Pigmented Lesion on the Sole: When to Biopsy and How to Interpret
A 45-year-old woman has a 9 mm asymmetric, irregularly bordered, variably pigmented macule on the plantar arch, first noticed 2 years ago and slowly enlarging. Dermoscopy shows a parallel ridge pattern in one area and a diffuse pigmented network elsewhere. She has no personal or family history of melanoma. She is Fitzpatrick V. A shave biopsy was performed elsewhere and reported as "compound nevus with mild atypia, margins not assessable."
1. Evaluate the dermoscopic findings and the biopsy report. What is the concern and why is the biopsy inadequate?
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Dermoscopy:
- Parallel ridge pattern on acral skin is highly concerning for acral melanoma in situ - pigment follows the ridges (eccrine duct openings) rather than the furrows. It is not specific (can occur in benign pigmentation and drug-related pigmentation), but in a changing, enlarging lesion it must be treated as suspicious.
- A diffuse network elsewhere is less specific but the combination in an enlarging 9 mm lesion raises concern.
Biopsy problem:
- A shave biopsy of an acral pigmented lesion is inadequate: it does not assess depth, cannot reliably evaluate the dermoepidermal junction architecture across the lesion, and "margins not assessable" means the lesion may be transected. Acral melanoma can be subtle and the shave may miss the diagnostic area.
- "Compound nevus with mild atypia" on a partial sample does not exclude melanoma; sampling error is a real risk.
Next step: excisional biopsy (or deep saucerization/punch excisional sampling) of the entire lesion with orientation, for full histologic assessment.
2. How does Fitzpatrick V skin affect your dermoscopic interpretation and your plan?
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In Fitzpatrick V skin, acral and mucosal pigmentation is common and often benign - including parallel furrow pattern, lattice-like pattern, and benign acral nevi. This raises the risk of over-calling benign pigmentation, but does not make the parallel ridge pattern benign.
Practical implications:
- Do not dismiss a parallel ridge pattern because of skin type; it remains the pattern most associated with acral melanoma and warrants biopsy in a changing lesion.
- Conversely, do not biopsy every acral macule - many are benign physiologic or nevus-related pigmentation. The decision hinges on change, size, border irregularity, and the specific dermoscopic pattern.
- Use excisional biopsy rather than shave, given the need to assess architecture and depth.
- Photograph and map the lesion for comparison if observation were chosen - but here the change and pattern favor biopsy.
3. The excisional biopsy shows melanoma in situ with a positive deep margin. Outline your next steps, including margin and staging considerations.
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Melanoma in situ with a positive margin after excision:
- Re-excision to achieve clear margins. For melanoma in situ, margins of 5 mm are commonly used (up to 1 cm for lentigo maligna-type or large lesions), balancing clearance against morbidity on the sole where tissue is limited.
- Staging: melanoma in situ is stage 0 - no sentinel lymph node biopsy is indicated (SLNB applies to invasive melanoma above a threshold depth, generally >0.8 mm or with other risk features). No imaging staging for in situ disease.
- Margin assessment on acral skin: because the lesion is on the sole, re-excision may be constrained; consider whether complete histologic assessment of the re-excision specimen is feasible and whether mapping/serial sections are needed.
- Surveillance: regular skin exams, education on acral self-examination, and documentation.
Counsel the patient that in situ disease has an excellent prognosis with complete excision, but recurrence risk means follow-up matters.