Poroid hidradenoma of the knee: a diagnostic challenge in imaging

Introduction

Poroid hidradenoma (PH) is a rare benign cutaneous adnexal tumor originating from either the eccrine or apocrine sweat glands and represents the rarest subtype of poroid neoplasms (1). PHs are hybrid tumors that combine the structural features of hidradenomas with the cytological characteristics of poromas (2). Although PH can occur in various regions of the body, it is most found on the head, neck, and trunk and affects patients of a wide age range (1). Herein, we report a rare case of PH arising in the knee, a highly unusual location, initially suspected to be a cystic lesion based on ultrasonography and magnetic resonance imaging (MRI), which posed significant diagnostic challenges.

Case presentation

A 22-year-old woman presented to our hospital with a slowly enlarging mass on the medial aspect of the right knee, first noted 4 years ago. The mass was elastic and soft without swelling, erythema, or tenderness. However, the overlying skin had a slightly dark reddish hue. The knee joint range of motion was normal and Tinel’s sign was negative. The patient had no notable medical history, and all blood test results were within normal limits. Ultrasonography (Figure 1A) revealed a mixed internal echotexture with areas of low and high echogenicity. Additionally, a distinct region of high echogenicity suggestive of a solid component was observed; however, no blood flow was detected in these areas (Figure 1B). MRI revealed a multilocular subcutaneous mass in contact with the dermis (Figure 2), demonstrating relatively homogeneous high signal intensity on T1-weighted imaging compared with adjacent muscle (Figure 2A) and hyperintensity on fat-suppressed T1-weighted (Figure 2C) and T2-weighted images (Figure 2B,2D), suggesting a cystic nature. The lesion showed a thickened wall and internal septa with a central nodular component that could be interpreted as a solid component (yellow arrow). Based on the imaging findings, the differential diagnoses included epidermal cysts, vascular malformations, lymphangiomas, and ganglions, rather than cutaneous tumors. At the patient’s request, surgical excision was performed for therapeutic and diagnostic purposes. Macroscopically, the tumor was dark red and measured 22 mm × 2 mm × 8 mm (Figure 3A). Histopathologically, the lesion formed a large cyst with hemorrhagic content, eosinophilic secretions, and cholesterol clefts within the septa and lumen (Figure 3B), along with focal papillary projections (Figure 3C). The tumor cells proliferated in sheets around a fibrous vascular core with hyalinization (Figure 3D). The proliferation of poroid cells with high nuclear-to-cytoplasmic ratios and nuclear grooves was observed, along with cuticular cells containing eosinophilic cytoplasm, forming small duct-like structures (Figure 3E). Immunohistochemically, the tumor cells showed diffuse positivity for nuclear protein in testis (NUT) (Figure 3F). Based on these pathological findings, the patient was diagnosed with PH. At the 1-year postoperative follow-up, there was no evidence of recurrence.

Figure 1 Ultrasound images showing (A) a predominantly hypoechoic mass with interspersed mildly hyperechoic areas (*) suggestive of solid components and (B) no vascular flow within the hypoechoic or mildly hyperechoic regions and mild flow within the septations.

Figure 2 MRI in the present case, including T1-weighted imaging (A), fat-suppressed T2 imaging (B,D), and fat-suppressed T1 imaging (C). White circles: marker placed at the time of imaging (A-C). The lesion had a thickened wall and internal septa, with a central nodular area suggestive of a solid component (yellow arrow). MRI, magnetic resonance imaging.

Figure 3 Macroscopic and microscopic findings. (A) Macroscopic and histopathological findings. (B-E) H&E and IHC staining of NUT-1 (F), magnification: ×20 (B), ×20 (C), ×100 (D) ×200 (F), ×400 (E). H&E, hematoxylin and eosin; IHC, immunohistochemistry; NUT-1, nuclear protein in testis 1.

All procedures performed in this case were in accordance with the ethical standards of the institutional and/or national research committee(s) and the Declaration of Helsinki and its subsequent amendments. Written informed consent was obtained from the patient for the publication of this article and the accompanying images. A copy of the written consent form is available for review by the journal’s editorial office.

