The Potential Utility of RAS Q61R Immunohistochemistry as a Screening Tool in Pre-operative Fine Needle Aspirates of Medullary Thyroid Carcinoma.

Medullary thyroid carcinoma (MTC) can either be sporadic, often via mutually exclusive RET or RAS alterations, or inherited via a RET germline alteration. Germline testing is recommended for all patients diagnosed with MTC. RAS p.Q61R immunohistochemistry (RASQ61R-IHC) can identify a subset of RAS-mutated MTCs on resections, but whether this could be applied pre-operatively to cytology specimens remains unclear. Herein, we assessed RASQ61R-IHC in a tri-institutional cohort of cytologic and histologic MTC specimens with available molecular and germline data. Thirty-four fine needle aspirates with cell blocks were identified between three institutions from 2009 to 2024 with corresponding histology. Tumor sequencing and germline data were recorded, if available. RASQ61R-IHC was scored on staining intensity with documentation of membranous accentuation. Sensitivity, specificity, positive predictive (PPV), and negative predictive values (NPV) were calculated. Of the MTCs, 29% were germline-mutated, and 71% were sporadic. Among all sporadic MTCs (n = 22), 41% were RET-altered, 27% were RAS-altered, and 31.8% did not have available data. With any RASQ61R-IHC staining considered positive, sensitivity, specificity, PPV, and NPV for detecting RAS p.Q61R-mutated MTCs were 100%, 72.7%, 45.4%, and 100%, respectively. Requiring a stain score of > 1 and/or membranous accentuation for a true positive changed sensitivity, specificity, PPV, and NPV to 100%, 100%, 100%, and 100%, respectively. RASQ61R-IHC membranous staining was 100% predictive of RET-negative germline testing. RASQ61R-IHC, when requiring a score > 1 and/or membranous stain accentuation for true positive, had high sensitivity and specificity for RAS p.Q61R mutation in cytologic and surgical MTC specimens. Moreover, RASQ61R-IHC is a rapid and inexpensive modality that could potentially tailor which MTC patients undergo germline testing.