Misdiagnosis of Intramuscular Abscess of Rectus Femoris: An Elusive Facade to a Grave Diagnosis

Introduction: Skeletal muscle abscess, such as that of the Rectus femoris, presenting as a primary finding of metastasis from a malignancy elsewhere is very rare. However, when present, lung malignancies are found to be the most infamous. Case report: A 74-year-old man presented with acute pain in the left groin, fever and a painful solitary swelling in the anterolateral aspect of the left thigh. Ultrasound and magnetic resonance imaging were suggestive of intramuscular abscess with fluid levels. The mass with purulent material was excised. All cultures were negative. Biopsy and further evaluation showed evidence of metastasis with themprimary site of carcinoma as lung. Metastasis to a skeletal muscle presenting as a primary finding in lung carcinoma is reported very rarely in literature. The patient died within 10 days of diagnosis of stage IV primary lung adenocarcinoma. Conclusion: Awareness of the possibility of muscle metastasis presenting as an acute painful soft tissue mass is necessary in cases of spontaneously appearing muscle abscess. This case report also highlights the importance of a definitive histopathological diagnosis in every abscess like collection, as a benign-looking mass could be a sign of a grave underlying disease.