Background: Focal myositis is a benign inflammatory pseudotumour of skeletal muscle, that usually presents in the extremities. We present what we believe is the first published case of a patient with a focal myositis of the pectoral region. Methods: A previously fit and well 34 year old Caucasian gentleman presented with a 3-month history of a progressively enlarging painful mass in the right pectoral region. He denied any trauma, prior masses, muscle dysfunction, symptoms of connective tissue disease, fever, rashes, acne or pustulosis. Clinical examination revealed a palpable, tender soft tissue mass measuring 4 cm x 5 cm, antero-inferior to the right sternoclavicular joint. There was no evidence of synovitis and the overlying skin was normal in appearance. The patient displayed normal muscle power and had no cervical or axillary lymphadenopathy. Investigations revealed ESR 43 mm/h (normal range < 20 mm/h), CRP 30mg/l (normal range < 10 mg/l), CK 85 U/l (normal range 24–195 U/l). Blood chemistry, bone profile, antinuclear antibodies, ENA, dsDNA, RF, immunoelectrophoresis, hepatitis serology, CMV and EBV were all negative. Chest X-ray was normal. A T2 weighted MRI of the thorax identified an area of oedema within the upper right pectoral muscle. The sternoclavicular and manubrio-sternal joints were not affected. A muscle biopsy demonstrated skeletal muscle with an eosinophilic inflammatory infiltrate, consistent with a diagnosis of focal myositis. Malignancy and TB were excluded. Results: The patient commenced prednisolone 20 mg daily. Upon review, the mass had clinically resolved and the patient was asymptomatic. The dose of steroid was then tapered over 4 months. Methotrexate 12.5 mg once weekly was initiated due to the reoccurrence of symptoms on lower doses of prednisolone. Conclusions: Focal myositis was first identified as a distinct clinicopathologic entity by Heffner et al. in 19771. In the 16 reported cases by the authors, myositis occurred in the limbs. Cases have subsequently been reported in the abdomen, head and neck. Focal myositis has no age or sex predilection and most patients deny any weakness or systemic symptoms. Clinically it can mimic the features of a soft tissue sarcoma. It may be mistaken for early manifestations of polymyositis or connective tissue disease. The aetiology is unknown. Trauma has rarely been identified as a precipitating factor. A viral mechanism has been suggested but no organism has been identified. Focal myositis has an excellent prognosis, with a high rate of spontaneous regression without recurrence. It can be treated effectively with glucocorticoids due to the inflammatory nature of the condition. We report to the best of our knowledge the first case described in the literature of a focal myositis in the pectoral muscle. Although focal myositis is a rare condition, it should be considered as a differential diagnosis for a mass presenting in any skeletal muscle group. Disclosure statement: All authors have declared no conflicts of interest.

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