Small (<4 cm), Unclassified Renal Cell Carcinoma Presenting with Initial Bone Metastasis: A Case of a Metastatic Lesion Missed at the Initial Diagnosis.

A 49-year-old man presented with an incidentally detected right renal mass on a health examination. The abdominal computed tomography and magnetic resonance imaging showed a 3-cm right renal mass suspected of being a hypovascular tumor, such as papillary renal cell carcinoma, and an osteoblastic metastatic lesion on the right iliac bone. However, we missed a bone lesion at the time of diagnosis. A laparoscopic radical nephrectomy was performed and the final pathology confirmed unclassified renal cell carcinoma. The follow-up imaging studies showed several neck lymph nodes and multiple bone metastases at the lumbar spine, right iliac bone, and left femur. Thirteen cycles of temsirolimus were administered to the patient, but follow-up positron emission tomography showed newly developed liver and left adrenal metastasis and increased bone metastasis. It is important to note that T1a renal cell carcinoma can present with distant metastasis and thus demands scrupulous examination even though the tumor size may be small.