Porokeratotic Adnexal Ostial Nevus (PAON) is an uncommon disorder characterized by abnormal keratinization affecting eccrine and follicular adnexa. Initially described separately as porokeratotic eccrine ostial and dermal duct nevus and porokeratotic eccrine and hair follicle nevus, the term PAON was introduced in 2009 to encompass these conditions. PAON presents with diverse clinical manifestations, ranging from asymptomatic keratotic papules and plaques to verrucous lesions. Typically appearing at birth, PAON can rarely manifest in adulthood. We report a case of a porokeratosis-spectrum lesion with features overlapping with PAON in a male patient in his 30s presenting with longitudinal splitting of the left thumbnail associated with an erythematous linear lesion extending from the nail matrix, alongside a scaly plaque on the proximal nail fold and dorsal thumb. This case highlights the importance of considering porokeratosis-spectrum disorders, including lesions with PAON-like features, in the differential diagnosis of monodactylous erythronychia and onychodystrophy, particularly due to its potential for subtle cutaneous involvement as was present in this case. This instance represents a rare presentation of a porokeratosis-spectrum lesion involving the nail unit epithelium and contributes to the expanding clinical spectrum of porokeratosis with PAON-like features.

Loncastuximab tesirine (LT) is an antibody-drug conjugate (ADC), a novel class of therapeutics that combines a target-specific monoclonal antibody with a cytotoxic agent via a chemical linker. LT has been approved in the United States as therapy for relapsed or refractory diffuse large B-cell lymphoma (DLBCL). Given the novelty of this drug, cutaneous adverse effects remain poorly characterized. In this case report, we present a 35-year-old male with relapsing and refractory DLBCL who developed a bullous eruption after his third cycle of LT. Physical exam revealed scattered erosions and erythema of the neck and extremities, along with multiple bullae and erosions on the hands and dorsal feet, and vesicles on the shin. Histopathological exam revealed basovacuolar alteration with epidermal dysmaturation and a re-epithelializing blister, features suggestive of toxic erythema of chemotherapy (TEC). Direct immunofluorescence was negative for immune complex deposition. To date, there are only three cases of LT-associated bullous eruptions in the literature, all demonstrating subepidermal blistering with granular IgA deposition along the basement membrane. This case lacks immunoglobulin deposition and instead exhibits TEC-like cytotoxic interface dermatitis. This case expands the clinicopathologic spectrum of LT-associated cutaneous toxicity and highlights a distinct non-immune mediated, TEC-like vesiculobullous presentation.

IgG4-related disease (IgG4-RD) is a systemic fibroinflammatory disorder characterized by dense infiltration of IgG4-positive plasma cells, fibrosis, and elevated serum IgG4 levels. Although various cutaneous manifestations of IgG4-RD have been described, and vasculitic changes have occasionally been reported, granulomatous vasculitis is not considered a typical histopathological feature of cutaneous IgG4-RD. We report a 93-year-old woman with a history of IgG4-RD who presented with long-standing subcutaneous nodules on both knees. Histopathological examination revealed dense lymphoplasmacytic infiltration with marked fibrosis, obliterative phlebitis, and granulomatous vasculitis characterized by vascular destruction with multinucleated giant cells. Immunohistochemistry demonstrated a marked increase in IgG4-positive plasma cells with a high IgG4/IgG-positive plasma cell ratio. Comprehensive clinical, serological, and histopathological evaluation excluded other causes of granulomatous vasculitis, including eosinophilic granulomatosis with polyangiitis, granulomatosis with polyangiitis, and sarcoid vasculitis. The lesions responded favorably to systemic corticosteroid therapy. To our knowledge, granulomatous vasculitis has not been recognized as a typical histopathological feature of cutaneous IgG4-RD. This case suggests that granulomatous vasculitis may rarely occur in cutaneous lesions of IgG4-RD, but additional cases are needed to confirm this association.

CD30-positive large-cell proliferations arising in patients with mycosis fungoides can be diagnostically challenging, particularly when they show variable T-cell receptor expression or gamma/delta (γδ) phenotype. We present two cases of mycosis fungoides (MF) with CD30-positive large cell transformation (LCT)/progression exhibiting γδ T-cell phenotypic evolution. Case 1, a 50-year-old woman, initially presented with hyperpigmented patches and papulonodular lesions showing epidermotropic small-cell MF alongside dermal CD30-positive large cells with a γδ phenotype. Over ten years, she developed recurrent cutaneous and eventually nodal disease with phenotypic shifts between γδ and αβ expression, while molecular studies confirmed a single clonal process across all biopsies. Potentially pathogenic variants in FAS and CD70 were identified. Case 2, a 72-year-old man with longstanding MF, developed CD30-positive LCT initially with an αβ phenotype that subsequently acquired γδ expression and DUSP22 rearrangement. Oncoscan microarray revealed shared copy-number alterations including CDKN2A/B loss across temporally distinct biopsies, and sequencing identified KMT2C and TET1 variants. Both cases illustrate that MF can exhibit dynamic T-cell receptor phenotypic evolution, posing significant diagnostic challenges with lymphomatoid papulosis and primary cutaneous γδ T-cell lymphoma. Clonal identity confirmation through molecular studies is essential for accurate classification.

Leukemia cutis (LC) refers to cutaneous infiltration by leukemic cells, most often by acute myeloid leukemia (AML) in adults and by lymphoblastic leukemia in children. Although uncommon, LC is a clinically significant manifestation of extramedullary disease and may precede, coincide with, or follow bone marrow involvement. Recognition of AML-associated LC is essential, as it can be diagnostically challenging due to histopathologic overlap with inflammatory dermatoses and other cutaneous hematolymphoid neoplasms. Recent cohort-based studies have systematically characterized AML involving the skin, demonstrating frequent immunophenotypic discordance between cutaneous and marrow blasts and recurrent genetic associations, with important diagnostic and prognostic implications for dermatopathologists. This review provides a practical framework for current and historical nomenclature, the diagnosis of cutaneous AML, and key histopathologic patterns and pitfalls.

