Mitotic counts in breast cancer should be standardized with a uniform sample area

Michael Bonert,Angela J Tate

doi:10.1186/s12938-016-0301-z

Abstract

BackgroundMitotic rate is routinely assessed in breast cancer cases and based on the assessment of 10 high power fields (HPF), a non-standard sample area, as per the College of American Pathologists cancer checklist. The effect of sample area variation has not been assessed.MethodsA computer model making use of the binomial distribution was developed to calculate the misclassification rate in 1,000,000 simulated breast specimens using the extremes of field diameter (FD) and mitotic density cutoffs (3 and 8 mitoses/mm2), and for a sample area of 5 mm2. Mitotic counts were assumed to be a random sampling problem using a mitotic rate distribution derived from an experimental study (range 0–16.4 mitoses/mm2). The cellular density was 2500 cell/mm2.ResultsFor the smallest microscopes (FD = 0.40 mm, area 1.26 mm2) 16% of cases were misclassified, compared to 9% of the largest (FD 0.69 mm, area 3.74 mm2), versus 8% for 5 mm2. An excess of 27% of score 2 cases were misclassified as 1 or 3 for the lower FD.ConclusionMitotic scores based on ten HPFs of a small field area microscope are less reliable measures of the mitotic density than in a bigger field area microscope; therefore, the sample area should be standardized. When mitotic counts are close to the cut-offs the score is less reproducible. These cases could benefit from using larger sample areas. A measure of mitotic density variation due to sampling may assist in the interpretation of the mitotic score.

Highlights

Mitotic rate is routinely assessed in breast cancer cases and based on the assessment of 10 high power fields (HPF), a non-standard sample area, as per the College of American Pathologists cancer checklist
The model assumes mitotic counting is a sampling problem. It assessed the classification and misclassification rates of 1,000,000 simulated breast specimens, using the sample areas for the extremes of the field diameter range (FD = 0.40 mm, 10 HPF = 1.26 mm2, FD = 0.69 mm, 10 HPF = 3.74 mm2) in the College of American Pathologists (CAP) checklist [6], as well as the areas of 5.00 mm2, which would be equivalent to 40 HPF at a FD 0.40 mm, or almost 14HPF for FD 0.69 mm
If one frames the comparison between the true mitotic score and the score generated by the simulated of mitotic count, as an inter-rater reliability problem, Cohen’s kappa is applicable as a measure

Summary

Introduction

Mitotic rate is routinely assessed in breast cancer cases and based on the assessment of 10 high power fields (HPF), a non-standard sample area, as per the College of American Pathologists cancer checklist. Tumour growth rate is a prognostic marker and can be evaluated by its correlate at the cellular level: mitoses. Mitotic counts are performed by a pathologist, counting mitotic figures at a high magnification. As mitotic figures are rare in relation to the number of cells, high power fields (HPF) of view are typically examined. As cellularity is time consuming to quantify, mitoses/area is often used instead of mitoses/cell. Considered as a sampling problem, mitotic counting is, typically biased in a number of ways: (1) many pathologists do not start the count until they have found one mitosis, Bonert and Tate BioMed Eng OnLine (2017) 16:28

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