Metastatic Malignancy Masquerading as Skin Rash.

Dermatomyositis (DM) is a rare autoimmune inflammatory myopathy characterized by specific skin findings and symmetric proximal skeletal muscle weakness. Many of the affected population have an underlying malignancy, which could naturally alter the condition's prognosis. This is a case of DM driven by adenocarcinoma of the lung, and the challenges this rare phenomenon led to, with regard to management. A 72-year-old man presented to the hospital with itchy, maculopapular, blanching lesions across the face, neck, upper chest, arms and hands. The condition was associated with peripheral edema and inflammatory erythema, particularly affecting the upper limbs. The patient also had rapid deterioration in limb muscle weakness. Initial investigations revealed significantly raised muscle injury and inflammatory markers. MRI of the arms demonstrated marked oedema involving the subcutaneous soft tissues as well as the muscles of both upper arms, indicative of DM/myositis. The patient was reviewed by the dermatology team, who gave a clinical diagnosis of DM. He received conservative management of the skin symptoms from dermatology, high-dose intravenous methylprednisolone, intravenous immunoglobulins from rheumatology and palliative management of painful rash and limbs. A thorax, abdomen and pelvis CT was also performed because of the association of DM with malignancy and unfortunately revealed a lung lesion, later confirmed to be an adenocarcinoma of lung origin on biopsy. In DM merely associated with cancer, the DM symptoms often precede the cancer diagnosis. In contrast, paraneoplastic DM symptoms generally develop after the cancer is already present, which is what led us to believe this was paraneoplastic in nature.  The patient was reviewed by the respiratory and oncology teams for management of his cancer. This proved to be especially challenging given that his pathology results suggested a response to immunotherapy would be high and durable in nature. However, given the severity of his DM and the level of immunosuppression required, immunotherapy could not be given first line because it would have significantly risked worsening his DM. He was then considered for chemotherapy after long and challenging multidisciplinary team discussions. However, before treatment could be initiated, the patient unfortunately passed away. Lung cancer causing DM is a rare phenomenon, which makes it difficult to decide how best to treat the patient. Based on his pathology results, this patient's particular cancer would have shown a good response to immunotherapy. However, this was not possible given the severity of his DM. Prognosis calculation in this situation also proved difficult.