Storiform collagenoma is a rare benign dermal neoplasm recently shown to harbor concurrent PTEN and PDGFRB mutations. While most are sporadic, a subset occurs in the context of Cowden syndrome. We describe two storiform collagenomas with unusual morphology including infiltrative growth pattern and perivascular myoid differentiation. Confirmation of concurrent PTEN and PDGFRB mutations in both cases allowed for a more confident diagnosis of storiform collagenoma in these cases and their distinction from dermatofibrosarcoma protuberans.

Malignant perineurioma, also referred to as perineuriomatous malignant peripheral nerve sheath tumor (MPNST), is an exceptionally rare neoplasm with limited clinicopathologic characterization, particularly in children. We report a 12-year-old boy with a 3.0-cm superficial right groin lesion. Histopathologic examination showed a spindle cell neoplasm with fascicular-to-swirling architecture in myxoid to collagenous stroma, moderate cytologic atypia and nuclear variability, mitotic activity, focal necrosis, and admixed inflammation. Epithelial membrane antigen, GLUT1, and claudin-1 positivity supported perineurial differentiation, while a broad negative immunohistochemical panel helped exclude potential mimics. Tumor-only sequencing demonstrated 17q loss with focal homozygous NF1 deletion, a BRCA2 frameshift, and low tumor mutational burden (3.4 mutations/Mb); RNA sequencing identified no pathogenic gene fusion. These molecular findings were interpreted as adjunctive rather than independently diagnostic. The tumor extended to the inked specimen edge, and subsequent management and follow-up information was unavailable. To our knowledge, this is the first reported superficial cutaneous malignant perineurioma/perineuriomatous MPNST in a pediatric patient.

New annular, indurated plaques arising during mogamulizumab therapy for cutaneous T-cell lymphoma (CTCL) present a diagnostic challenge, as they may raise concern for treatment-related eruption or disease progression. We report two patients with darker skin types who developed plaques that were morphologically distinct from their baseline presentations during mogamulizumab therapy. These morphologic changes may be subtle, manifesting as increased induration, textural change, or plaque thickening, and can be easily overlooked if not assessed in the context of prior disease morphology. Despite clinical changes suggestive of alternative pathology, histopathologic evaluation in both cases demonstrated persistent or progressive mycosis fungoides. Immunophenotyping and T-cell receptor gene rearrangement studies further supported lymphoma involvement rather than drug-induced eruptions. These cases illustrate clinicopathologic discordance, in which changes in clinical morphology do not reliably reflect the underlying pathology. Accurate classification requires a structured, algorithmic approach incorporating repeat biopsy, histopathologic assessment, immunophenotyping, molecular studies, and comparison with prior clinical and pathologic findings over time as well as appropriate restaging. Recognition of this diagnostic pitfall, particularly in patients with darker skin types, is essential to guide appropriate management during mogamulizumab therapy.

Indeterminate cell histiocytosis (ICH) is a clonal proliferative disorder of mononuclear phagocyte cells that shows features of both dendritic and histiocytic cells. Only around 100 cases of ICH were reported, and among them, only 24% of reported cases involve pediatric patients, with a mean age at diagnosis of 50 years. We report an exceptionally rare case of solitary ICH in a 4-year-old boy. A 4-year-old visited our department presenting a solitary asymptomatic bean-sized reddish nodule on upper abdomen for 1 year. Initially thought to be keloid or dermatofibroma, an excisional biopsy was performed. Histopathological examination revealed dense cellular infiltration in the dermis, characterized by medium-sized mononuclear histiocytes displaying abundant glassy eosinophilic cytoplasm and indented or folded nuclei. Immunohistochemistry studies confirmed positivity for S-100 protein and CD1a, coupled with negativity for CD207/Langerin. Electron microscopy revealed the absence of Birbeck granules, leading to the diagnosis of ICH. With no remarkable findings in laboratory and imaging studies, the patient is undergoing regular follow-up to monitor any potential recurrence or development of related conditions. While most cases of skin-restricted ICH follow an indolent course, recognizing the reported association between ICH and other hematologic neoplasms is crucial. Therefore, awareness of ICH is essential, and in cases of suspicion, a skin biopsy should be performed for an accurate diagnosis.

Cutaneous adverse reactions associated with tattoos exhibit various histopathologic patterns, ranging from allergic or interface dermatitis to sarcoid-like granulomatous inflammation. Immune checkpoint inhibitors (ICIs) are also well known to induce granulomatous cutaneous eruptions, usually with sarcoid-like features. Interestingly, ICIs can enhance immune responses at previous antigen exposure sites, including tattoos. Herein, we report a case of a non-caseating granulomatous tattoo reaction associated with red pigment in the context of ICI therapy. Histopathologically, the granulomas exhibited a tuberculoid architecture characterized by prominent peripheral lymphoplasmacytic inflammation rather than the relatively "naked" granulomas more typical of sarcoidal reactions. This case highlights the morphologic heterogeneity of granulomatous reactions arising within tattoos during ICI therapy and underscores the complex interaction between host immunity, tattoo pigment, and immunotherapy. Awareness of this pattern may help prevent misclassification as cutaneous sarcoidosis or infection and guide clinical care for patients on ICIs who develop such granulomatous reactions.