Section thickness and mitotic counts in ovarian mucinous carcinoma. Methodological study with scanning confocal microscopy.

To study the effect of section thickness on mitotic counts, paraffin sections of 28 cases of mucinous ovarian carcinomas were analyzed. Mitotic counts, estimated with the number of mitoses per mm2 of neoplastic epithelium (M/Vv index) and per mm3 from optical sections and through the whole section thickness were done. Section thickness was measured with scanning confocal microscopy from each specimen of 3 series of sections with different nominal thicknesses (5, 8, and 10 microns). Section thickness varied considerably within each series of sections. The sections of the 5-microns group tended to be thicker and the sections of the 10-microns group thinner than expected on the basis of their nominal values. As expected, mitotic counts through the section gave higher M/Vv index values than counts done using optical sections. M/Vv values obtained using optical sections increased with increasing nominal section thickness. The study suggests that, when the section thickness cannot be measured, it is advisable to do mitosis counting by using optical sections in lightly stained tissue sections in the thickness range of 8-15 microns.