Ambiguous Acral Lentiginous Melanocytic Lesion After Shave Biopsy
A 47-year-old woman has a 6 mm asymmetric, irregularly pigmented macule on the right heel that appeared 18 months ago. A shave biopsy was performed by a community dermatologist. The report describes a confluent lentiginous proliferation of atypical melanocytes with pagetoid scatter and 'no definitive dermal invasion'; margins are positive. Immunohistochemistry shows diffuse PRAME nuclear positivity and retained p16. The lesion is reviewed at your multidisciplinary conference. On examination, there is a healing shave site with residual pigment at the edge; no inguinal lymphadenopathy. She has no personal or family history of melanoma, and she is Fitzpatrick type IV. The pathologist asks whether this is an acral nevus or acral lentiginous melanoma.
1. What are the limitations of the biopsy, and what is your next diagnostic step?
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A shave biopsy is inadequate for a pigmented acral lesion because it distorts architecture and cannot reliably measure Breslow thickness or distinguish a junctional component from dermal invasion. The report already shows concerning features, including confluent lentiginous atypia and pagetoid scatter, but 'no definitive dermal invasion' may reflect sampling rather than biology. Diffuse strong PRAME positivity supports melanoma but is not perfectly specific; retained p16 does not exclude melanoma, because p16 loss is absent in many melanomas. The next step is a full-thickness diagnostic procedure at the same site, such as a deep punch or narrow excisional biopsy, with expert dermatopathology review and clinical photographs. Correlate with dermoscopy and the residual pigment at the margin. Do not perform a wide excision until the lesion has been completely sampled and Breslow thickness is known.
2. If the repeat biopsy shows Breslow 0.6 mm, no ulceration, and 2 mitoses/mm², how do you stage and what surgical and nodal management do you recommend?
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Under AJCC eighth edition, a melanoma less than 0.8 mm thick with at least one mitosis/mm² is T1b; with no nodal or distant disease, the stage is IB. The 2 mitoses/mm² upstages the lesion despite the low Breslow thickness. Recommend wide local excision with a 1 cm margin, accepting that the heel location may require reconstruction or a graft to preserve function. Discuss sentinel lymph node biopsy: it is generally offered for T1b melanoma because the mitosis rate increases nodal risk, although the absolute benefit is small and the decision should be shared. If sentinel node biopsy is chosen, it can be coordinated with the wide excision using radiocolloid and blue dye. Refer to a melanoma multidisciplinary team, discuss surveillance, and document that the original shave biopsy was insufficient for staging.
3. How would you counsel her about the PRAME and p16 results while awaiting the repeat biopsy?
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Explain that PRAME and p16 are adjuncts, not stand-alone diagnoses. Diffuse PRAME positivity raises concern for melanoma, but it can occur in some benign melanocytic lesions, and retained p16 does not rule out melanoma. The shave biopsy did not provide a full-thickness sample, so the most important missing information is the true architecture and Breslow depth. Avoid declaring the lesion benign or malignant until complete sampling and expert review are done. Acknowledge the uncertainty, schedule the repeat biopsy promptly, and explain that the final plan, including margins and possible lymph node sampling, depends on the full-thickness pathology. Document the discussion and provide a copy of the plan.