Paraneoplastic dermatomyositis associated with metastatic leiomyosarcoma of unknown primary.

BACKGROUND: Sarcomas are rare and heterogeneous tumours of mesenchymal origin, with over 100 histological subtypes. Paraneoplastic dermatomyositis has rarely been described in sarcoma. This is the first documented case of paraneoplastic dermatomyositis in a patient with metastatic leiomyosarcoma.

CASE PRESENTATION: A 43-year-old female diagnosed with metastatic leiomyosarcoma of unknown primary presented with a mild rash in sun-exposed areas of her face and upper chest, with no other neuromuscular symptoms. This rash resolved with systemic treatment with doxorubicin for metastatic leiomyosarcoma. Imaging assessment confirmed overall stable disease after chemotherapy completion. She presented acutely 2 months later with new onset rash in a shawl-like distribution, periorbital oedema and proximal muscle weakness. Based on the characteristic cutaneous signs and symmetrical proximal muscle weakness, abnormal electromyography and raised skeletal muscle enzymes with a positive anti-transcription intermediary factor-1 gamma antibody result, a diagnosis of paraneoplastic dermatomyositis was made. Re-evaluation of her metastatic leiomyosarcoma revealed disease progression. Second-line chemotherapy was commenced once the dermatomyositis was controlled on steroid therapy. Systemic anti-cancer therapy was again associated with mild improvement in dermatomyositis symptoms.

DISCUSSION: Paraneoplastic dermatomyositis heralded disease progression after first-line chemotherapy; however, in hindsight, subtle cutaneous features were present at sarcoma diagnosis. The temporal relationship between paraneoplastic dermatomyositis and metastatic leiomyosarcoma is key in this case, as fluctuations in dermatomyositis severity correlated with growth of metastatic disease. Understanding this relationship may provide clues for tumour progression and prompt timely initiation of anti-cancer therapy. It is important to recognise that in addition to the more common cancers associated with paraneoplastic dermatomyositis, it can also occur in rarer tumours such as leiomyosarcoma.