Introduction
Basaloid follicular hamartoma (BFH) is a rare benign neoplasm of the hair follicle. BFHs have clinical and histopathologic similarities to basal cell carcinomas (BCCs), and they both involve mutations in the patched gene (PTCH-1).1 Here, we present a case of a patient with BCCs with smoothened (SMO) mutations developing in a Blaschkoid BFH on the left chest.
Case report
A 35-year-old man presented to undergo Mohs micrographic surgery (MMS) for a biopsy-proven pigmented superficial, nodular, and infiltrative BCC on the central chest. The lesion had been present for 2 months and had developed within a birthmark (Fig 1).
Fig 1.
Initial evaluation in the dermatology clinic showed a 7-mm pigmented papule on the central chest within the medial component of a band of firm, slightly verrucous light brown papules extending from the sternum to left clavicle in a Blaschkoid distribution.
He had a previous history of BCC that had also developed in the birthmark and was treated with excision 4 years prior.
Upon histopathologic examination, the MMS specimen showed 2 distinct basaloid proliferations with areas of basaloid epithelioid cells in strands and islets forming a radial pattern felt to be more consistent with a BFH, and other areas of confluent basaloid cells emanating from the epidermis with retraction artifact and surrounding inflammation consistent with superficial BCC (Fig 2).
Fig 2.
Mohs specimen from the central chest demonstrating 2 distinct basaloid neoplasms. On the left are confluent basaloid cells emanating from the epidermis with retraction artifact and on the right are basaloid epithelioid cells in strands and islets forming a radial pattern. Hematoxylin and eosin, ×20.
There were no areas of frank BCC present after 4 stages of MMS, and the defect was repaired with a linear closure. Given the clinicopathologic conundrum this case presented, a shave biopsy was taken from the left medial clavicle (Fig 3) and processed for frozen section examination to elucidate the pathology of the patient’s birthmark. This specimen had the same features as the MMS specimen, with findings of both BFH and BCC (Fig 4), and the area was treated with electrodessication and curettage.
Fig 3.
Demarcation of shave biopsy site on the left medial clavicle distant from the Mohs site.
Fig 4.
Shave biopsy specimen from the left medial clavicle demonstrating 2 distinct basaloid neoplasms. On the left are findings consistent with benign follicular hamartoma and on the right are those of superficial basal cell carcinoma. Hematoxylin and eosin, ×40.
FoundationOne CDx testing was performed on the initial biopsy specimen as well as the biopsy obtained at the time of MMS. Multiple gene variants were identified with the most clinically relevant being SMO p.L412F. Other gene variants included APC R2226, KIT exon 13, AMER1 R531, RAC1 P29S, and TERT promoter-146C. PTCH-1/2 mutations were notably absent. We elected to monitor the patient with regular skin examinations and treat symptomatic lesions if they develop.
Discussion
BFH is a benign tumor of hair follicles with histopathologic findings of anastomosing basaloid cells. It may present as single or multiple papules, a single linear unilateral plaque, or generalized with and without an associated disorder. It shares histopathologic characteristics with BCC and is often difficult to distinguish histologically.1
PTCH-1 and SMO are receptors involved in the hedgehog pathway and work to regulate cell cycle and differentiation. When active, SMO activates a cascade to signal for transcription of genes. A mutation in either the PTCH-1 tumor suppressor or the SMO proto-oncogene leads to an increased risk of abnormal cellular proliferation and carcinogenic effects. Both PTCH-1 and SMO mutations have been implicated in the pathogenesis of BCC,2,3 whereas PTCH-1 mutations are typically associated with BFH.4 Few isolated cases of segmental/linear BFH have been described with SMO variants.5
The possible risk of developing BCC within a BFH is potentially due to shared somatic mutations within the hedgehog pathway, which creates a permissive environment for unregulated cell growth and proliferation. Although BFHs are predominantly caused by PTCH-1 mutations, our case adds to a growing body of evidence linking SMO pathway activation to follicular neoplasms and highlights the importance of surveilling for malignant transformation in these patients.
Conflicts of interest
None disclosed.
Footnotes
Funding sources: None.
Patient consent: The authors attest that they have obtained written consent from patient/s, their legal guardian/s or person/s with legal authority, for their photographs and medical information to be published in print and online and with the understanding that this information may be publicly available. Patient consent forms were not provided to the journal but are retained by the authors to be made available upon request.
IRB approval status: Not applicable.
References
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