Abstract: Mucinous tubular and spindle cell carcinoma (MTSCC) is a rare sub-type of renal cell carcinoma (RCC). It has been considered to be a kind of “indolent” tumor with lowgrade fashion, weak invasive capacity and relatively favorable prognosis. However, in the current case, a 3.7 cm×2.8 cm spherical mass with contrast enhancement was found in the left kidney incidentally by computed tomography (CT) in a 60-year-old male patient. A lesion in the right humerus (2.1 cm×1.6 cm×3.1 cm) was found at the same time without any symptoms or sign of pathological fracture by magnetic resonance (MR) imaging. Further positron emission tomography (PET)/CT scan which was ordered immediately after admission suggested multiple bone destruction including skull, pelvis, sternum, right humerus and femur, left scapula, multiple vertebrae and libs. Pathological examination after radical nephrectomy and palliative resection with internal fixation of the lesion in the right humerus indicated that both renal (3.0 cm×3.0 cm×2.5 cm) and bone lesions were MTSCC with the features of high-grade ovoid epithelioid cells, cord-like spindle cells and mucinous matrix under light microscope. The diagnosis of renal MTSCC concurrent with multiple bone metastasis was made. This case report suggested the necessity of general evaluation, especially bone scan for possible distant metastasis, as MTSCC might present unexpected advanced behaviors without any orthopedic symptoms. The behavior of bone metastasis might be associated with male and elderly age. MTSCC has similar enhancement features to papillary RCC on CT scan. As results, attentions are needed to differentiate MTSCC from papillary RCC as they both tend to show lesser enhancement degrees than cortex. Rather than exhibiting a dedifferentiating appearance, the pathological characteristics of bone metastasis lesion were close to those of primary renal lesion. The reason of distant metastasis to the bone remained unclear, negative expression of cytokeratin (CK) 7 might be attributed to. Though immunotherapy, chemotherapy and target therapy could all be methods for systematic therapies, procedures to remove renal lesions and prevent skeletal related events are still highly recommended.

Key words: Kidney neoplasms, Mucinous tubular and spindle cell carcinoma, Neoplasm metastasis, Bone neoplasms