Myxoid Onychomatricoma with Unusual CD99 Expression: A Report of a Rare Case and Literature Review

Abstract

Onychomatricoma (OM) is a rare, benign fibroepithelial tumor originating from the nail matrix, with different clinical and histological variants documented in existing studies. Myxoid OM is an even rarer subtype, characterized by a prominent fibromyxoid component in histopathology. We report a case of a 34-year-old male patient presenting with clinical manifestations consistent with typical OM. Histopathological examination revealed a biphasic tumor composed of an epithelial component and a prominent myxoid stroma, suggesting a diagnosis of myxoid OM. Notably, the tumor exhibited diffuse CD99 positivity, an extremely rare finding in OM. Histopathologically, myxoid OM shows considerable morphological overlap with superficial acral fibromyxoma (SAFM). Our case illustrates that CD99 positivity cannot serve as an absolute criterion to exclude OM; cautious interpretation of this marker is required to avoid diagnostic pitfalls in the differential diagnosis of these two entities. This case expands the immunophenotypic spectrum of myxoid OM and offers a valuable reference for its differential diagnosis. Furthermore, we include and provide a descriptive review to summarize the clinicopathological features and key differential diagnostic considerations of OM, with a focus on diagnostic pitfalls arising from rare histological variants and atypical immunophenotypes, to facilitate accurate differential diagnosis of subungual tumors.

Introduction

Onychomatricoma (OM) is a rare, nail-specific benign tumor arising in the nail matrix anatomical compartment.¹ While the classic histological subtype is relatively well characterized, several rare variants have been identified in recent years, which often pose significant diagnostic challenges due to their overlapping features with other subungual tumors.^2–5 These diagnostic challenges maybe result in delayed treatment, unnecessary aggressive interventions, or permanent nail deformity, underscoring the critical need for improved recognition of these rare variants and their atypical immunophenotypic profiles.

Myxoid OM is a rare histological variant of OM. Since the term was coined by Stewart et al in 2015, one additional case using this diagnostic label has been reported by Pessoa e Costa et al (2020).^6,7 Classically, CD99 negativity has been considered a highly consistent immunohistochemical feature of OM, and this marker has been widely used to distinguish OM from superficial acral fibromyxoma (SAFM), which typically shows diffuse CD99 expression.⁸ Notably, this diagnostic distinction has been called into question by a recent study from Perrin et al, who reported a single case of SAFM-like OM with diffuse CD99 immunoreactivity, thereby challenging the utility of CD99 as a high-specificity differential marker between OM and SAFM.⁹

Herein, we present a case of CD99-positive Myxoid OM. We describe the clinical, imaging, histopathological, and immunohistochemical features of this rare case, and provide a descriptive review of the literature focusing on the epidemiological characteristics, clinical manifestations, imaging features, molecular pathogenesis, histological spectrum, immunohistochemical markers, differential diagnosis, treatment, and prognosis of OM, with particular emphasis on the diagnostic pitfalls associated with histological variants and unusual immunohistochemical findings.

Case Report

A 34-year-old otherwise-healthy male presented with a 1-year history of progressive thickening and yellowish discoloration of the right ring fingernail. The condition occurred insidiously without any history of trauma or infection. He had no prior treatment and denied any pain, tenderness, or functional impairment of the finger, but was concerned about the progressive cosmetic deterioration. There was no family history of nail disorders or skin tumors. Physical examination revealed significant thickening, yellowish discoloration, and excessive longitudinal curvature of the lateral nail plate of the right ring finger, accompanied by obvious longitudinal ridges and subungual splinter hemorrhages. No Hutchinson sign, periungual erythema, or tenderness was noted (Figure 1A). The remaining nails were unremarkable. Dermoscopic examination of the nail plate showed thickening of the free edge, with multiple internal honeycomb-like spaces filled with yellowish keratinous material (Figure 1D and E). Initial magnetic resonance imaging (MRI) detected no definite abnormal lesion in the nail unit. Follow-up high-frequency ultrasonography revealed a well-circumscribed, relatively regular hypoechoic lesion at the nail matrix, measuring approximately 5.3 mm × 3.3 mm × 3.5 mm, with no evidence of bony invasion or infiltration into adjacent soft tissues (Figure 1F). After obtaining informed consent, the patient underwent nail plate avulsion and complete tumor excision under digital nerve block. Intraoperatively, a relatively well-circumscribed, whitish tumor was identified at the nail matrix, with verrucous projections at the distal portion (Figure 1B). The ventral side of the avulsed nail plate was smooth, and irregular depressions were visible on the proximal surface of the nail plate; these depressions appeared to be consistent with the projections of the tumor. Pathology showed that the tumor exhibited a fibroepithelial structure. The surface of the tumor was composed of papillary onychomatrical epithelium, with its base extending into the underlying tissue to form cord-like structures or deep, large invaginations, and its upper end showing papillomatous hyperplasia, presenting a characteristic V-shaped structure. The stroma within the tumor villi and base was fibrovascular, and a dense, hypercellular fibromyxoid zone was present in the deep aspect of the lesion (Figure 2A). No significant nuclear pleomorphism, mitotic activity, or necrosis was identified. Alcian blue staining was strongly positive in the myxoid stromal areas, confirming the presence of abundant acidic mucopolysaccharides (Figure 2B). Immunohistochemical staining showed diffuse and strong positivity for CD34 in the stromal spindle cells (Figure 2C). The epithelial component showed strong nuclear LEF-1 expression (Figure 2D). Diffuse cytoplasmic CD99 positivity was observed in the lesional spindle stromal cells (Figure 2E). S-100 protein was negative in both the epithelial and stromal components. Based on these clinical, histopathological, and immunohistochemical findings, a diagnosis of myxoid OM with aberrant CD99 expression was made.

