Becker's naevus — SAQ

Stem

A 16-year-old boy presents with a 12 cm, unilateral, light-brown to dark-brown, irregularly bordered patch with coarse dark terminal hairs confined to the patch on his left shoulder and upper chest. The lesion was first noticed at age 13 and has progressively enlarged and darkened, with the hair developing over the last year. He is otherwise well and has no past medical history. He is distressed by the appearance and avoids swimming.

Questions

a) What is the most likely diagnosis and what clinical features support it? (2 marks)

b) Outline the pathophysiology, including the histological features you would expect on biopsy. (3 marks)

c) Describe your stepwise management plan for this patient. (3 marks)

d) What associated conditions must you actively screen for, and what is the prognosis? (2 marks)

Model answer

a) Diagnosis (2 marks) — Becker's naevus (Becker melanosis). Supporting features: pubertal onset; unilateral, large, geographic brown patch; hypertrichosis confined to the patch; classical shoulder/chest distribution in an adolescent male.^[3]

b) Pathophysiology & histology (3 marks) — Benign, androgen-dependent cutaneous hamartoma of three native structures: epidermis (acanthosis, papillomatosis, increased basal-layer melanin without increased melanocyte number), hair follicle (coarse terminal hair) and arrector pili smooth muscle (hyperplasia, SMA-positive). Up-regulated androgen receptors in basal keratinocytes and follicular outer root sheath explain pubertal onset, male predominance and hypertrichosis. No naevus cells (distinguishes it from a melanocytic naevus). Paradominant inheritance (postzygotic somatic mutation) explains the unilateral, mosaic distribution.^[2][3]

^[2]c) Management (3 marks) — Step 1: confident diagnosis and reassurance — benign, no malignant potential, stabilises after puberty. Step 2: camouflage cosmetics if desired. Step 3: Q-switched laser (Nd:YAG 1064 nm, ruby 694 nm, alexandrite 755 nm) for pigmentation — variable, partial, may recur. Step 4: long-pulsed laser (alexandrite, diode, Nd:YAG) for the hypertrichosis — better response. Step 5: combination protocols give the best cosmetic result. Excision is rarely appropriate (scar worse than lesion). No role for topical bleaching agents or retinoids. Counsel on realistic, partial improvement and risk of post-inflammatory hyperpigmentation.

^[1]d) Associations & prognosis (2 marks) — Screen for Becker's naevus syndrome: examine the ipsilateral breast (hypoplasia), spine (scoliosis, vertebral anomalies) and limbs (asymmetry); image if any anomaly found. Prognosis: excellent — benign, stabilises after puberty, persists lifelong, no malignant transformation, no impact on lifespan; quality of life determined by cosmetic outcome and counselling.

References3ShowHide

1. [1]Zhong Y, et al. Lasers for Becker's nevus Lasers Med Sci, 2019.PMID 30762191
2. [2]Urbani CE, et al. Paradominant inheritance, supernumerary nipples and Becker's nevus: once again! Eur J Dermatol, 2001.PMID 11701421
3. [3]Patel P, et al. Sebaceus and Becker's Nevus: Overview of Their Presentation, Pathogenesis, Associations, and Treatment Am J Clin Dermatol, 2015.PMID 25782676