Original StudyUse of the Ciliation Index to Distinguish Invasive Melanoma From Associated Conventional Melanocytic NeviLang, Ursula E. MD, PhD*,†; Love, Nicholas R. PhD*; Cheung, Christine BS*; McCalmont, Timothy H. MD†; Kim, Jinah MD, PhD*,‡Author Information *Section of Dermatopathology, Department of Pathology, Stanford University Medical Center, Stanford, CA; †Division of Dermatopathology, Department of Dermatology and Pathology, University of California at San Francisco, San Francisco, CA; and ‡Department of Dermatology, Stanford University Medical Center, Stanford, CA. Correspondence: Ursula E. Lang, MD, PhD, Division of Dermatopathology, Department of Pathology, University of California San Francisco, 1701 Divisadero St. Suite 280, San Francisco, CA 94115 (e-mail: [email protected]). This work has been generously supported by a Dermatology Foundation Research Grant. The authors declare no conflicts of interest. The American Journal of Dermatopathology: January 2020 - Volume 42 - Issue 1 - p 11-15 doi: 10.1097/DAD.0000000000001459 Buy Metrics Abstract Our understanding of melanoma precursors and progression to melanoma has developed as a result of advances in the field of molecular diagnostics. We now better understand the potential for genetic heterogeneity within a single lesion. Combined tumors can pose a diagnostic challenge when deciding the line between benign and malignant, which in turn has direct implications for patient management. Primary cilia (PC) are ubiquitous sensory organelles that have essential functions in cellular proliferation, differentiation, and development. The ciliation index (percentage of ciliated melanocytes) has been shown to reliably differentiate melanoma, which fail to ciliate, from melanocytic nevi, which retain PC. We therefore analyzed the potential for using the ciliation index to differentiate benign and malignant components in combined melanocytic lesions. We collected patient samples (n = 10) of unequivocal combined lesions with both melanoma and associated nevus components. Melanocytes were highlighted with SOX10 and costained with gamma-Tubulin and acetylated alpha-Tubulin to highlight the basal body and cilium, respectively. The number of melanocytes retaining cilia under high-power microscopy was examined. The melanoma component had average of 4% ciliation (SD: 7%), whereas the associated nevus component was significantly higher with 59% ciliation (SD: 17%). These data show that PC may be a reliable means of distinguishing benign from malignant components within a single tumor. The ciliation index may be a helpful tool in distinguishing challenging cases of combined lesions of melanoma in situ with a dermal nevus component from invasive melanoma, thus promoting improved staging and clinical management. Copyright © 2019 Wolters Kluwer Health, Inc. All rights reserved.