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Diagnosis and Discussion: Case 1163

Final Diagnosis

BASALOID NEOPLASM IN A BACKGROUND OF NEVUS SEBACEOUS (ORGANOID NEVUS OF JADASSOHN).

RESECTION MARGINS ARE FREE OF THE BASALOID LESION.

ADJACENT INCIDENTAL INTRADERMAL MELANOCYTIC NEVUS.

Comment: “Overall, given the histologic and IHC findings, we considered that the basaloid proliferation best represents a variant of basal cell carcinoma.”

Discussion

Nevus sebaceous is a hamartoma of major skin components including ample sebaceous glands without cellular atypia but often with architectural distortion.  They are most commonly found on the face and scalp. These lesions are benign, but there is a low chance (<10%) of transformation into secondary neoplasms. Various large retrospective studies have identified that the most common benign secondary neoplasms are trichoblastoma and syringocystadenoma papilliferum, while the most common malignant secondary tumor is basal cell carcinoma.[1,2,3,4] Notably, these studies found that malignant transformation occurred almost exclusively in adult patients, while very few children developed secondary neoplasms, all of which were benign tumors.

In this case, the diagnostic challenge was to distinguish whether this lesion should be called a trichoblastoma or a basal cell carcinoma, which is less likely to occur in children in this context. Several studies have pointed out the challenges of distinguishing these lesions especially in the context of nevus sebaceous.[4,5]  Trichoblastoma is a benign biphasic neoplasm with dual differentiation towards follicular germinative epithelium and follicular papillae. They can exhibit a variety of histological patterns, some of which are very similar to a conventional basal cell carcinoma or a Merkel cell carcinoma. Notably, trichoblastomas have been shown to retain the presence of CK20-positive Merkel cells within the lesion. They may exhibit calcification or keratin horn cysts, which may elicit a foreign body giant cell reaction.

Molecular features may be helpful in distinguishing these two lesions. Nevus sebaceous can transform into BCC in about 1% of cases primarily through HRAS mutations, which promote cell growth and potential tumor development [6].  Basal cell carcinomas also may carry alterations in PTCH1, as well as TP53 and SMO. Some trichoblastomas have been found to carry alterations in CTNNB1, HRAS, and PTCH1.

Notably this case demonstrated both HRAS and PTCH1 alterations. However, it was still classified as a basal cell carcinoma based on histological features: this lesion did not have any retained CK20-positive Merkel cells, and the Ki67 percentage was quite high, favoring a malignant diagnosis.

References

  1. Cribier B, Scrivener Y, Grosshans E. Tumors arising in nevus sebaceus: A study of 596 cases. J Am Acad Dermatol. 2000 Feb;42(2 Pt 1):263-8. doi: 10.1016/S0190-9622(00)90136-1. PMID: 10642683.
  2. Idriss MH, Elston DM. Secondary neoplasms associated with nevus sebaceus of Jadassohn: a study of 707 cases. J Am Acad Dermatol. 2014 Feb;70(2):332-7. doi: 10.1016/j.jaad.2013.10.004. Epub 2013 Nov 20. PMID: 24268309.
  3. Hsu MC, Liau JY, Hong JL, Cheng Y, Liao YH, Chen JS, Sheen YS, Hong JB. Secondary neoplasms arising from nevus sebaceus: A retrospective study of 450 cases in Taiwan. J Dermatol. 2016 Feb;43(2):175-80. doi: 10.1111/1346-8138.13070. Epub 2015 Sep 12. PMID: 26361884.
  4. Karimian P, Zargar Javaheri N, Gharaei Nejad K, Eftekhari H, Rafiei E, Rafiei R. Histopathological Features of Nevus Sebaceous and Superimposed Skin Tumors. J Skin Cancer. 2025 May 26;2025:3828133. doi: 10.1155/jskc/3828133. PMID: 40458116; PMCID: PMC12129612.
  5. Sellheyer K, Cribier B, Nelson P, Kutzner H, Rütten A. Basaloid tumors in nevus sebaceus revisited: the follicular stem cell marker PHLDA1 (TDAG51) indicates that most are basal cell carcinomas and not trichoblastomas. J Cutan Pathol. 2013 May;40(5):455-62. doi: 10.1111/cup.12107. Epub 2013 Mar 14. PMID: 23489134.
  6. Krakowski AC, Hafeez F, Westheim A, Pan EY, Wilson M. Advanced basal cell carcinoma: What dermatologists need to know about diagnosis. J Am Acad Dermatol. 2022 Jun;86(6S):S1-S13. doi: 10.1016/j.jaad.2022.03.023. PMID: 35577405.