S245
Clinical - Breast
ESTRO 2026
Digital Poster Highlight 1557 Breast cancer: Exploring the impact of radiotherapy-dose in muscles on treatment- induced arm morbidity Lotte N Veldt 1 , Mariam Hammad 1 , Anna Dinkla 1 , Omar Bohoudi 1 , Desiree HJG van den Bongard 1,2 1 Radiotherapy Oncology, AmsterdamUMC, Amsterdam, Netherlands. 2 Cancer Center Amsterdam, AmsterdamUMC, Amsterdam, Netherlands Purpose/Objective: Breast cancer is the most common malignancy among women. Postoperative radiotherapy improves locoregional control and survival, but may lead to long-term side effects in surrounding tissues, including arm morbidity, such as pain, stiffness, reduced mobility and muscle weakness. It is increasingly recognized that arm morbidity might be associated with radiation dose in muscles, though evidence remains limited. This study explores the association between radiation dose to specific shoulder and thoracic muscles and radiotherapy- induced arm morbidity, using an AI-based model for automated muscle segmentation. Material/Methods: Data from 79 female breast cancer patients enrolled in the BREAST-ART trial (NCT05727553) was analyzed. Radiotherapy target volumes included whole breast, partial breast, and/or axillary lymph nodes. Arm
Results: Accuracy of automated segmentation was lowest for the serratus anterior and several arm muscles (deltoid, triceps brachii, teres major, coracobrachialis), which can be explained by the model being trained on patients positioned with their arms down. The corrected segmentations were subsequently used to enhance AI model performance for patients treated with both arms abducted.Pain in extremity was significantly associated with higher radiation doses to the pectoralis minor, deltoid, and trapezius.Detailed results are presented in Table 1. No significant associations were found for reduced arm mobility, chest wall pain, or arm oedema. Patients receiving locoregional radiotherapy demonstrated overall higher muscle doses.
morbidity was assessed using standardized questionnaires at baseline, one month, and
three months post-radiotherapy. Outcomes included pain in extremity, arm oedema, reduced arm mobility, and chest wall pain.Eleven muscles were evaluated: pectoralis major/minor, serratus anterior, deltoid, infraspinatus, supraspinatus, subscapularis, teres major, triceps brachii, coracobrachialis, and trapezius. Muscles were automatically segmented on planning CTs by an AI model and manually corrected (Figure 1). Corrected segmentations were used to retrain the model and improve performance. Dosimetric parameters (mean dose, maximum dose, V5–V25) were extracted for each muscle. Associations between muscle dose and arm morbidity were analyzed using Mann–Whitney U-tests, logistic regression, ROC analysis, and Spearman’s correlation. A p-value of <0.05 was considered
statistically significant.
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