inner/outer breast quadrant location did not significantly affect PSS scores (p > 0.05). No statistically significant correlations were found between radiation dose parameters (total radiation dose or V10-V40 values) and PSS values.
Conclusion: The PMaj, PMin, and SA all receive a high radiation dose during post lumpectomy adjuvant RT, with the PMin demonstrating the highest mean dose and the highest percent of muscle exposed to at least 10 Gy – 40 Gy of radiation. Boost location also affected PMin mean dose. No significant correlations were found between radiation dose parameters and PSS in this small study cohort. Further investigation is warranted to quantify the radiation dose to key shoulder muscles in order to identify breast cancer survivors at a high risk of developing shoulder dysfunction.