Eccrine Poroma of the Thigh: An Unusual Diagnosis

Sir, Eccrine poroma is a benign adnexal neoplasm of eccrine or apocrine lineage having poroid or terminal ductal differentiation. It usually presents as solitary, slow-growing, soft to firm papules, plaques or nodules on areas of high eccrine gland density like the palms and soles, neck, chest, forehead, nose and scalp. It is notorious for having varied morphological presentations resulting in diagnostic difficulty. When approaching such lesions, a clinician needs to be aware of the same, so that such lesions can be subjected to histopathological evaluation, which is usually sufficient to cinch the diagnosis. Occasionally, further immunohistochemistry might be needed.[1] A 66-year-old male presented with a 0.4 mm × 0.4 mm, firm nodule on his right thigh for 6 months, which was tender to touch [Figure 1]. The overlying skin was slightly erythematous but otherwise unremarkable. Differentials of neuroma, neurilemmoma and leiomyoma were considered and an excisional biopsy was done. Histopathology demonstrated proliferation of compact cuboidal monomorphous cells with small basophilic nuclei and a fibrotic, vascular stroma with foci of ductal differentiation [Figure 2 a–c]. A final diagnosis of eccrine poroma was made.Figure 1: (Arrow): 0.4 × 0.4 mm well-circumscribed nodule on right thighFigure 2: (a–c): Histopathology using haematoxylin and eosin stain (10× and 40×) showing compact cuboidal cells with small monomorphous nuclei with foci of ductal differentiation suggestive of eccrine poromaPoroma is a benign cutaneous neoplasm arising from the acrosyringium. The mean age at diagnosis is 68.3 years with a male-to-female ratio of 15:1[2], which was similar to our patient. First reported by Pinkus in 1956, poromas comprise 0.1% of all primary skin tumours.[3] Usually, poromas present as a slow-growing, solitary, soft to firm plaque or nodule on palms, soles or the scalp. They are well-circumscribed and characteristically surrounded by a shallow, 'moat-like' depression. However, this feature may not always be present. Due to their varied clinical presentations, poromas can be a diagnostic challenge, especially when presenting at atypical sites or with uncommon symptoms. Aside from being a rare dermatological entity, mentions are even sparser in medical literature outside of dermatology.[2,3] Notably, Wong et al.[3] at the hand-and-foot clinic, reported three cases presenting with a chronic, solitary plaque on the plantar foot. The first one remained asymptomatic for 14 years and then ulcerated, while the other two were associated with bleeding on contact and difficulty walking. These reports underscore the polymorphic nature of poromas: Ranging from protruding, purple, violaceous retroauricular mass (resembling atypical nevus/melanoma) to tender, nodular lesion on the eyelid (resembling basal cell carcinoma) to fleshy plaque on soles (resembling haemangioma).[2-4] Differentials are multiple and vary depending on morphology and location. For the garden variety of poromas presenting as solid papules on the face, common differentials are syringoma, haemangioma, pyogenic granuloma, seborrheic keratosis, verruca, basal and squamous cell carcinoma.[5] Their clinicopathologic findings are elucidated in Table 1.Table 1: Clinical features and histopathology of common differentials of poromaHistopathology shows broad anastomosing bands arising from the epidermis and extending into the dermis. The tumour consists of uniform, cuboidal cells with basophilic nuclei and may contain vacuolated areas due to glycogen. Ductal lumina and cystic spaces maybe present, and are lined by eosinophilic, periodic acid Schiff (PAS) positive, diastase-resistant cuticles similar to eccrine sweat ducts. Various forms include completely intraepidermal (hidroacanthoma simplex), intraepidermal with microcysts, intraepidermal with macrocysts, completely dermal (dermal duct tumour), pigmented, irritated, chondroid, syringomatous and focally malignant.[6] This is to be differentiated from porocarcinoma, which shows infiltrative growth and atypia. Notably, irritated poromas may also show foci of necrosis, atypia or mitosis.[7] Up to 0.01% can progress to malignant Porocarcinoma, further highlighting the need for timely diagnosis and treatment.[8] Histopathology is relatively straightforward, so the clinician's knowledge and attention to detail while forming differentials can go miles in proper clinicopathological correlation. Eccrine poroma can have varying clinical presentations and can mimic other proliferative and malignant lesions. It is imperative to do a biopsy in such cases and immunohistochemistry might be further required to typify the lesion. Treatment is optional, but removal is imperative if the lesion is symptomatic or bothersome to the patient. For superficial lesions, electrosurgical removal can be performed, while deep lesions require surgical excision. This case is being reported as eccrine poroma is a rare diagnosis and chronic lesions can progress to malignancy. This case was also unusual owing to the atypical presentation at an uncommon site, and the tenderness, which is not commonly seen with poromas. Declaration of patient consent The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.