Original articleDetection of mitotic figures in thin melanomas—Immunohistochemistry does not replace the careful search for mitotic figures in hematoxylin-eosin stain
Introduction
Tumor thickness is the most powerful independent prognostic criterion in primary melanoma. In thin melanoma, the mitotic rate was considered one major prognostic indicator by the American Joint Committee on Cancer (AJCC) guidelines.1, 2, 3, 4 The presence of ≥1 mitosis in the dermal tumor part results in a shift to pT1b.4, 5 Several studies have found that patients with melanoma less than 1 mm with additional risk factors, such as mitoses, might benefit from sentinel lymph node biopsy.6, 7, 8, 9 The evaluation of mitotic figures is still a matter of debate. In reviewing pathology sections, it may be difficult to unequivocally identify a mitotic figure in a melanocyte versus another cell in the dermis. Moreover, high interobserver variability has been reported.10, 11, 12 The antibody phosphohistone H3 (PHH3, ser10) has been identified as a reliable and sensitive marker to detect mitotic figures in every stage.13, 14, 15 To differentiate mitoses in melanocytes from mitoses in other cells with certainty, double staining for Melan A and PHH3 are helpful.11 The aim of our study was to evaluate the reproducibility of the mitotic count in hematoxylin-eosin (H&E)-stained slides and to determine the benefit of immunohistochemical staining with PHH3 in the subgroup of pT1 melanomas. Moreover, we looked into whether the extension of the “hot spot” to the epidermal tumor part has an influence on the number of stage-relevant mitoses.
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Patients and methods
All anonymized clinical and histologic data were stored in a database. All examinations were performed in a blinded fashion. Our study was approved by the local ethics commission.
Patient and tumor characteristics: relation to the mitotic rate
We found a significant correlation between the number of dermal mitoses and Breslow thickness (Mann-Whitney U test; P < .001) and between the mitotic count and Clark level (Jonckheere-Terpstra test; P < .001). We found an equal distribution of mitoses between gender (χ2; P = .09) and age (Mann-Whitney U test; P = .620). The details of the patients and tumor characteristics are given in Table I.
Mitotic figures in H&E—reproducibility in re-evaluation
In routine histology (H&E) of the first cohort (150 pT1a and pT1b melanomas), dermal mitoses were
Discussion
Since the implementation of the mitotic rate in the AJCC classification for thin melanoma as a criterion with a high prognostic impact,2, 4 the correct evaluation and reproducibility of the detection of mitoses has remained a matter of debate.11, 12, 17, 18 In contrast to others,11, 12 we found a high interobserver reproducibility of the mitotic rate with H&E (routine diagnostics vs re-evaluation) with respect to the tumor stage. When comparing the mitotic rate gained by re-evaluation with that
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Funding sources: None.
Parts of this work were presented as a poster at the meeting of the Dermatohistopathologic Working Group German Society of Dermatology, 2014.
Conflicts of interest: None declared.