A Case of Renal Abscess Mimicking Metastatic Lesion in a Patient with Lung Carcinosarcoma

Renal abscesses require prompt diagnosis and appropriate intervention, as they can be life-threatening. However, diagnosis based solely on clinical findings is often challenging. We present the case of a 69-year-old woman with left renal masses on follow-up computed tomography (CT) after surgery for pT2aN0M0 lung carcinosarcoma. The masses were localized only in the left kidney without suspected metastatic lesions at other sites. The patient was referred to our department for further evaluation and treatment under a diagnosis of suspected metastatic lung carcinosarcoma of the left kidney. On enhanced CT, the left renal masses, the largest of which had a diameter of 40×36 mm had thick irregular walls gradually enhanced by the contrast media and an internal low-attenuation area. The masses showed heterogeneous signal intensity with a pseudocapsule on T2-weighted magnetic resonance imaging. Clinical symptoms such as fever or costovertebral angle tenderness were absent, and blood and urine tests were not sufficiently inflammatory to suggest a renal abscess. Histopathological findings on CT-guided renal biopsy revealed only inflammatory tissue and no tumor cells. However, because lung carcinosarcoma metastatic nodules could not be ruled out, laparoscopic left nephrectomy was performed for a definitive diagnosis and curative intent. The pathological diagnosis was renal abscess without malignant lesions. Here, we present a case of renal abscess mimicking metastatic lesions in a patient with lung carcinosarcoma. Accurately differentiating renal abscesses from metastatic renal tumors before treatment is often difficult. Renal abscess diagnosis should be considered through a comprehensive evaluation of the clinical findings of individual cases.