Discussion

Clinically, PH has been reported in patients aged 13–81 years, with a mean age of 57 years. A slight male predominance has been noted (41 men and 32 women). The most commonly affected sites are the head, neck, and trunk (1). In contrast, PH arising in the knee is extremely rare, with only one other case reported to date (3). Pain or tenderness is observed in approximately one-third of cases, and most tumors measure <5 cm in diameter (1), although larger lesions of up to 7 cm have also been documented (4). The symptom duration varies widely, ranging from 0.1 to 40 years, with an average of 4.4 years (1). The mass in the present case had been present for 4 years and showed gradual enlargement, consistent with the behavior of a benign tumor. However, some low-grade sarcomas may present with indolent clinical courses and small size, mimicking benign lesions (5). Therefore, imaging and histopathological evaluations are essential for a definitive diagnosis rather than clinical findings alone.

Next, regarding the imaging findings: since there are no reports analyzing MRI characteristics based on a large number of cases, inference must be drawn from previous ultrasound findings (6,7). The cystic component is expected to show high signal intensity on T2-weighted images without enhancement, whereas the solid component is thought to demonstrate heterogeneous high T2 signal with enhancement. In the present case, the lesion was located in the superficial subcutaneous layer, not in a position to exclude cutaneous tumors. However, because the MRI signal resembled that of a cystic lesion, imaging suggested not only cystic diseases such as epidermal cyst, ganglion, or lymphatic malformation but also adnexal tumors (including sweat gland carcinoma) and squamous cell carcinoma arising from epidermal cysts (8-11). Since the imaging appearance of PH varies greatly depending on the ratio of solid to cystic components, it is likely difficult to distinguish on imaging. Cystic tumors such as ganglion or epidermal cyst often resemble the MRI findings observed in this case (8,9). On the other hand, when rupture or infection is complicated in ganglion or epidermal cyst, internal blood flow may be detected, producing findings similar to PH with a larger solid component. For PH diagnosis, non-invasive examinations other than MRI—such as dermoscopy and line-field confocal optical coherence tomography—are also considered useful (12,13). However, dermoscopic findings are known to resemble those of various benign and malignant skin tumors; thus, careful interpretation is required.

PH is a nodular, mixed-type tumor typically confined to the dermis, often lacking continuity with the overlying epidermis (1,2). It exhibits features of both poromas and hidradenomas. Poroid cells are small, with uniform nuclei and scant cytoplasm, whereas cuticular cells, often lining ductal structures, possess eosinophilic cytoplasm (1,2). A key differential diagnosis based on histopathology is digital papillary adenocarcinoma, a malignant sweat gland tumor that commonly arises in the digits (14). PH and digital papillary adenocarcinoma share several features, including well-defined borders, solid-cystic architecture, presence of mitotic figures, and minimal cytologic atypia. These similarities, combined with the limited tissue obtained from biopsy samples, make histological differentiation challenging. However, accurate distinction is critical as digital papillary adenocarcinoma carries the risk of local recurrence and distant metastasis (14). Recent advances in molecular pathology have revealed that a Yes-associated protein 1 (YAP1)-NUTM1 fusion, frequently observed in PH, is absent in digital papillary adenocarcinoma. This finding has led to the use of NUT-1 immunohistochemistry as a valuable diagnostic tool. PH typically shows NUT-1 positivity, whereas digital papillary adenocarcinoma does not (14). Nevertheless, caution is warranted, as NUT-1 positivity may also be observed in porocarcinoma, malignant PH, and other NUT-associated cutaneous tumors (14). Therefore, interpretation should be made in the context of histological and clinical findings.

The standard treatment for PH is surgical excision, with the recommended resection including the overlying epidermis and subcutaneous fat tissues (1,2). Although definitive data are lacking, recurrence is considered extremely rare, and reports of malignant transformation are virtually nonexistent (1,2). However, the natural history of PH and appropriate follow-up duration remain unclear and warrant further investigation.

We reported a case of PH that posed significant diagnostic challenges based on ultrasonographic and MRI findings. Imaging suggested a cystic lesion, such as a ganglion or epidermal cyst; however, these impressions likely reflected the characteristic solid and cystic architecture of PH. Varying the proportions of these components can lead to a broad differential diagnosis. Given the low risk of recurrence or malignant transformation, surgical excision serves both diagnostic and therapeutic purposes and is considered a reasonable approach.

Acknowledgments

None.

Footnote

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://qims.amegroups.com/article/view/10.21037/qims-2025-1-2681/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All procedures performed in this case were in accordance with the ethical standards of the institutional and/or national research committee(s) and the Declaration of Helsinki and its subsequent amendments. Written informed consent was obtained from the patient for the publication of this article and the accompanying images. A copy of the written consent form is available for review by the journal’s editorial office.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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