Round Light Brown Skin Spots: Differential Diagnosis & When to Biopsy

Common Causes of Circumscribed Brown Macules

Entity	Key Features	Clinical Clues
Solar lentigo (sun spot)	1–10 mm, flat, uniform tan-brown, sharp borders	Chronic sun exposure; face, hands, forearms; >40 yr
Ephelides (freckles)	1–3 mm, light brown, seasonal (darken summer)	Fair skin, red hair, MC1R variants; childhood onset
Café-au-lait macule (CALM)	>5 mm, uniform light brown, smooth/coast of California border	≥6 CALMs >5 mm (prepubertal) or >15 mm (adult) → NF1
Melasma	Symmetric, irregular patches, facial (malar, forehead, upper lip)	Women, pregnancy, OCP, sun; hormonal + UV
Congenital melanocytic nevus	Present at birth/early infancy, grows proportionally	Size categories: small <1.5 cm, giant >20 cm (melanoma risk)
Seborrheic keratosis	Stuck-on, waxy, verrucous, tan-black	>50 yr; "barnacles of aging"; benign
Pigmented actinic keratosis	Rough, scaly, erythematous base	Sun-damaged skin; precancerous (SCC risk)
Lentigo maligna / melanoma in situ	Irregular color, asymmetric, progressive	Chronic sun; face/neck; ABCDEs → biopsy
Mycosis fungoides (patch stage)	Poikilodermatous, scaly, "cigarette paper" wrinkling	Chronic, non-sun-exposed; biopsy + TCR gene rearrangement

When to See a Dermatologist

• New or changing lesion (ABCDE: Asymmetry, Border, Color, Diameter >6 mm, Evolution).
• Multiple CALMs (screen for NF1).
• Personal/family history of melanoma.
• Immunosuppression.

Dermoscopy improves diagnostic accuracy; biopsy for definitive histology.