Figure 1.

Clinical and imaging features of onychomatricoma (OM). (A) Marked thickening, yellowish discoloration, and pronounced longitudinal curvature of the nail plate on the lateral aspect of the right ring finger. (B) Intraoperative view showing a relatively well-circumscribed whitish tumor arising from the nail matrix, with verrucous projections at its distal margin. (C) Clinical appearance at 6-month follow-up. Dermoscopic features: (D) Longitudinal nail ridges and subungual splinter hemorrhages are visible within the nail plate. (E) Thickened nail free edge containing multiple honeycomb-like cavities filled with yellow keratin debris. (F) Transverse high-frequency ultrasound (23 MHz) showing a well-circumscribed hypoechoic lesion (yellow arrow).

Figure 2.

Histopathological and immunohistochemical features of onychomatricoma (OM). (A) Hematoxylin–eosin staining shows a fibroepithelial tumor with papillary onychomatrical epithelium, basal cord-like proliferations, deep invaginations, and superficial papillomatous hyperplasia forming typical V-shaped contours; the stroma is fibrovascular with a hypercellular fibromyxoid zone in the deep lesion, and no nuclear pleomorphism, mitoses, or necrosis are seen. (B) Alcian blue staining is strongly positive in the myxoid stroma. (C) Stromal spindle cells show diffuse and strong CD34 positivity. (D) Epithelial cells show strong nuclear LEF-1 expression. (E) Moderate diffuse CD99 expression with cytoplasmic localization is noted in approximately 80% of the lesional stromal cells. Original magnification: ×40 for panel (A); ×100 for panels (B)–(E).

Immunohistochemical staining was performed on a fully automated IHC stainer (CNT330, Celnovte, Zhengzhou, Henan, China) with EDTA buffer (pH 8.0) for heat-induced epitope retrieval (HIER, 40 min), and visualized with 3,3’-diaminobenzidine (DAB) as the chromogen. Internal tissue controls within each section were evaluated for all antibodies to confirm staining specificity. For CD34, an additional external positive control was included to standardize staining intensity across batches. The primary antibodies used were as follows: CD99 (clone PCB1, ready-to-use, Zhongshan Golden Bridge, Beijing, China), CD34 (clone QBEnd/10, ready-to-use, Celnovte, Zhengzhou, Henan, China), LEF-1 (clone EB310, ready-to-use, Maixin, Fuzhou, Fujian, China), and S-100 (clone 4C4.9, ready-to-use, Maixin, Fuzhou, Fujian, China).

At the 6-month postoperative follow-up, the affected nail achieved favorable regeneration with good cosmetic outcome, and the patient reported high satisfaction with the treatment result (Figure 1C).

