EXTRAORDINARY CASE REPORT Locally Invasive Dermal Squamomelanocytic Tumor With Matrical Differentiation: A Peculiar Case With Review of the Literature Nemanja Rodi  c, MD, PhD,* Janis M. Taube, MD,*† Paul Manson, MD,‡ Manisha Patel, MD,† James W. Patterson, MD,§ and Gulsun Erdag, MD† Abstract: Dermal-based combined squamous and melanocytic neo- plasms are emerging clinicopathologic entities that tend to appear on sun-exposed areas of elderly patients. The biologic behavior of such cutaneous neoplasms remains uncertain because of their rarity. Histopathologic differential includes the following diagnostic entities: (1) dermal squamomelanocytic tumor, (2) melanocytic matricoma, and (3) rare histologic variant of pilomatrical carcinoma, the so-called pilomatrical carcinoma with intralesional melanocytes. Herein, we present a novel case of locally invasive dermal squamomelanocytic tumor. A 72-year-old man presented with a pigmented papule on nasal ala that was clinically concerning for basal cell carcinoma. Histopath- ologic evaluation demonstrated atypical melanocytic cells architectur- ally and intimately intermixed with single units and clusters of atypical squamous cells. Most notable feature of this case is focal matrical differentiation and locally invasive tumor growth, characterized by multifocal perineural invasion. Key Words: dermal squamomelanocytic tumor, melanocytic matri- coma, pilomatrical carcinoma with intralesional melanocytes, perineural invasion (Am J Dermatopathol 2013;35:e72–e76) INTRODUCTION Certain inherited tumor syndromes, such as xeroderma pigmentosum, show a propensity for the formation of both squamous cell carcinomas and malignant melanomas. However, the simultaneous neoplastic transformation of intermixed squamous cell carcinoma and malignant melanoma is distinctly rare, despite the fact that solar irradiation is a main etiological factor in the tumorigenesis of both neoplasms. Whereas the occasional melanocytic colonization of squamoproliferative and basal cell carcinomas has been reported, 1 dermal squamomelanocytic tumors are a rare diagnostic entity, which was first described by Pool et al 2 in 1999. Only 11 cases of dermal squamomelanocytic tumors have been documented in the English literature (Table 1). 2–9 Dermal squamomelano- cytic tumors demonstrate 2 phenotypically distinct but inter- mingled populations of malignant keratinocytes and melanocytes. As a result of the rarity of these tumors, there is no consensus on their classification and biologic behavior. Another equally rare diagnostic entity is melanocytic matricoma, with only 14 cases reported to date. 10–22 Histolog- ically, this neoplasm is comprised of pigmented dendritic melanocytes and admixed epithelial cells. Except in 3 cases, most melanocytic matricomas demonstrate no connection to the overlying epidermis and grow in a predominantly endo- phytic fashion as well-circumscribed nodules composed of supramatrical cells and intermixed pigmented dendritic mel- anocytes. 10 The lack of granulomatous reaction and the abrupt transition between supramatrical cells and small round aggre- gates of rare shadow cells with concentric keratin formation around a lucent central portion are often seen in matrical carcinomas. Finally, the third example of combined squamous and melanocytic neoplasms is pilomatrical carcinoma with intra- lesional melanocytes. 23 This neoplasm is an extremely rare histologic variant of a World Health Organization recognized diagnostic entity, with only 5 cases reported to date. 15,23–25 Similar to the 2 above-mentioned entities, these neoplasms consist of basiloid cells with prominent nucleoli and inter- mixed pigmented melanocytes but more often exhibit multi- nodular architecture with areas of necrosis. Herein, we report the case of a locally aggressive dermal squamomelanocytic tumor, which was characterized by focal matrical differentiation and multifocal perineural invasion of the squamous component. RESULTS A 72-year-old man was seen at the Department of Dermatology Outpatients Center at Johns Hopkins Hospital as he recently discovered an enlarging skin lesion on his right nasal ala. The patient reported significant sun exposure over his lifetime with a history of boating and fishing and also reported not using sunscreen on a regular basis. The patient denied any family history of skin cancer. Of note, the patient had been seen previously in our dermatology clinic. Eight years ago, he was initially seen for extensive actinic keratoses From the Departments of *Pathology; †Dermatology; and ‡Plastic Surgery, Johns Hopkins Hospital, Baltimore, MD; and §Department of Pathology, Division of Dermatopathology, University of Virginia Health System, Charlottesville, VA. Supported by Intradepartmental study funds from the Johns Hopkins Hospital, Departments of Pathology (to N.R.) and Dermatology (to J.M.T. and G.E.). The authors declare no conflict of interest. Reprints: Nemanja Rodic, MD, PhD, Department of Pathology, Johns Hopkins University School of Medicine, Carnegie 401, Baltimore, MD 21205 (e-mail: nrodic1@jhmi.edu). © 2013 Lippincott Williams & Wilkins e72 | www.amjdermatopathology.com Am J Dermatopathol  Volume 35, Number 4, June 2013