The Utility of RAS Q61R immunohistochemistry for Medullary Thyroid Carcinoma in Cytologic and Histologic Specimens

Summary: 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 Q61R immunohistochemistry can identify a subset of RAS Q61R-mutated MTCs on resections, but whether this could be applied pre-operatively to cytology specimens remains unclear. Herein, we assessed RAS Q61R immunohistochemistry 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 2009-2024 with corresponding histology. Tumor sequencing and germline data were recorded, if available. RAS Q61R immunohistochemistry 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 sporadic MTCs, 20.8% were RAS Q61R-mutated. With any RAS Q61R staining considered positive, the sensitivity, specificity, PPV, and NPV for detecting RAS Q61R-mutated MTCs were 100%, 90.9%, 66.7%, and 100%, respectively. The addition of strong membranous accentuation changed sensitivity, specificity, PPV and NPV to 100%, 100%, 100%, and 100%, respectively. RAS Q61R membranous staining was 100% predictive of RET-negative germline testing. RAS Q61R immunohistochemistry, when requiring membranous stain accentuation for a positive interpretation, had high sensitivity and specificity for RAS Q61R mutation in both cytologic and surgical MTC specimens. Moreover, the immunostain is a rapid and inexpensive modality that can potentially tailor which MTC patients undergo germline testing.