Collision tumor of spindle cell lipoma arising from perirenal tissue and acquired cystic disease‐associated renal cell carcinoma: A rare case report

INTRODUCTION: Collision tumors are a rare phenomenon defined as two or more histologically distinct tumors that are in contact with each other. CASE PRESENTATION: The patient was a man with a history of end‐stage diabetic nephropathy under hemodialysis treatment for 15 years. A plain computed tomography scan showed a 4.3 cm mass with obscured margins in the right perirenal fat of the lower pole kidney. On T(2)‐weighted magnetic resonance imaging, the lesion showed heterogeneous signal intensity with a partially cystic component. A radical nephrectomy was performed. Histopathological and immunohistochemical examination revealed collision tumors constituted of a spindle cell lipoma covering the kidney surface underneath the perirenal fat and diffusely distributed acquired cystic disease‐associated renal cell carcinoma in the renal parenchyma. CONCLUSION: We report the first case of collision tumors comprising spindle cell lipoma and acquired cystic disease‐associated renal cell carcinoma.