Discussion

OM was first described in 1992, with fewer than 200 cases reported globally to date.^1,10 Myxoid OM is a rare histological subtype of OM. Stewart et al formally proposed and named this variant in 2015, and only two well-documented cases under this specific designation have been reported in the international English-language literature so far.^6,7 To better understand the clinical, imaging, and pathological characteristics of OM and its subtypes, we performed a descriptive literature review and present an integrated discussion combined with the present case. The review was performed across PubMed, Web of Science, and Embase covering publications released between 1992 and 2026. The search terms were “onychomatricoma” and “myxoid onychomatricoma”. Original clinical case reports, clinicopathological studies, and relevant reviews published in English were included. Duplicate publications, animal studies, and articles irrelevant to nail apparatus diseases were excluded. The eligible literature was summarized and synthesized to support the analysis in subsequent sections.

Epidemiological Characteristics

OM affects a wide age range (4–93 years), predominantly occurring in adults, particularly the elderly, but may also present in children and adolescents.^11,12 OM shows no significant sex predilection and affects all races.¹³ Most cases affect only one digit, and simultaneous involvement of multiple digits is relatively rare. Many studies have suggested that the incidence of OM is higher in the fingers than in the toes.^14,15 However, this observation may be subject to bias, as hands are more visible and socially prominent in daily interactions.

Clinical Manifestations

OM is typically slow growing and asymptomatic without functional impairment. Most cases present without pain or functional loss, while mild pain or progressive pain may occur in a minority of patients.¹⁴ The classic clinical tetrad of OM includes nail plate thickening, transverse or longitudinal overcurvature, chromonychia (leukonychia, xanthonychia, or leukoxanthonychia), and splinter hemorrhages (more prominent proximally).¹⁵ Additional manifestations may comprise longitudinal ridging, plate fracture, nail dystrophy, distal tubular structures, erythronychia, melanonychia, paronychia, and dorsal pterygium.^16,17 Given overlapping clinical presentations of subungual lesions, OM is frequently initially misdiagnosed, mimicking onychomycosis, SAFM, subungual melanoma, and subungual squamous cell carcinoma. To improve diagnostic accuracy, a comprehensive evaluation combining clinical assessment, imaging examination, and histopathological with immunohistochemical analysis is essential.

Imaging Features of OM

Dermoscopy, a first-line non-invasive tool, is used to identify pathognomonic honeycomb or woodworm-like cavities containing yellowish material at the nail plate free edge that correspond to tunnel-like spaces, along with consistent findings including longitudinal parallel white or yellow bands, proximal splinter hemorrhages, and nail thickening. These features are closely associated with the tumor’s fibroepithelial digitations within the nail plate.^2,7 High-frequency ultrasound typically shows a well-circumscribed hypoechoic, hypovascular solid lesion localized to the nail matrix with hyperechoic linear dots or lines representing tumor digitations projecting into the interplate space.^18,19 This is accompanied by secondary findings of nail plate thickening, upward displacement of the nail plate, enlargement of the interplate space, and increased nail bed thickness,^19,20 with color Doppler revealing a hypovascular pattern within the lesion and the distal phalangeal bone margin usually being unremarkable.^18,20 Magnetic resonance imaging (MRI), which is rarely performed but provides detailed anatomical delineation, shows a tumor extending from the nail matrix with a hypointense proximal core and hyperintense distally oriented finger-like projections on T2-weighted images, with axial images clearly demonstrating holes in the nail plate and tumor digitations.^7,20 Although, in some cases, it may only show mild thickening with edematous changes in the nail and subungual region.¹⁹ Finally, in vivo confocal microscopy has been applied in four reported cases, revealing longitudinal dark areas and bright/gray lines corresponding to canalicular structures within the distal nail plate as well as splinter hemorrhages, offering additional preoperative diagnostic value.²⁰ In our case, the lesion was not identified on initial MRI but was clearly detected using high-frequency ultrasound. This observation raises the possibility that high-frequency ultrasound may offer better diagnostic sensitivity for small superficial OM lesions. Given its broad clinical availability and cost efficiency, high-frequency ultrasonography could serve as a useful first-line cross-sectional imaging modality for suspected OM.

Pathogenesis and Molecular Genetics of OM

The pathogenesis of OM has been the subject of intense debate in recent years, particularly regarding its histogenesis and the nature of its epithelial component. Traditionally, OM was classified as a biphasic fibroepithelial neoplasm in which both the proliferating nail matrix epithelium and underlying stromal spindle cells were considered neoplastic components.¹ However, molecular studies published in 2023 have challenged this paradigm, providing strong molecular evidence supporting the hypothesis that OM is a predominantly mesenchymal neoplasm, with the overlying epithelial component representing a secondary reactive hyperplasia induced by the neoplastic stroma.^9,21

