Context.— Hypertrophic lichen planus (HLP) is a variant of lichen planus that can be difficult to diagnose based on histopathologic features alone. Thus, patient clinical history and clinicopathologic correlation are essential considerations to make the correct diagnosis. Objective.— To discuss the clinical and histologic presentation of HLP and provide a thorough review of commonly encountered mimickers in the differential diagnosis. Data Sources.— Data were derived from a literature review, personal clinical and research experiences, and a review of cases in the archives of a tertiary care referral center. Conclusions.— In general, HLP involves the lower extremities and is characterized by thickened, scaly nodules and plaques that are often pruritic and chronic in nature. HLP affects both males and females and is most common in adults 50 to 75 years of age. Unlike conventional lichen planus, HLP tends to have eosinophils and classically displays a lymphocytic infiltrate most concentrated around the tips of rete ridges. The differential diagnosis for HLP is broad and encompasses numerous entities in many different categories, including premalignant and malignant neoplasms, reactive squamoproliferative tumors, benign epidermal neoplasms, connective tissue disease, autoimmune bullous disease, infection, and drug-related reactions. Therefore, a high index of suspicion must be maintained to avoid a misdiagnosis and potential inappropriate treatments.

Dermatofibrosarcoma protuberans (DFSP) is a cutaneous sarcoma with a high propensity for local invasion and recurrence. Although it is a rare event, the occurrence of multiple tumors in a single patient raises a diagnostic dilemma, as metastatic disease should be differentiated from multiple primary malignant events. In more than 90% of DFSP, a pathogenic t(17;22) translocation leads to the expression of COL1A1::PDGFB fusion transcripts. Karyotype analysis, fluorescence in situ hybridization, and RT-PCR can be useful ancillary studies in detecting this characteristic rearrangement, and sequencing of the fusion transcript can be used to support a clonal origin in metastatic and multifocal disease. However, previous reports have demonstrated variable sensitivity of these assays, in part due to the high sequence variability of the COL1A1::PDGFB fusion. Here, we report a patient who developed two distinct DFSP tumors over the course of 7 years. Chromosomal microarray analysis identified distinctive genomic alterations in the two tumors, supporting the occurrence of multiple primary malignant events.

Pleomorphic dermal sarcoma (PDS) is a rare cutaneous/subcutaneous neoplasm of purported mesenchymal differentiation that exists along a clinicopathologic spectrum with atypical fibroxanthoma (AFX). While PDS and AFX share histopathologic and immunohistochemical features, PDS exhibits deeper tissue invasion and has a higher rate of metastasis and local recurrence than AFX. Given its aggressive clinical course, early recognition and clinical management of PDS are essential for optimizing patient outcomes. This review aims to provide a brief overview of the clinicopathologic and molecular features, prognosis, and treatment of PDS.

Context.— Myelodysplasia cutis is an emerging concept in cutaneous neoplasia. Many of these cases were previously included under the umbrella of histiocytoid Sweet syndrome. However, with the advent of next-generation sequencing, cutaneous involvement by myelodysplastic syndrome is being increasingly recognized. Objective.— To review histiocytoid Sweet syndrome and myelodysplasia cutis and discuss our current understanding of these entities. Additionally, to discuss how next-generation sequencing can be applied in the evaluation of cutaneous infiltrates of immature histiocytoid cells. Data Sources.— The English-language literature from 2005 to 2023 on the topic of histiocytoid Sweet syndrome and myelodysplasia cutis was reviewed. Conclusions.— Biopsy specimens showing infiltrates of histiocytoid, immature myeloid cells may represent cutaneous involvement by myelodysplastic syndrome. Close clinical correlation is recommended in these cases. Recent studies suggest that next-generation sequencing is useful in separating myelodysplasia cutis from true histiocytoid Sweet syndrome. This distinction has important implications for patients.

