Mitotic Figure Counts Are Significantly Overestimated in Resection Specimens of Invasive Breast Carcinomas

Hans-Anton Lehr; Candice Rochat; Cornelia Schaper; Antoine Nobile; Sherien Shanouda; Sandrine Vijgen; Arnaud Gauthier; Ellen Obermann; Susana Leuba; Marcus Schmidt; Curzio Ruegg C; Jean-Francois Delaloye; Nectaria Simiantonaki; Stephan C Schaefer

Disclosures

Mod Pathol. 2013;26(3):336-342. 

In This Article

Abstract and Introduction

Abstract

Several authors have demonstrated an increased number of mitotic figures in breast cancer resection specimen when compared with biopsy material. This has been ascribed to a sampling artifact where biopsies are (i) either too small to allow formal mitotic figure counting or (ii) not necessarily taken form the proliferating tumor periphery. Herein, we propose a different explanation for this phenomenon. Biopsy and resection material of 52 invasive ductal carcinomas was studied. We counted mitotic figures in 10 representative high power fields and quantified MIB-1 immunohistochemistry by visual estimation, counting and image analysis. We found that mitotic figures were elevated by more than three-fold on average in resection specimen over biopsy material from the same tumors (20±6 vs 6±2 mitoses per 10 high power fields, P=0.008), and that this resulted in a relative diminution of post-metaphase figures (anaphase/telophase), which made up 7% of all mitotic figures in biopsies but only 3% in resection specimen (P<0.005). At the same time, the percentages of MIB-1 immunostained tumor cells among total tumor cells were comparable in biopsy and resection material, irrespective of the mode of MIB-1 quantification. Finally, we found no association between the size of the biopsy material and the relative increase of mitotic figures in resection specimen. We propose that the increase in mitotic figures in resection specimen and the significant shift towards metaphase figures is not due to a sampling artifact, but reflects ongoing cell cycle activity in the resected tumor tissue due to fixation delay. The dwindling energy supply will eventually arrest tumor cells in metaphase, where they are readily identified by the diagnostic pathologist. Taken together, we suggest that the rapidly fixed biopsy material better represents true tumor biology and should be privileged as predictive marker of putative response to cytotoxic chemotherapy.

Introduction

For some patients to benefit from aggressive chemotherapy for invasive breast carcinoma, many patients are currently being treated with little or no benefit. Enormous effort is hence being directed towards the identification of those patients who will benefit from chemotherapy and those who will not. As cytotoxic chemotherapy targets proliferating cells,[1] pathologists focus on the proliferative activity of tumors, as assessed by mitotic figure counts or by the immunohistochemical detection of the cell-cycle-specific antigen Ki-67.[2] In fact, the mitotic figure count is a key element of the time-honored grading of invasive breast carcinomas and has been routinely assessed in breast carcinomas for more than five decades.[3,4] Since the advent of core-needle biopsies to diagnose breast tumors, many authors have compared the histological findings obtained in biopsy material with the findings obtained in resection specimen. As far as the tumor grade is concerned, these studies have reported a tendency to upgrade carcinomas in resection specimen when compared with the initial diagnosis made on the biopsy material, and several authors have noted that the upgrade in resection specimen is due solely or to a large extent to an increase in mitotic figure counts.[5–11] Explanations for this phenomenon have been sought in intratumoral heterogeneity and in sampling artifacts, suggesting that the biopsy material is either (i) too small for proper mitotic figure counting or (ii) taken at random from anywhere within the tumor and not necessarily from the more proliferative tumor periphery.[7–11] It has consequently been suggested that discrepancies in tumor grades could be reduced by increasing the number and the size of the biopsy cores,[9,10] but no data have ever been published in support of that idea. As a consequence of this concept, several authors have suggested that in case of grade discrepancies between biopsy and resection specimen, therapeutic decisions should be based on the grade obtained in the resection specimen.[7,12]

Herein, we propose a different explanation for the discrepancy in mitotic figure counts between biopsy and resection material. We assessed the proliferative activity of 52 invasive ductal carcinomas and confirm that the number of mitotic figures significantly increased in resection specimen by a factor of more than 3 over the counts in biopsy material, whereas at the same time, the pan-cell-cycle-specific marker MIB-1 (targeting Ki-67) yielded comparable results for biopsies and resection specimen. We also found that the proportion of post-metaphase figures (anaphase and telophase) among the counted mitotic figures in resection specimen is less than half of those found in biopsies. On the basis of these observations, we propose here that the delayed formalin fixation of resection specimen allows cell cycle activities to continue in the resected tissue for up to many hours, resulting in a preferential arrest of mitoses in metaphase, where they are readily identified by the pathologist. As a consequence of this reasoning, we propose that the mitotic figure count in the rapidly fixed biopsy cores better represents the tumor biology and should be used as a basis for therapeutic decisions.

processing....