Mitotic counts in one high power field in breast core biopsies is equivalent to counts in 10 high power fields

Mitotic rate is an important prognostic predictor in invasive breast carcinoma. Current guidelines recommend counting mitoses from 10 contiguous high power fields (HPFs) in the core biopsy. We propose a method to score mitotic activity in 1 HPF at the most mitotically active area of the tumour edge, or the interface between invasive tumour and benign breast tissue. We propose a score of 1, 2, or 3, corresponding to ≤1, 2, or ≥3 mitoses in 1 HPF, respectively. A total of 141 breast core biopsies with corresponding surgical excisions were blindly examined. We counted the number of mitotic figures in 1 HPF and in 10 contiguous HPFs in the core biopsy and compared with the mitotic count from 10 contiguous HPFs in the excision which is considered the gold standard. Concordance rates and interobserver agreement rates were calculated. The concordance rate was 82.3%, 78.7% and 82.3% between 1 HPF versus 10 HPFs in the core biopsy, 1 HPF in the core biopsy versus 10 HPFs in the excision and 10 HPFs in the core biopsy vs 10 HPFs in the excision, respectively. In the core biopsy, all three investigators agreed in 73.8% and 83.7% of the cases using the 1 HPF method and the 10 HPFs method, respectively; in the excision specimen, agreement was reached in 82.3% of the cases. The 1 HPF method showed similar concordance rate and interobserver agreement compared to the conventional method in the prediction of the mitotic score in the excision in all score groups. When stratified by mitotic score, the 1 HPF method predicted superior correlation with excision in the score 1 group than the 10 HPFs method, but not in the score 2 or 3 groups. From these findings we conclude that the proposed 1 HPF method can be used in clinical practice to grade invasive breast carcinomas in core biopsies, with the possibility of being utilised in small biopsies with less than 10 HPFs of invasive carcinoma.