Congenital asymptomatic depressed plaque on the upper back in a young man

Question

What is your diagnosis?

Answer

Diagnosis: Medallion-like dermal dendrocytic hamartoma (MLDDH)

Discussion

Histopathology revealed normal epidermis with abnormally arranged thickened collagen in the dermis encircling sebaceous glands and hair follicles. Concentric proliferation of nucleated spindle-shaped cells was seen in the dermis prominently around hair follicles [Figure 2]. These hair follicles were miniaturised and lacked keratinisation. Further characterisation of these spindle cells on immunohistochemistry (IHC) revealed positivity for CD34 [Figure 3].

MLDDH, also referred to as plaque-like dermal fibroma (PDF), is a clinico-pathologically distinct, benign lesion arising from dermal dendrocytes. Rodríguez-Jurado et.al. proposed this entity and reported three females presenting with medallion-shaped, well-defined, slightly atrophic, and asymptomatic congenital lesions.¹ All patients showed spindle cell proliferation with CD34 and factor XIII positivity.¹ Dermal dendrocytes are bone marrow-derived cells of the monocyte-macrophage system with a role in immunologic surveillance as the antigen-presenting cells. They are often concentrated along vessels and nerves in the deep dermis, expressing factor XIIIa, and are routinely negative for CD34 expression.² However, a distinct subset of CD34-positive dermal dendrocytes in the periadnexal region of reticular dermis has been recognised with an obscure function. Hamartomas arising from these dermal dendrocytes have been described with variable clinical and immunopathologic features.² The pathogenesis is unclear, involving a clonal population of dermal dendrocytes and may involve a postzygotic mutation leading to cutaneous mosaicism.³ It typically presents as an asymptomatic congenital skin lesion, most commonly located on the trunk, and can measure several centimetres in size. The morphology ranges from an atrophic wrinkled reddish-brown plaque, as in MLDDH, to red-brown nodular plaque or multiple papules and plaques in other dermal dendrocytic hamartomas. Histopathology demonstrates perivascular and perineural spindle cell proliferation in the dermis along with epidermal atrophy.² Mast cells can be increased with concentration around spindle cells.² Immunohistochemically, the spindle cells are uniformly positive for CD34 with variable factor XIIIa positivity.² Due to the benign nature of the lesion, simple excision or expectant management is advised.In cases where surgical excision is not feasible, soft tissue augmentation techniques such as autologous fat grafting for depressed plaques may offer a viable cosmetic alternative.

The common differentials include atrophic dermatofibrosarcoma protuberans (DFSP), congenital smooth muscle hamartoma, aplasia cutis congenita, and other dermal dendrocytic tumours. Atrophic DFSP can represent an early stage of the typical DFSP, although it has been recognised as a distinct variant. It is an intermediate-grade mesenchymal neoplasm, which presents as an atrophic small depressed plaque quiescent for many years.⁴ The lesion is slowly evolving and can subsequently develop small hard nodules over the atrophic surface over a span of 10-20 years, evolving into typical forms.⁴ The lesion presents most often over the trunk in 40-50% cases and requires surgical resection as the standard treatment.⁴ Histopathologically, atrophic DFSP shows reduced dermal thickness with ill-defined spindle cell proliferation in storiform distribution with positive staining for CD34 and vimentin.^4,5 Translocation of t(17;22) (q22;q13) leading to fusion of COL1A1 gene with PDGFB gene can be demonstrated by reverse transcriptase polymerase chain reaction (RT-PCR) in almost all cases of atrophic DFSP.⁵ Aplasia cutis congenita or congenital absence of skin most often presents as an ulcerative lesion over the scalp in 80% with a characteristic hair collar. Lesions are usually non-inflammatory, well-demarcated and can be variable depending upon size, depth and scarring.⁶ Although it may initially present with ulceration, the healed cases look like a scar with a well-demarcated outline lacking dermal appendages and surrounded by a hair collar sign, similar to MLDDH. Truncal aplasia is rare, and bilaterally symmetrical lesions are associated with foetus papyraceus.⁶ Congenital smooth muscle hamartoma is characterised by smooth muscle proliferation in the reticular dermis. The presentation varies from hyperpigmented or skin coloured plaque over the trunk to a patch with follicular papules or as an atrophic hyperpigmented infiltrated plaque. Localised hypertrichosis with increased induration on rubbing (pseudo-Darier sign) can be observed. Histopathology is diagnostic with basal layer hyperpigmentation and numerous bundles of smooth muscle in the reticular dermis.

All these differentials can present as depressed atrophic plaques; however, the congenital onset, slowly progressive nature, well-defined morphology, and characteristic histopathological features with CD34-positive spindle cells around adnexal structures favour a diagnosis of medallion-like dermal dendrocytic hamartoma. Recognising MLDDH is important as it spares the patient from unnecessary aggressive surgery.