Final diagnosis:

Mitotically Active Compound Nevus of Pregnancy and Nodal Nevi Misdiagnosed as Stage III Melanoma

Discussion:

About 10% of pregnant women develop changes to preexisting moles; including itching, increase in size and/or increase in pigmentation. During pregnancy, nevi present on the anterior surface of the body show increase in size more significant compared to nevi in other locations. Benign melanocytic proliferations during pregnancy can show worrisome features concerning for malignant melanoma. Up to 4 mitoses/mm2 including in the deep dermis have been previously noted in some reports.

In our case, the lesion showed histologic features worrisome for melanoma, including sheet-like growth and increased mitotic activity which lead to the misdiagnosis rendered by the outside institution. Several studies have shown that melanocytic nevi in pregnancy tend to have minimal to no cytologic atypia along with prominent cytological and architectural maturation, which was present in our case. Nodal nevi are relatively common, with reported incidence reaching up to 22% in some studies.  

Awareness of such phenomenon coupled with careful histopathologic examination and potentially genetic testing is crucial to render the correct diagnosis and avoid unnecessary treatment.

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List of References:

1. Bieber AK, Martires KJ, Driscoll MS, Grant-Kels JM, Pomeranz MK1, Stein JA. Nevi and pregnancy. J Am Acad Dermatol. 2016 Oct;75(4):661-666.

2. Chan MP, Chan MM, Tahan SR. Melanocytic nevi in pregnancy: histologic features and Ki-67 proliferation index. J Cutan Pathol. 2010 Aug;37(8):843-51.

3. Caron M. Grin, M.D., Department of Dermatology, MC 6230, University of Connecticut Health Center, 263 Farmington Avenue, Farmington, CT 06030, USA. Does pregnancy alter melanocytic nevi? J Cutan Pathol. 2001 Sep;28(8):389-92.

4. Driscoll MS, Grant-Kels JM. Nevi and melanoma in pregnancy. Dermatol Clin. 2006 Apr;24(2):199-204.

5. Piana S, Tagliavini E, Ragazzi M, Zanelli M, Zalaudek I, Ciarrocchi A, Valli R. Lymph node melanocytic nevi: pathogenesis and differential diagnoses, with special reference to p16 reactivity. Pathol Res Pract. 2015 May;211(5):381-8.

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