This study is a comparative analysis of the prevalence, absolute number and aggregation status of bone marrow micrometastases (BMM) between breast (n = 234) and gastric (n = 102) cancer patients based on a standardized number of I X lo6 bone marrow-derived cells per patient. Additionally, expression of the epithelial cell adhesion molecule E-cadherin was analyzed on disseminated tumor cells. A positive BMM status was demonstrated in 88f234 breast and 45/ 102 gastric cancer patients. The presence of CKI 8+ cells positively correlated with parameters of advanced tumor progression in breast, but not in gastric cancer. Interestingly, 25.2% of the nodenegative patients already had micrometastatic cells in the bone marrow at diagnosis. Regarding the absolute number of CK18+ cells and the frequency of CK18+ cell clusters, no significant difference was found between the 2 tumor types. However, clusters consisting of more than I0 CKI 8+ cells (type II clusters) were present exclusively in breast cancer patients. Additionally, co-expression of CK18 and E-cadherin was detectable in 15/21 micrornetastases-positive breast but in only I / 9 gastric cancer patients. While prevalence of micrometastatic cells in bone marrow is discussed as an early indicator for systemic disease, aggregation status and a certain antigen profile might be indicative for site-specific differences in the manifestation pattern of solid metastases.o 1996 Wiley-Liss, Inc.In most carcinoma patients a local curative resection can be achieved. The fate of these patients, however, depends on the capacity of primary tumor cells to disseminate to distant organs in an early stage of cancer. In this situation the tumor disease may already be considered systemic. Identification of tumor cell dissemination on the single-cell level, termed "micrometastases," is a direct approach to defining the disseminative potential of an individual tumor and may be a useful tool for selecting groups of patients at high risk for tumor recurrence. Immunocytochemical approaches to identifying micrometastatic cells in bone marrow have been used in various types of carcinoma (for review see Riethmiiller and Johnson, 1992). Several studies reported a positive correlation between bone marrow micrometastases (BMM) status, tumor progression and survival in breast, gastric and lung cancer (Berger et al., 1988;Dearnaley et al., 1991;Mansi et at., 1991;Schlimok et al., 1991; Die1 et al., 1992;Pantel et al., 1993; Harbeck et al., 1994). A positive BMM status was identified in a multivariate analysis as an independent prognostic factor in colorectal carcinoma (Lindemann et al., 1992). These findings pushed the clinical acceptance of the evaluation of BMM status, which is proposed as an optional subdivision for the revised version of the existing T-, N-and M-categories in 1997 (P. Hermanek, personal communication).The present study compares BMM status in breast (n = 234) and gastric (n = 102) cancer patients, regarding prevalence, absolute number and aggregation status of micrometastatic c...