Tumour-to-tumour metastasis: a rare cause of apparent intratumoural heterogeneity in conventional clear cell renal cell carcinoma

Tumour-to-tumour metastasis is a rare phenomenon and few case reports exist that describe tumor-to-tumor metastases to and from clear cell renal cell carcinoma.1 In order for metastases to become established, tumour cells must be shed, survive in circulation, and implant, grow, and establish vascularity in a distant site - which in our case was within another primary tumour.2 Tumours are, by definition, environments that promote growth of neoplastic cells. Clear cell renal cell carcinoma, in particular, provides a unique pro-tumour environment, in part due to its molecular characteristics, affording metastatic tumours the opportunity to survive and grow.3 We describe a case of metastatic breast carcinoma that was found within a conventional clear cell type renal cell carcinoma. Further, this case illustrates the potential for sampling errors with percutaneous biopsies of renal masses and highlights the need for pathologists to consider the rare possibility of tumour-to-tumour metastasis when confronted with tumours showing striking morphologic heterogeneity.