Pigmented lesion on the sole with discordant dermoscopy and history
A 58-year-old woman presents with a 9 mm asymmetric, irregularly bordered pigmented macule on the right plantar arch, first noticed 2 years ago. She reports no change in size but occasional itching. Dermoscopy shows a parallel furrow pattern centrally with an area of parallel ridge pattern at the periphery and a small blue-white veil. She has no personal or family history of melanoma. The lesion is on a non-weight-bearing area. Your clinic has a dermatoscope but no confocal microscopy; the next available pigmented lesion clinic appointment is 6 weeks away, and the patient is anxious about waiting. She asks whether it can simply be watched.
1. How do the dermoscopic findings modify your pretest probability, and what is the appropriate next step?
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The parallel furrow pattern alone is a benign acral pattern, but the presence of a parallel ridge pattern (pigment on the ridges rather than furrows) is the dermoscopic hallmark of acral melanoma in situ and overrides the reassuring central pattern. A blue-white veil further raises concern for invasion or regression. This converts a low pretest probability into a high one, regardless of the 2-year stability (acral melanoma in situ can be slow-growing and clinically stable for years).
Next step: do not watch and wait. Perform an excisional/punch or deep saucerization biopsy of the most suspicious area (the ridge-pattern periphery), with orientation and clinical correlation, and send for histopathology with immunohistochemistry (S100, Melan-A, SOX10, HMB-45). Refer urgently to the pigmented lesion/dermato-oncology clinic rather than the routine 6-week slot. Photograph and map the lesion. Document the discussion.
2. The biopsy of the ridge-pattern focus reports 'increased melanocytes along the basal layer with focal pagetoid spread and no dermal invasion; margins involved.' How does this change management, and what are the pitfalls?
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This is consistent with acral melanoma in situ (or a severely atypical junctional melanocytic proliferation). Margin-involved in situ disease requires re-excision to achieve clear margins (typically 5 mm clinical margins for in situ melanoma, adjusted for anatomic constraints on the sole). Pitfalls:
- Sampling error: a single biopsy may miss an invasive component; the re-excision specimen must be thoroughly examined (step sections, IHC).
- Histologic mimics: acral nevi can show pagetoid scatter and lentiginous proliferation; correlation with the parallel ridge pattern and clinical size/border is essential. If the diagnosis is equivocal, expert dermatopathology review is warranted.
- Anatomic constraints: plantar skin re-excision can be morbid; involve plastic surgery if margins are difficult.
- Follow-up: even after clear margins, acral melanoma in situ has recurrence risk; clinical and dermoscopic surveillance of the scar and remainder of the foot is needed. Discuss the possibility of amputation only if margins cannot be achieved.