Feasibility of Pap-derived ctDNA for detection of sporadic and Lynch-associated endometrial cancer.

10503 Background: Patients (pts) with Lynch Syndrome (LS) have high risk of endometrial cancer (EC) and are recommended risk-reducing hysterectomy (RR-Hys/BSO) due to lack of effective screening. As plasma-derived circulating tumor DNA (plasma-ctDNA) lacks adequate sensitivity in EC, we sought to evaluate Pap-derived ctDNA (Pap-ctDNA) as a novel, non-invasive assay for EC detection. Methods: Plasma-ctDNA and Pap-ctDNA were obtained from pts with EC undergoing surgery or LS undergoing RR-Hys/BSO under an IRB-approved protocol. Paired tumor/normal sequencing of ECs was compared to genomic findings from plasma-ctDNA and Pap-ctDNA. Somatic variants in plasma and Pap samples were genotyped using matched tumor tissue, with average allele frequency (VAF) calculated. Tumors were assessed for MSI and/or MMRD via MSISensor or IHC. In LS pts, pathology for precursor lesions was assessed. Results: 19 pts (16 EC, 3 LS) underwent prospective collection of 42 samples (20 plasma, 22 Pap). 56% of ECs were low-grade endometrioid and 81% early-stage (I/II) (Table). 12.5% of ECs were MMRD due to MLH1 hypermethylation. Median yield of Pap-ctDNA was higher than plasma-ctDNA (91ng vs. 19.5ng; p=0.0004). 94% of ECs had mutations covered within target regions of both assays, with mutations detected in 93% of Pap-ctDNA, but only 33% of plasma-ctDNA. In pts with mutations detected in both assays, median VAF was higher in Pap-ctDNA vs. plasma-ctDNA in all 5 cases (Table; p =0.012). Among the 13 pts with early-stage EC, 92% of Pap-ctDNA vs 23% of plasma-ctDNA were positive. Of 3 LS pts, one had a focus of MMRD atypical hyperplasia concordant with germline MMR mutation; plasma and Pap-ctDNA were negative. Conclusions: Pap-ctDNA from EC pts results in higher ctDNA yield than plasma-based testing. Tumor mutations were detected in >90% of Pap-ctDNA even in early-stage EC, versus only 23% of plasma-ctDNA samples. Pap-ctDNA for early-detection of EC is feasible and is a promising tool for average and high-risk individuals with potential applicability in other neoplasms. EC characteristics. ID Histology Stage MSS/ MMRD Plasma ctDNA yield (ng) Pap ctDNA Yield (ng) Avg VAF mutations-Plasma Avg VAF mutations-Pap 1 G2 Endometrioid IA MSS 19.49 36.83 - - 2 G1 Endometrioid IA MSS 57.27 144.97 - 0.00014 3 G1 Endometrioid IA MSS 44.3 179.2 - 0.006 4 Carcinosarcoma IIIC MSS 23.78 28.21 0.0062 0.279 5 Serous IV MSS 38.45 17.03 0.0046 0.403 6 G1 Endometrioid IA MSS 13.76 21.38 - 0.004 7 Mesonephric-like II MSS 21.34 31.3 - 0.123 8 G2 Endometrioid IA MMRD- MLH1 hypermeth 10.67 185.5 0.0092 0.119 9 G1 Endometrioid IA MSS 23.92 27.35 - 0.049 10 Serous IA MSS 15.61 110 -