A dark plantar macule in a patient with Fitzpatrick type V skin
A 34-year-old woman with Fitzpatrick type V skin is seen for a routine skin check. She mentions a mole on the sole of her right foot that her husband says looks darker than a year ago, though she is not certain it has changed. Examination shows a 9 x 7 mm asymmetric, irregularly bordered, dark brown-to-black macule on the non-weight-bearing arch of the right plantar foot. Dermoscopy shows areas of parallel ridge pattern, with irregular thick brown-black bands running along the ridges, plus a focal area of diffuse structureless gray-black pigmentation; the furrows are spared. There is no ulceration, bleeding, or nodularity, and no inguinal lymphadenopathy. She has no personal or family history of melanoma, no prior biopsies, and no immunosuppression. She is anxious about scarring on the foot and asks whether we can just watch it for six months.
1. Interpret the dermoscopic findings and justify what you do next.
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The parallel ridge pattern - pigment running along the ridges of glabrous skin - is the finding that matters. Benign acral nevi and acral melanocytic hyperplasia typically show parallel furrow, lattice-like, fibrillar, or homogeneous patterns confined to the furrows. The ridge pattern is the melanoma pattern, with reported specificity above 95% and sensitivity in the 80s; the focal diffuse structureless gray-black area is a second melanoma-associated feature. Focal ridge pigmentation can occur in benign lesions, so pattern must be weighed with morphology: this lesion is 9 mm, asymmetric, irregularly bordered, and reportedly darkening.
Observation is not defensible. Acral melanoma in situ is easily under-recognized, delay is the main driver of poor outcome at this site, and the lesion is small enough for straightforward excision. The next step is full-thickness excisional biopsy for histology - not a shave, not a punch at the periphery, and not serial photography. A negative dermoscopic exam elsewhere and a normal nodal exam do not change that. Explain to her that a scar on the arch of the foot is functionally trivial compared with the risk of missing a melanoma, and that the pathology result guides everything afterward.
2. Describe the biopsy you perform and the technical errors to avoid.
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Excise the entire lesion with a 2-3 mm clinical margin using a fusiform elliptical incision oriented along the relaxed skin tension lines; the arch skin is relatively thin and usually closes primarily. Take the specimen down to subcutaneous fat. A deep, full-thickness specimen is mandatory because Breslow thickness and mitotic rate cannot be assessed on a transected base, and mis-staging changes both wide-local-excision margins and sentinel node discussions.
Avoid: a shave or curette (partial sampling, unreadable base); a 3 mm punch placed centrally (leaves residual pigmentation and may miss an eccentric invasive focus); diagnostic cautery; and crushing the specimen with forceps. Orient the specimen for the pathologist with a suture or clip, and supply the clinical and dermoscopic description - acral lentiginous melanoma has a lentiginous intraepidermal component that is sometimes undercalled without context.
If a lesion were too large for primary excision, a full-thickness incisional biopsy from the most atypical area would be second-best, accepting that Breslow may still be unmeasurable. Document the dermoscopic pattern in the chart, since it supports the histologic interpretation.
3. Pathology returns acral lentiginous melanoma, Breslow 1.2 mm, non-ulcerated, 1 mitosis/mm2. What are the staging and local treatment steps, and how do you counsel about sentinel lymph node biopsy?
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This is T2a disease. Next steps are wide local excision with 1-2 cm margins; for 1.01-2 mm, 1 cm is acceptable and is reasonable on the foot where tissue is limited, and re-excision can often be combined with the sentinel node procedure in one anesthetic.
Sentinel lymph node biopsy should be offered and discussed: it is recommended at T2a and above, is primarily prognostic, and determines eligibility for adjuvant therapy or trials. The risk of a positive node at 1.2 mm without ulceration is roughly 10-15%, and the procedure itself carries small risks of seroma, infection, and lymphedema. It is not therapeutic in its own right, so patient preference after informed discussion is legitimate.
Routine staging imaging is not required for asymptomatic stage IB, though baseline imaging is often obtained if adjuvant therapy is being considered. Counsel her that acral melanoma is not behaviorally different stage for stage - late diagnosis at this site explains much of the prognosis gap - and that she needs lifelong total skin exams plus self-examination of palms, soles, and nail units. Genetic testing is not indicated without family history. Reassure about foot function after wide excision.