A pedunculated nodule over upper lip

Question

What is your diagnosis?

Diagnosis

Tubular apocrine adenoma

Discussion

Tubular apocrine adenoma (TAA) is a rare benign adnexal neoplasm of apocrine derivation and usually presents as a painless slowly growing firm nodule. The most common site of involvement is the scalp (58%), despite the limited distribution of apocrine glands over this area. Besides the scalp, TAA can occur at other sites like the face, neck, upper and lower extremities, trunk and vulva. TAA was first reported by Landry and Winkelman and is mostly found in isolation and rarely in association with organoid lesions like naevus sebaceous and syringocystadenoma papilliferum (SCAP).¹ The mean age at diagnosis is 48.9 ± 22.1 years and it is more common in females with a female-to-male ratio of 2:1.²

The exact origin of tubular apocrine adenoma is unknown. It is hypothesised to originate from primary epithelial germ cells, pluripotent appendageal cells or preexisting glands. Fisher proposed that TAA could be a minor variant of SCAP. Another theory based on histopathological similarities proposes that the superficial part of the lesion is formed by SCAP, while the deeper intradermal portion has characteristics akin to tubular papillary adenoma (TPA).³ Various adnexal tumours, such as TAA, papillary eccrine adenoma, apocrine cystadenoma, SCAP and TPA, exhibit notable similarities in their morphology, despite their distinct characteristics.^4,5 Interestingly, about one-third of these tumours exhibit a composite nature, where histological features from two or three different types of adnexal tumours can coexist concurrently. To unite this diverse group of tumours, the term “tubulopapillary cystic adenoma with apocrine differentiation” has been introduced.⁵ Consequently, the term “tubular papillary adenoma” has been employed to describe lesions that fall within a spectrum ranging from papillary eccrine adenoma to tubular apocrine adenoma, as precise differentiation between these two entities can sometimes be challenging.⁵ Certain authors suggest the possibility of intermediate differentiation in adnexal tumours, which aligns with this idea. The emerging concept of “apo-eccrine glands” might prove valuable in this context as it helps explain the hypothesis of intermediate differentiation. It is worth noting, however, that these glands are not ubiquitous but present in specific areas of the body like the axillae, anogenital region, periumbilical region, areolae, external auditory canals, and the eyelids.^4,5 Histologic features of TAA are characteristic and help in confirming the diagnosis. It is characterised by well-defined lobules of tubular structures within the dermis that may extend into the subcutis.² The tubules are lined by two rows of cuboidal to columnar epithelial cells with decapitation secretion. There is minimal stroma which is usually fibrotic.^4,5 Rarely, TAA may transform into apocrine adenocarcinoma.^4,5 Complete surgical excision of the lesion to negative margins is usually curative.

In conclusion, the diagnosis and classification of sweat gland tumours pose challenges due to the subtle variations in the morphologies of different entities and the overlapping histopathological features. We report the occurrence of this rare tumour over the upper lip.