Abstract
Tumor-to-tumor metastasis is the presence of a metastatic deposit within a second malignant primary, and its identification could be challenging, particularly when the latter malignancy has not yet been discovered. Renal cell carcinoma is one of the known recipients. Here we present a 52 years old woman who presented with a small nodule in the left kidney, which had a biphasic cell proliferation, chromophobe type, and signet-ring cell type for which we suggested to investigate the presence of a second tumor of gastrointestinal origin. We present this case for the rarity of its presentation, for the peculiar histological symbiosis we found between the two tumor entities, and for the challenging diagnosis due to the presence of an occult aggressive primary tumor.