EP1050 - Perianal Basal Cell Carcinoma In The Setting Of NIGRO Protocol Treated Squamous Cell Carcinoma: A Case Report

Purpose/Background: Basal cell carcinoma (BCC) in the anal and perianal region is extremely rare and makes up about 0.2% of anorectal neoplasms. Cases of radiation induced BCC have been reported, however no literature has been published regarding primary anal BCC in areas of previously irradiated anal carcinomas. Furthermore, differentiating between BCC and basaloid squamous cell carcinoma (BSCC) using morphologic and immunohistochemical (IHC) characteristics is challenging (Table 1). The patient was a 70-year-old female with a history of hypertension, hypothyroidism, no surgical history, an unknown family history of colorectal cancer or other malignancies, and no history of smoking. In 2020 she was diagnosed with anal squamous cell cancer with a negative metastatic workup.

Methods/Interventions: The patient completed NIGRO protocol, however after 2 years in remission the patient developed a new anal margin lesion. An excisional biopsy revealed BCC with positive margins. A wide local excision (WLE) with negative margins was completed. Nine months later the patient presented with ulcerated mucosa at the anal verge. PET-CT showed uptake in the rectum consistent with recurrence, as well as in the right presacral region with regional lymph node involvement. Exam under anesthesia (EUA) and biopsy revealed BSCC. A salvage abdominoperineal resection (APR) was completed with pathology and HPV testing confirming invasive basaloid squamous cell carcinoma. Of note, the original SCC treatment, WLE, EUA and APR were all performed at different institutions utilizing different multidisciplinary tumor boards.

Results/Outcomes: WLE histologic sections showed an ulcerated tumor consisting of nests of basaloid cells with peripheral cells focally "budding" from the basal surface of the epidermis and atypical mitotic figures. IHC staining was positive for Ber-EP4, p16 (without HPV confirmation testing), and CD56, but negative for squamous cell markers p40 and p63. Findings were consistent with BCC. EUA biopsy and the surgical specimen histological sections showed cells arranged in sheets and large nests, cells with high nuclear to cytoplasmic ratios, no clefting artifact, focal areas that showed peripheral basal palisading, frequent atypical mitotic figures, and coagulative necrosis. Stains were weakly positive for p63, positive for P16, BCL2, MOC31, and diffusely positive for BerEP4 (Table 2).

Conclusion/Discussion: We present a novel case of BCC diagnosed in an area of prior irradiated SCC. IHC and morphological results, including diffuses expression of Ber-EP4, BCL-2 positivity, and basal nuclear palisading are significantly associated with BCC; however, in this case they were present in the BSCC surgical specimen. The overlap of the pathologic features in the patients' specimens highlights the difficulty in differentiating BCC from BSCC. Hybridization of high-risk HPV is critical in confirming the diagnosis of BSCC. Treatment algorithms differ depending on the diagnosis of BCC or a basaloid variant, which affect treatment and prognosis.

Morphological and immunohistochemical features of BCC and BSCC
Timeline of events