Relationship of standardized mitotic indices to other prognostic factors in breast cancer

To examine the relationship between mitotic index (MI), calculated from direct microscopic counts, and other prognostic features in breast cancer.

Mitotic index was based on direct microscopic observations of mitotic figures in 10 consecutive microscopic fields, and the average cell number was determined by counts of population density in three of those fields. Tumor grade and type were established from tissue sections, whereas metastases were detected in lymph node biopsy, chest roentgenograms, and bone scan. Estrogen receptor (ER) and progesterone receptor (PgR) levels were determined by flow cytometry.

The MI for 242 patients ranged from 0.2 to 37.6, with a mean of 5.8 mitoses per 1000 cells. More than 85% of the tumors with an MI below 1.0 were diploid and contained an S-phase fraction of 6.7% or less. In contrast, more than 75% of tumors with an MI above 5.0 were aneuploid with more than 6.7% of cells in S-phase. There was an inverse relationship between ER and PgR status and MI. Eighty percent of tumors with an MI less than 1.0 were both ER and PgR positive while only 25% of those with an MI above 10.0 were both ER and PgR positive. Receptor-positive tumors with high S-phase and MI values had ER and PgR levels below 100 fmol/mg.

Lower MI values calculated from direct cell counts are correlated with negative node status, diploid DNA content, low S-phase fraction, and positive receptor status. Thus, there is a significant relationship between objective MI values and several other factors that predict the probability of breast tumor recurrence.