Pityriasis rosea is an acute, self-limited exanthem that typically occurs in adolescence and young adulthood, classically featuring ovoid erythematous and scaly lesions on the trunk and proximal extremities. While its cause is not definitively known, the classic form of pityriasis rosea may result from the reactivation of latent human herpesvirus (HHV) infections (HHV-6 and HHV-7). Interestingly, drug eruptions that clinically and/or histopathologically resemble pityriasis rosea have also been reported. These pityriasis rosea-like drug eruptions tend to occur at an older age and have a shorter duration than the classic type. As there are different management paradigms, the distinction between classic pityriasis rosea and the mimicking drug eruption is important to recognize. Herein, we report a case of a pityriasis rosea-like drug eruption that occurred in association with imatinib mesylate treatment for chronic myeloid leukemia. We also review the clinicopathologic features of reported cases of pityriasis rosea-like drug eruption, including those due to imatinib. While the clinical morphology of the cutaneous drug-related eruption mimics the lesions seen in classic pityriasis rosea, the presence of unique histopathologic findings, including necrotic keratinocytes, interface dermatitis, and eosinophils, may aid in distinction.

The conjunctiva is a thin mucous membrane covering the anterior portion of the eye. Similar to the skin, the conjunctiva has normal resident melanocytes along the basal layer of the epithelium and these cells can give rise to melanocytic neoplasms of the conjunctiva. A variety of pigmented tumors can involve the conjunctiva, ranging from benign to malignant. Given the sensitive anatomic location and potential for some conjunctival melanocytic lesions to progress to melanoma, timely clinical recognition and correct histologic diagnosis of these tumors are essential in order to initiate the appropriate treatment and optimize outcomes. Clinicopathologic correlation is a vital step in this process. In this review article, the authors aim to provide an up-to-date concise, yet detailed, summary of the clinical, histopathologic, and molecular findings as well as a brief mention of management options for each type of conjunctival melanocytic lesion that may be encountered by pathologists and dermatopathologists.

Mesenchymal tumours with melanocytic expression can pose a diagnostic challenge because they frequently demonstrate both morphological and immunohistochemical overlap with other cutaneous melanocytic neoplasms. Therefore, they present potential pathological pitfalls that may lead to a misdiagnosis of malignant melanoma. Mesenchymal neoplasms that closely mimic melanoma include malignant melanotic nerve sheath tumour (melanotic schwannoma), epithelioid schwannoma, malignant peripheral nerve sheath, cutaneous syncytial myoepithelioma, clear cell sarcoma of soft tissue, and perivascular epithelioid cell tumour. Awareness of these melanoma mimics is necessary for establishing the correct diagnosis so that the appropriate clinical management can be rendered to the patient. This in-depth review highlights key diagnostic features and molecular genetics and also discusses the differential diagnosis and treatment of mesenchymal tumours that exhibit melanocytic expression.

Malakoplakia is a rare chronic inflammatory condition that most commonly involves the urogenital tract. Cutaneous malakoplakia is extremely rare and many patients diagnosed with skin involvement are immunosuppressed. While the clinical presentation of cutaneous malakoplakia is variable, the histopathologic features are quite distinct and include sheets of closely packed dermal histiocytes with foamy-appearing cytoplasm and Michaelis-Gutmann bodies that are positive with certain immunohistochemical stains. While the exact pathogenesis of malakoplakia is unknown, it has been associated with certain bacterial infections. Treatment generally involves a combination of surgery and antimicrobial agents and/or modulation of immunosuppressant therapy if appropriate. Herein, the authors report a unique case of cutaneous malakoplakia arising in a patient on chronic immunosuppressive therapy for the management of pyoderma gangrenosum.

Cutaneous vascular tumors constitute a heterogeneous group of entities that share overlapping morphologic and immunohistochemical features, which can be diagnostically challenging for pathologists and dermatopathologists. Our understanding and knowledge of vascular neoplasms have improved over time, resulting in both a refinement of their classification by the International Society for the Study of Vascular Anomalies (ISSVA) and an improvement in the accurate diagnosis and clinical management of vascular neoplasms. This review article aims to summarize the updated clinical, histopathological, and immunohistochemical characteristics of cutaneous vascular tumors, as well as to highlight their associated genetic mutations. Such entities include infantile hemangioma, congenital hemangioma, tufted angioma, spindle cell hemangioma, epithelioid hemangioma, pyogenic granuloma, Kaposiform hemangioendothelioma, retiform hemangioendothelioma, pseudomyogenic hemangioendothelioma, Kaposi sarcoma, angiosarcoma, and epithelioid hemangioendothelioma.