Analysis of false multiple primary cancers: Hiroshima Tumor Tissue Registry's experience. Hiroshima Tumor Tissue Registry, Acting Committee.

During the period 1978-84 a total of 34,422 cases with malignant tumors were accessed by the Hiroshima Tumor Tissue Registry. Of these, 3,136 (9.1%) were classified as having multiple primary cancers (MPC). A detailed review was conducted on 1,148 cases selected from the latter group, and 330 (29%) were found to have been falsely registered as having MPC. "False" MPC cases were classified into six major categories according to the type of error: Category I--invasions or metastases occurring at the same or adjacent site within three months of the primary tumor, misclassified as separate primaries; Category II--invasions or metastases occurring at the same or adjacent site more than three months of the primary tumor, misclassified as separate primaries; Category III--metastases to distant sites, misclassified as separate primaries; Category IV--tumors lacking a clear description of location, resulting in registrations as "false" MPC; Category V--tumors with revisions or corrections in their pathological diagnoses resulting in false MPC; Category VI--tumors with errors in coding and other operational procedures resulting in false MPC. The cases in Categories I, II and III together comprised 52% of all false MPC cases (18%, 23% and 11%, respectively). In many of these cases, it was difficult to reject the possibility of one tumor being a metastasis of the other. This suggests that specific criteria may help reduce the number of false MPC cases of these types. Cases in Categories IV, V and VI represented 24%, 14% and 9%, respectively, of all false MPC cases. Errors encountered in these cases may be prevented by the concentrated efforts of clinicians and hospital pathologists as well as by an improvement in the operation of the registry.