Colonic carcinoma metastasis to the thyroid is rare. Here the authors present the case of an 81-year-old lady who presented with metastatic colonic adenocarcinoma in her thyroid gland. This case is unique as it is the first to demonstrate metastasis from the colon to the thyroid with no other site involvement. The use of cytokeratin immunohistochemical staining is reviewed along with the current perspectives on the concept of skip metastasis.

The early and clinically occult spread of viable tumour cells throughout the body is increasingly considered as a hallmark of cancer progression, because recent data suggest that these cells are precursors of subsequent distant relapse. Using monoclonal antibodies to epithelial cytokeratins or tumour-associated cell-membrane glycoproteins, individual carcinoma cells can be detected in cytological bone marrow preparations at frequencies of 10-5 to 10-6. Prospective clinical studies have shown that the presence of these immunostained micrometastatic cells in bone marrow, as a frequent site of overt metastases, is prognostically relevant with regard to relapse-free period and overall survival. This screening approach might therefore be used to improve tumour staging and to guide stratification of patients for adjuvant therapy in clinical trials. Another promising clinical application is the use of these micrometastatic cells to monitor response to adjuvant therapies, which at present can be assessed only retrospectively after an extended period of clinical follow-up. This review summarises current data on the clinical significance of occult metastatic breast cancer cells in bone marrow.