Unmasking the Myositis: A Diagnostic Clue to Occult Breast Cancer.

Dermatomyositis (DM) is an idiopathic inflammatory myopathy characterised by proximal muscle weakness and pathognomonic cutaneous features. A well-recognised but underappreciated association exists between DM and underlying malignancy, termed cancer-associated myositis (CAM). The prevalence of malignancy in DM patients is high, with breast, ovarian, lung, and gastrointestinal cancers most commonly implicated. However, the detection of occult malignancy in young women presenting with DM remains particularly challenging due to atypical epidemiology and the limitations of standard imaging in dense breast tissue. This report presents the case of a woman in her third decade who was initially diagnosed with idiopathic DM based on heliotrope rash, Gottron papules, proximal limb weakness, elevated creatine phosphokinase (CPK), and serological positivity for anti-Mi-2 antibody. Despite the initiation of corticosteroids and methotrexate, the patient showed clinical deterioration over four weeks. Discovery of a firm left axillary lymph node on physical examination prompted further workup. Axillary node biopsy confirmed metastatic estrogen receptor-positive(ER+)/progesterone receptor-positive (PR+)/human epidermal growth factor receptor 2 (HER2)-neu+ breast adenocarcinoma. Conventional mammography and breast ultrasonography had been negative, while breast MRI identified a 1.8 mm lesion in the lower inner quadrant. Fluorodeoxyglucose (FDG)-PET/CT confirmed locoregional disease without distant metastasis. Following neoadjuvant chemotherapy, surgery, and radiation, the patient achieved complete remission of her myositis, with normalisation of creatine phosphokinase (CPK) levels, resolution of rash, and recovery of muscle strength, confirming the paraneoplastic aetiology. This case underscores the imperative to pursue malignancy screening in DM patients unresponsive to immunosuppressive therapy, regardless of age. It highlights the critical role of thorough physical examination and the superiority of breast imaging in young women with dense breasts.