The pathologist and the residual tumor (R) classification.

The R classification, adopted in 1987 by the UICC, denotes absence or presence of residual tumor after treatment. Residual tumor may be localized in the area of the primary tumor and/or as distant metastases. R0 corresponds to resection for cure or complete remission. R1 to microscopic residual tumor, R2 to macroscopic residual tumor. The R classification takes into account clinical and pathological findings. A reliable classification requires the pathological examination of resection margins. The R classification has considerable clinical significance, particularly being a strong predictor of prognosis. General and specific procedures for performing pathological R classification on resection specimens of different organs will be described. New methods in R classification comprise imprint cytology, cytolocial examination of ascites, examination of bone marrow biopsy. The importance of these methods will have to be established in the future.