Malignant mesothelioma associated with low pulmonary tissue asbestos burdens: a light and scanning electron microscopic analysis of 18 cases.

Most malignant mesothelioma cases are associated with pulmonary asbestos body (AB) counts significantly greater than those of the general population. However, the question remains whether malignant mesothelioma cases associated with "normal" AB counts (i.e., indistinguishable from the general population) represent background incidence levels or are, actually, asbestos related. We performed AB counts (by light microscopy) and mineral fiber analysis (by scanning electron microscopy) in 18 mesothelioma cases with AB counts within our normal range (0 to 20 AB/G wet lung) and in 19 "control" cases. Our study demonstrated that approximately one-third (6 of 18) of the mesothelioma cases have asbestos fiber burdens greater than 95% of the control levels. These results suggest that these six mesothelioma cases may be asbestos related despite AB counts similar to those of the general population. An asbestos etiology was suggested in three additional cases, but too few amphibole fibers were identified in these cases to be certain of a value above background. The remaining nine cases showed no evidence of an asbestos etiology. Electron microscopic analysis of pulmonary mineral fibers may be required to differentiate asbestos-related mesotheliomas from non-asbestos-related cases when AB counts are within the range of background values.