High-grade Endometrial Carcinomas With Solid Basaloid Morphology and Geographic Necrosis Lacking Definitive Pilomatrix-like Features: Clinicopathologic Characteristics Including Aggressive Behavior and Novel Molecular Events.

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High-grade endometrioid carcinomas occasionally demonstrate solid basaloid morphology with geographic necrosis (SB-GN). This pattern is among the defining features of pilomatrix-like high-grade endometrioid carcinoma (PiMHEC), a recently proposed tumor type which is additionally characterized by the presence of shadow cells, abnormal beta-catenin/CTNNB1 mutations, strong CDX2 expression, and poor outcomes. Clinicopathologic overlap between PiMHEC and other high-grade endometrial cancers with SB-GN has not been established. We screened 300 endometrial carcinomas on tissue microarray for SB-GN histology and performed a detailed whole-section morphologic review, immunohistochemical analysis, and next-generation sequencing on all cases bearing this pattern. Four (1.3%) demonstrated SB-GN. All 3 with clinical follow-up had extremely aggressive behavior despite being MMR-deficient; in contrast, only 27% of other MMR-deficient high-grade carcinomas recurred. One SB-GN case met most of the previously outlined diagnostic criteria for PiMHEC including abnormal beta-catenin/CTNNB1 (p.S37P variant) and strong CDX2 expression; notably, however, shadow cells were absent. This case also demonstrated a KRAS p.A59T pathogenic variant. The other 3 cases also lacked shadow cells; the 2 with sequencing data bore no CTNNB1 abnormalities but showed likely oncogenic variants involving the pilomatrixoma-associated gene FGFR2. All 3 cases with molecular results also bore somatic Notch pathway (NOTCH1/NOTCH2/NOTCH3) variants. The single case treated with immunotherapy showed complete and sustained response with regression of bone metastases despite abnormal beta-catenin/CTNNB1, which has been associated with immunotherapeutic resistance. These data suggest that the SB-GN pattern may connote a poor prognosis even in the absence of overt pilomatrix-like differentiation, and that novel molecular events may have implications for the treatment of these tumors.