Association between the rates of synchronous and metachronous metastases: analysis of SEER data

Patients with cancer are usually staged based on the presence of detectable regional and/or distant disease. However, staging is inexact and cM0 patients may have microscopic metastases (cM0pM1) that later cause relapse and death. Since the clinical tools used to stage patients are fairly similar for different tumors, the ratio of the rates of metachronous to synchronous metastases should be similar for different tumors (hypothesis #1). Improvements in diagnostic tools should have caused the ratio of metachronous-to-synchronous metastases to have decreased over time (hypothesis #2). Finally, the fraction of patients with either metachronous or synchronous metastases should have declined over time due to increased screening and earlier diagnoses (hypothesis #3). To test these hypotheses, Surveillance, Epidemiology, and End Results (SEER) data from 1973-1998 were analyzed for 19 solid tumors. A linear relationship was seen between the rates of metachronous and synchronous metastases, with modestly strong correlation coefficients, consistent with hypothesis #1. Over time, changes in staging methods have not significantly altered the ratio of metachronous/synchronous metastases, contrary to hypothesis #2. Also over time, a decrease in the number of patients with metastases was found, consistent with hypothesis #3. Therefore, the rate of anticipated metachronous metastases can be estimated from the rate of clinically evident metastases at presentation. Changes in screening/staging of disease over time may have reduced the overall fraction of patients with metastases.