Another core point of ongoing controversy concerns the precise mesenchymal cell of origin of OM. Shim et al have proposed that OM arises from specialized onychofibroblasts within the so-called “onychodermis”, which is a purported nail-specific mesenchymal structure located beneath the nail matrix and nail bed epithelium, characterized by CD10 and CD34 expression and unique transcriptional signatures including RSPO4, MSX1, and BMP5.^22,23 However, this view has been vigorously challenged by Perrin et al, who argue that the concept of the onychodermis lacks a valid microanatomical basis and demonstrate that the nail unit mesenchyme consists of only two distinct zones (the proximal matrix mesenchyme and distal nail bed mesenchyme) with no evidence of a separate specialized compartment, and that CD10 expression is not specific to any nail mesenchymal subpopulation.²⁴

While the precise mesenchymal cell of origin remains debated, a key recurrent molecular feature of OM has been consistently identified: the recurrent loss of chromosome 13q encompassing the RB1 gene locus. Perrin et al demonstrated this finding in all eight OM cases tested using array comparative genomic hybridization (aCGH), including all histological variants.⁹ Notably, RB1 deletion was detected in both classic and almost all variant forms of OM in all tested cases, indicating that this molecular alteration is a consistent and early event in OM tumorigenesis regardless of histological appearance. This was independently confirmed by Liu et al, who used fluorescence in situ hybridization (FISH) to show that RB1 loss is restricted to the mesenchymal component of OM, while the overlying epithelium retains normal RB1 expression.²¹ These findings support the inclusion of OM within the expanding family of 13q/RB1-deleted soft tissue tumors, which also includes SAFM, spindle cell/pleomorphic lipoma, mammary-type myofibroblastoma, acral cellular angiofibroma, and pleomorphic fibroma.

Histological Spectrum of OM

OM demonstrates a far broader histological spectrum than was initially recognized, encompassing multiple well-characterized variants as well as rare morphological deviations. The classic phenotype accounts for the majority of documented cases, while several atypical variants have been systematically described and are widely accepted. These atypical forms frequently represent diagnostic pitfalls, particularly in fragmented biopsy specimens lacking characteristic nail plate changes. The core clinicopathological features of all recognized OM subtypes are summarized (Table 1).

Table 1.

Histopathological Subtypes, Typical Immunophenotypes, and Key Differential Diagnoses of Onychomatricoma (OM)

Subtype	Core Histopathologic Features	Typical Immunophenotype	Key Differential Diagnoses
Classic OM	Biphasic fibroepithelial architecture with prominent finger-in-glove fibroepithelial digitations	Stromal: CD34 (diffuse +); Epithelial: AE1/AE3 (+), CK5/6 (+), LEF-1 (nuclear +); Typically negative: CD99 (−)	Onychomycosis; nail dystrophy9,26
Myxoid OM	Abundant myxoid stromal deposition with scattered spindle cells and preserved biphasic architecture	Stromal: CD34 (diffuse +), factor XIIIa (focal +); Typically negative: CD99 (−), S100 (−), EMA (−); Special stain: colloidal iron/Alcian blue (+)	SAFM; digital myxoid cyst; superficial angiomyxoma6,7
Pleomorphic OM	Degenerative nuclear pleomorphism and multinucleation in stromal cells; no atypical mitoses or necrosis	Stromal: CD34 (diffuse +), p16 (diffuse +), CD10 (focal +); Typically negative: CD99 (−), EMA (−), pancytokeratin (−), S100 (−)	Pleomorphic SAFM; superficial CD34-positive fibroblastic tumor; subungual squamous cell carcinoma5,32
Proliferating OM	Biphasic fibroepithelial architecture; lobulocystic squamous proliferation with keratogenous spheres; epithelial dysmaturation without significant atypia, mitoses, or necrosis	Stromal: CD34 (diffuse +), CD10 (focal +); Typically negative: CD99 (−), EMA (−), S100 (−), p16 (−), p53 (−)	Subungual squamous cell carcinoma; onycholemmal carcinoma; conventional onychomatricoma; onychocytic matricoma3
Micropapilliferum OM	Biphasic fibroepithelial architecture; papillated epithelial hyperplasia; keratogenous spheres with ring-pattern matrical keratinization; absent proximal cavitation with tiny distal nail plate cavities	Stromal: CD34 (diffuse +), CD10 (focal perivascular +); Typically negative: CD99 (−), EMA (−), S100 (−), BerEp4 (−)	Conventional onychomatricoma; onychocytic matricoma; onychocytic carcinoma; keratotic basal cell carcinoma2
Pigmented OM	Biphasic fibroepithelial architecture with papillary matrical projections; intralesional melanin deposition, stromal melanophages, and pigment incontinence	Stromal: CD34 (diffuse +), CD10 (+)	Subungual melanoma; pigmented ungual Bowen’s disease; onychocytic matricoma; pigmented onychomycosis17,34–36

Notes: CD99 negativity is typical but not absolute for OM, and rare cases of CD99-positive OM have been documented in the literature.

