Dermatomyositis as a paraneoplastic sign of breast cancer: a case report and literature review.

INTRODUCTION: Dermatomyositis (DM) is a rare autoimmune inflammatory disorder characterized by skin and muscle involvement and is well known for its association with malignancy. Clinically amyopathic dermatomyositis (ADM) is a distinct subtype that presents with characteristic cutaneous manifestations in the absence of clinically significant muscle weakness, which may delay diagnosis. ADM can occur as a paraneoplastic syndrome and warrants thorough evaluation for underlying malignancy. CASE PRESENTATION: A 39-year-old Iranian premenopausal woman presented with a one-year history of a neglected right breast mass and progressive pruritic erythematous skin lesions involving the trunk and upper extremities. Physical examination revealed no muscle weakness. Imaging demonstrated a large right breast mass with bilateral axillary lymphadenopathy. Core needle biopsy confirmed grade 3 invasive ductal carcinoma (ER-positive, PR-positive, HER2-negative). Laboratory tests showed mild elevated inflammatory markers and muscle enzymes. Skin biopsy findings were consistent with dermatomyositis. Corticosteroid therapy was ineffective. A diagnosis of paraneoplastic clinically amyopathic dermatomyositis was established. Following initiation of neoadjuvant chemotherapy with doxorubicin and cyclophosphamide, cutaneous lesions resolved completely after the first treatment cycle. The patient subsequently underwent surgery and adjuvant radiotherapy, with no recurrence during follow-up. DISCUSSION: DM may represent a paraneoplastic manifestation of breast cancer and poses diagnostic challenges, particularly in the absence of muscle involvement. Steroid resistance and rapid resolution following chemotherapy support a tumor-driven immune mechanism. CONCLUSION: Early recognition of paraneoplastic ADM and prompt cancer-directed therapy are essential for optimal outcomes.