Combined utility of p16 and BRAF V600E in the evaluation of spitzoid tumors: Superiority to PRAME and correlation with FISH

Abstract

Background: Spitzoid melanocytic neoplasms are diagnostically challenging; criteria for malignancy continue to evolve. The ability to predict chromosomal abnormalities with immunohistochemistry (IHC) could help select cases requiring chromosomal evaluation.

Methods: Fluorescence in situ hybridization (FISH)-tested spitzoid neoplasms at our institution (2013-2021) were reviewed. p16, BRAF V600E, and preferentially expressed antigen in melanoma (PRAME) IHC results were correlated with FISH.

Results: A total of 174 cases (1.9F:1M, median age 28 years; range, 5 months-74 years) were included; final diagnoses: Spitz nevus (11%), atypical Spitz tumor (47%), spitzoid dysplastic nevus (9%), and spitzoid melanoma (32%). Sixty (34%) were FISH positive, most commonly with absolute 6p25 gain (RREB1 > 2). Dermal mitotic count was the only clinicopathologic predictor of FISH. Among IHC-stained cases, p16 was lost in 55 of 134 cases (41%); loss correlated with FISH positive (p < 0.001, Fisher exact test). BRAF V600E (14/88, 16%) and PRAME (15/56, 27%) expression did not correlate with FISH alone (p = 0.242 and p = 0.359, respectively, Fisher exact test). When examined together, however, p16-retained/BRAF V600E-negative lesions had low FISH-positive rates (5/37, 14%; 4/37, 11% not counting isolated MYB loss); all other marker combinations had high rates (56%-75% of cases; p < 0.001).

Conclusions: p16/BRAF V600E IHC predicts FISH results. "Low-risk" lesions (p16⁺ /BRAF V600E^- ) uncommonly have meaningful FISH abnormalities (11%). PRAME may have limited utility in this setting.

Keywords: BRAF; FISH; PRAME; Spitz; malignant; p16.

FIGURE 1

Table 5, Case 3: 3‐year‐old boy with ear lesion, Category 1, FISH positive, called atypical Spitz tumor. (A) Sections show a mostly intradermal melanocytic proliferation with ulceration and inflammatory crust (H&E, ×20); left inset shows p16 is retained in a checkboard pattern (p16, ×40); right inset shows BRAF V600E is negative (BRAF V600E, ×40). (B) Cells are arranged in sheets, small nests, and cords with scattered dermal mitotic figures (arrows) (H&E, ×400). (C) Scattered multinucleated cells are noted (arrows) (H&E, ×400). (D) Cells are fairly uniform, with abundant amphophilic cytoplasm, open chromatin, irregular nuclear contours (arrows), and prominent nucleoli (H&E, ×400).

FIGURE 2

Table 5, Case 4: 11‐year‐old boy with a forearm lesion, Category 1, FISH positive, called spitzoid melanoma. Sections show a large, mostly intradermal melanocytic proliferation with brisk inflammation (H&E, ×10); left inset shows p16 is retained diffusely (p16, ×40); middle inset shows BRAF V600E is negative (BRAF V600E, ×40); right inset shows Ki67 highlights a portion of the atypical melanocytes (Ki67, ×200). (B) Cells are arranged in sheets and nests with impaired maturation, chronic inflammation, and patchy, irregular pigmentation (arrows) (H&E, ×200). (C) Cells range from large to small with multinucleation, abundant amphophilic cytoplasm, open chromatin, irregular nuclear contours with multilobation, and prominent nucleoli (arrows) (H&E, ×200). (D) Focal junctional activity is noted with rare pagetoid cells (arrows) (H&E, ×200).

FIGURE 3

Table 5, Case 5: 20‐year‐old woman with a shoulder lesion, Category 1, FISH negative, called spitzoid melanoma. (A) Sections show a broad compound melanocytic proliferation with dermal fibrosis and inflammation. Cells are arranged in large and small nests and cords (H&E, ×10). (B) Junctional nests show patchy pigmentation (arrow) and pagetoid spread (arrow). Cells have irregular nuclear contours with multilobation and prominent nucleoli (H&E, ×200). (C) Nuclei range from large to small with open to dense chromatin and moderate amphophilic cytoplasm (H&E, ×200). (D) p16 was positive (p16, ×100); inset shows BRAF V600E was negative (BRAF V600E, ×100). (E) Melan‐A/Ki67 multiplex staining highlighted an elevated dermal proliferative index (Melan‐A red/Ki67 brown, ×100).

FIGURE 4

Table 5, Cases 7–9—(A, B) Case 7: 13‐year‐old boy with a thigh lesion, Category 2, FISH positive, called atypical Spitz tumor. (A) The lesion shows a large intradermal, nested melanocytic proliferation with impaired maturation (inset, H&E, ×10); cells are fusiform and uniform with amphophilic cytoplasm, open to coarse chromatin, prominent nucleoli, and scattered mitotic figures (arrows) (H&E, ×400). (B) Case 8: 21‐year‐old woman with an abdominal lesion, Category 3, FISH positive, called dysplastic nevus with spitzoid features. (B) The lesion is a compound melanocytic proliferation with irregular nests, bridging of nests, and melanoderma (inset, H&E, ×10); cells are whorled within nests, and show a mix of cells with open chromatin and pale, dustily pigmented cytoplasm as well as more hyperchromatic nuclei (H&E, ×200). (C, D) Case 9—37‐year‐old woman with a calf lesion, Category 4, FISH negative, called spitzoid melanoma. (C) The lesion is a compound melanocytic proliferation with chronic inflammation (inset, H&E, ×40) and lateral pagetoid spread (arrows) (H&E, ×200). (D) Cells have abundant amphophilic cytoplasm, and enlarged nuclei with inclusions, irregular nuclear membrane contours/multilobation (arrows), and prominent nucleoli (H&E, ×400).