Abbreviations: OM, onychomatricoma; SAFM, superficial acral fibromyxoma; EMA, epithelial membrane antigen; CK, cytokeratin; LEF-1, lymphoid enhancer-binding factor 1; BerEp4, epithelial antigen BerEP4; (+), positive immunoreactivity; (−), negative immunoreactivity; diffuse, widespread positive staining; focal, focal positive staining.

Beyond the well characterized histological subtypes listed in the table, OM exhibits a broad morphological spectrum. Importantly, transitional forms between different histological variants have been observed. Some cases show classic OM areas adjacent to myxoid or collagenous regions. This finding suggests a morphological continuum among OM subtypes. Some OM cases display SAFM-like histological features. These lesions are characterized by deep dermal predominance, fibromyxoid stroma and minimal epithelial papillomatosis. They may closely mimic SAFM on histological examination.⁹ However, this morphological overlap does not equate to pathogenetic equivalence. Liu et al found that RB1 loss is confined to the mesenchymal component of OM. They interpret these similarities as evidence of a histological continuum between OM and SAFM. They thus classify OM as a purely mesenchymal tumor.²¹ Perrin and Coutts hold a different view. Their team first identified RB1 deletion in OM. Nevertheless, they maintain that OM and SAFM are fundamentally distinct entities. They emphasize that OM retains true biphasic onychogenic differentiation as a fibroepithelial neoplasm. The two tumors can be reliably distinguished by clinical features, stromal architecture and LEF-1 expression pattern.²⁵

Myxoid OM is defined by the presence of a prominent myxoid stroma that replaces the typical fibrous stroma, while retaining the characteristic biphasic architecture and finger-in-glove projections. Our case and all previously reported cases of myxoid OM have presented with the classic clinical features of OM, including nail thickening, yellow discoloration, and honeycomb cavities at the free edge, suggesting that this histological alteration does not substantially alter the clinical phenotype, though this observation requires validation in larger cohorts.^6,7

Immunohistochemical Markers of OM

The immunohistochemical profile of OM has been progressively refined over the past three decades, with several markers emerging as key diagnostic tools. Early studies identified CD34 as a consistent marker of OM stromal cells, with diffuse positivity observed in nearly 100% of cases.²⁶ This finding remains a cornerstone of OM diagnosis, as it helps distinguish OM from other subungual tumors such as fibrokeratoma and warts, which are typically CD34-negative.

More recently, LEF-1 has emerged as the most specific marker for OM, with nuclear expression restricted to the onychomatrical epithelial component in all tested cases.^9,21 The specificity of LEF-1 for OM is further supported by its role as a downstream effector of the Wnt/β-catenin signaling pathway, which is proposed to be constitutively activated in the epithelial component by RSPO4 secretion from onychofibroblasts.^27,28 However, this pathogenetic interpretation remains debated, as independent molecular studies have yielded conflicting data regarding the extent of Wnt/β-catenin pathway involvement in OM. In contrast, SAFM and other subungual mesenchymal tumors are consistently LEF-1-negative, even when they overlie the nail matrix.²⁵

Nestin has been reported as a sensitive marker for SAFM. Perrin et al demonstrated a complete absence of nestin immunoreactivity in tumor cells in all 20 cases of OM, and consistent with prior studies, subungual SAFM exhibits diffuse cytoplasmic nestin staining.²⁹ This staining profile shows a predominantly reciprocal distribution relative to the well-characterized LEF-1 immunophenotype: OM exhibits diffuse nuclear LEF-1 positivity in onychogenic epithelium, whereas SAFM shows absent LEF-1 expression in tumor cells, with positivity restricted solely to residual native nail matrix epithelium.⁹ Available studies have observed that the LEF-1/nestin diagnostic panel achieves high diagnostic concordance for distinguishing OM and SAFM.^9,25,29

