In the Business of Counting Mitoses

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In the Business of Counting Mitoses

This, the first of three summaries covering new strategies for staging melanoma, examines mitotic rate.

Breslow thickness has proved to be the most accurate predictor of biologic behavior in cutaneous malignant melanoma. With the worldwide increase in the incidence of this tumor, specialists continue to look for better predictive factors. These investigators examined the prognostic value of mitotic rate (MR; defined as the number of mitotic figures per square millimeter of tumor tissue). A related measure, the mitotic index, reflects the ratio of cells in mitosis to cells not in mitosis.

A single dermatopathologist reviewed the cases of 650 consecutive patients with invasive melanoma enrolled in the Connecticut tumor registry over 28 months. Seventeen clinical and histopathologic variables were evaluated: histologic type, Clark level, microscopic satellites, regression, Breslow thickness, tumor ulceration, vascular invasion, MR, mitotic index, tumor-infiltrating lymphocytes, vertical growth phase, age, sex, body site, solar elastosis, coexisting nevus, and lymphoid response. In the multivariate-regression analysis, tumor thickness, moderate mitotic index (range, 1–6), and mitotic index >6 were independent prognostic factors. Tumor ulceration became an independent prognostic factor only when MR was omitted from the analysis (relative risks: in the model with tumor ulceration but no MR, 2.4; in the model with MR but no tumor ulceration, 14.5).

Comment: The identification of prognostic factors to guide the choice of suitable treatment strategies is a goal in the clinical management of various tumor types. For melanoma, two histologic findings have been used to define stage -- Breslow thickness and tumor ulceration. If additional studies confirm the value of mitotic rate as an independent prognostic factor, it might be considered in the next revision of melanoma staging.

— Angelica Selim, MD

Published in Journal Watch Dermatology July 12, 2005