Mucin-Poor Mucinous Tubular and Spindle Cell Carcinoma of the Kidney Presented with Multiple Metastases Two Years after Nephrectomy: An Atypical Behaviour of a Rare, Indolent Tumour

Background Mucinous tubular and spindle cell carcinoma (MTSCC) is a rare type of renal cell carcinoma, whose clinical behaviour and metastatic potential have not been fully elucidated to date. There are only a few metastatic cases in the literature, which all either featured sarcomatoid differentiation or were synchronously metastasised at diagnosis. Case Presentation We report a case of a 49-year-old male with end-stage kidney disease on dialysis, presenting with multiple osseous metastases of a mucin-poor variant of MTSCC of the kidney, without sarcomatoid differentiation, two years after bilateral nephrectomy for papillary renal cell carcinoma (RCC) at a curable stage. After retrospectively reexamining the initial nephrectomy specimens, the tumour of the right kidney was also diagnosed as a mucin-poor variant of MTSCC, while the tumour of the left kidney was confirmed as a papillary RCC. Conclusions It is proposed that MTSCC can be associated with end-stage renal disease and that particularly the mucin-poor variant is easily confused with papillary renal cell carcinoma, as happened in this case. Although it is considered as a relatively indolent malign entity, it can metastasise even years after successful primary surgical treatment. This implies, besides accurate diagnosis, that MTSCC patients should be monitored closely in the follow-up period.