Traditionally, CD99 was considered a reliable negative marker for OM, and its absence was widely used to support the diagnosis of OM over SAFM.^26,30,31 However, our case and the previously reported case of CD99-positive SAFM-like OM demonstrate that this marker is not absolutely specific for distinguishing these two entities.⁹ This lack of absolute specificity is not entirely surprising, as CD99 expression is known to be highly variable across the entire spectrum of 13q/RB1-deleted soft tissue tumors. In the context of SAFM, CD99 positivity rates have been consistently reported to be in the range of 70%–85% across multiple independent studies: The original description by Fetsch et al documented CD99 expression in 84.6% (11/13) of SAFM cases.²⁹ Meanwhile, Cullen et al demonstrated CD99 immunoreactivity in 75% (9/12) of cases, with moderate or strong homogeneous staining intensity,⁸ indicating that approximately 15%–25% of SAFM cases will be CD99-negative, creating a significant diagnostic overlap with the historically described CD99-negative phenotype of OM. The identification of CD99-positive OM cases further erodes the discriminatory value of this marker, and this bidirectional variability in CD99 expression in both tumors creates substantial diagnostic pitfalls.

Differential Diagnosis and Diagnostic Pitfalls

OM is prone to diagnostic delay or misdiagnosis due to its rarity and low clinical suspicion. Common misdiagnoses include onychomycosis, subungual squamous cell carcinoma, melanoma, fibrokeratoma, and verruca, potentially resulting in avoidable nail damage and unnecessarily aggressive interventions. Histological variants of OM further heighten diagnostic risks: pleomorphic OM may be misinterpreted as fibroma or low-grade sarcoma owing to cytologic atypia,³² proliferating OM can simulate squamous or onycholemmal carcinoma,³ and myxoid OM may be mistaken for SAFM, with the rare occurrence of CD99-positive OM further compounding this diagnostic difficulty.

The differential diagnosis between OM and SAFM represents a key challenge in nail unit pathology, especially when specimens are fragmented or clinical data are limited. In classic cases with intact tissue and adequate clinical context, the distinction is generally straightforward. Clinically, OM typically causes marked nail plate thickening with multiple tunnel-like cavities, and the lesion is usually not visible prior to nail plate avulsion. In contrast, SAFM more often leads to nail plate thinning, deformation, cracking, or fracture, with a visible or palpable subungual mass appreciable through the attenuated plate. Histologically, the fundamental distinction lies in tissue architecture: OM is a biphasic fibroepithelial neoplasm with onychogenic digitations extending into the nail plate, whereas SAFM is a purely mesenchymal proliferation with no epithelial components.

To mitigate these pitfalls, strict clinicopathological correlation is indispensable: pathologists should include OM in the differential diagnosis for any subungual CD34-positive spindle cell neoplasm regardless of myxoid changes or CD99 expression, and actively solicit clinical details regarding nail plate morphology, while clinicians should provide comprehensive clinical information and submit the intact nail plate with the specimen whenever feasible, as the characteristic tunneling pattern is highly pathognomonic for OM.

Treatment and Prognosis

The mainstay of treatment for OM is complete surgical excision, typically performed with partial or total nail avulsion.¹³ Mohs micrographic surgery may be used as a matrix-sparing alternative.³³ Local recurrence is uncommon after complete surgical excision. Nail dystrophy may occur depending on the extent of nail matrix resection, but most patients achieve acceptable cosmetic results with proper surgical technique.^7,10,13,33 Good nail regrowth and a satisfactory cosmetic outcome were also observed in our case at the 6-month follow-up.

Conclusion

We report a rare case of myxoid OM exhibiting aberrant diffuse CD99 immunoreactivity. A key take-home message is that CD99 positivity should not automatically exclude OM when morphological and clinicopathologic findings support a diagnosis of OM. This case further expands the known immunohistochemical spectrum of OM and highlights the diagnostic pitfalls posed by histological variants of OM with unusual immunophenotypes, particularly the high risk of misdiagnosis as SAFM. Taken together, this case and the reviewed literature underscore the importance of a multimodal diagnostic approach integrating clinical presentation, imaging features, histological architecture, and a targeted immunohistochemical panel rather than reliance on any single finding.

Funding Statement

This study was not supported by any funding.

Ethical Approval and Consent to Publish

The publication of this case report was not required the approval from the Ethics Committee of Chengdu Integrated TCM & Western Medicine Hospital. Written informed consent was obtained from the patient for the publication of this case report and any accompanying images.

Disclosure

The authors declare that they have no conflict of interest.