Breast cancer radiotherapy in patients with autoimmune or cancer-associated myositis: balancing oncological benefit and immune-mediated risk

BACKGROUND: Breast cancer radiotherapy and autoimmune myositis intersect in a clinically complex setting in which paraneoplastic disease, pre-existing inflammatory myopathy, and radiation-related muscle injury may overlap. Dermatomyositis is the idiopathic inflammatory myopathy most strongly associated with malignancy, and breast cancer is one of the solid tumours most frequently reported in cancer-associated myositis, particularly in anti-TIF1γ- and anti-NXP2-positive subsets. METHODS: A structured narrative review of PubMed and Scopus was performed from database inception to March 2026. Evidence was synthesised qualitatively owing to limited and heterogeneous literature. RESULTS: Muscle symptoms during breast cancer treatment may reflect autoimmune, paraneoplastic, radiation-induced, fibrosis-related, or recall myositis. MRI is the most informative modality for pattern-based assessment, with FDG PET-CT and ultrasound providing complementary value in selected cases. Although direct disease-specific evidence is sparse and derives largely from retrospective series, mixed connective tissue disease cohorts, radiobiological studies, and case reports, radiotherapy does not appear to represent an automatic contraindication when disease activity, multidisciplinary evaluation, and conformal planning are taken into account. CONCLUSIONS: Breast cancer radiotherapy in patients with autoimmune or cancer-associated myositis requires structured differential diagnosis rather than categorical avoidance. Diagnostic assessment should integrate timing, field distribution, imaging, serology, systemic features, and treatment sequence. Further studies should refine risk stratification, muscle-specific dose constraints, and biomarkers distinguishing radiation-related from autoimmune or paraneoplastic myositis.