1. ¹Department of Pathology, Massachusetts General Hospital, Boston, MA, USA
2. ²Department of Surgery, Massachusetts General Hospital, Boston, MA, USA
3. ³Department of Surgery, Harvard Medical School, Boston, MA, USA
4. ⁴Department of Pathology, Harvard Medical School, Boston, MA, USA
5. ⁵Department of Surgery, Keck School of Medicine, University of Southern California, Los Angeles, CA, USA

Correspondence: Dr JS Michaelson, Division of Surgical Oncology, Cox Building Room 626, Massachusetts General Hospital, 100 Blossom Street, Boston, MA 02114, USA. E-mail: michaelj@helix.mgh.harvard.edu

Revised 15 June 2005; Accepted 7 September 2005; Published online 8 November 2005.

Abstract

There has been much uncertainty as to whether metastasis requires mutation at the time of spread. Here, we use clinical data to calculate the probability of the spread of melanoma and breast cancer cells. These calculations reveal that the probability of the spread of cancer cells is relatively high for small tumours (1 event of spread for every 500 cells for melanomas of 0.1 mm) and declines as tumours increase in size (1 event of spread for every 10⁸ cells for melanomas of 12 mm). The probability of spread of breast cancer cells from the lymph nodes to the periphery is 1 event of spread for every 10⁸ cells in the nodal masses, which have a mean diameter of 5 mm, while the probability of spread of cancer cells from the breast to the periphery when the primary masses are 5 mm is also 1 event of spread for every 10⁸ cells. Thus, the occurrence of an event of spread from the breast to the lymph nodes appears not to increase the propensity of the progeny of those cells to spread from the lymph nodes to the periphery. These values indicate that the spread of human breast cancer and melanoma cells is unlikely to occur by a mechanism requiring mutation at the time of spread.