Background: LS is among the most common cancer (CA) syndromes affecting 1:280 individuals and portends high lifetime CA risk. Clinical trials are currently evaluating whether frameshift peptide vaccination (FSPVAX) could reduce LS CA risk. FSPVAX enhances immune response to diverse FSP neoantigens in MSI-H tumors and has shown promising results in an MSH2-mutant mouse model and metastatic MSI-H CRC. However, negative public attitudes towards vaccination and evidence suggesting low uptake of ASA chemoprevention (CP) among patients with LS could undermine success of this novel therapy. Here, we benchmark attitudes towards FSPVAX for CA prevention against colonoscopy (COLO) and aspirin or NSAID chemoprevention (CP). Methods: The PREVENTLynchII survey (IRB 20-8014) was distributed electronically to 1) participants in the Fox Chase Cancer Center (FCCC) Risk Registry and 2) members of two online LS support groups. Patients were invited to complete the one-time survey after providing informed consent. Demographic and personal CA history were collected. Attitudes were measured on 9-point scales indicating low vs high agreement with belief statements: e.g. “Colonoscopy is a convenient way to lower my risk of LS-related CA.”Results: Overall 296 participants at FCCC, nationally, and internationally completed the survey. Median age was 52.6 years [23-78], 84% female, and 95% White. Affected genes included MLH1 (20.3%), MSH2/EPCAM (35.5%), MSH6 (28.7%), and PMS2 (15.5%). Use of CP was uncommon (32.1% and 5.1% respectively). Stratified analyses of COLO perceptions by sex, age, and personal history of CA showed no differences. Interestingly, US pts viewed ASA as more convenient than non-US, and with lower risk of side effects (p=0.007), but were no more likely to use or recommend CP. Compared to COLO, FSPVAX was overall viewed as more convenient, but side effect concerns were higher, and perceptions of efficacy were lower (all p<0.001), while compared to CP, FSPVAX was less convenient and fostered greater concerns about side effects, but was viewed as having higher potential preventive efficacy (all p<0.001). FSPVAX perceptions showed high variability: pts >50 (p=0.003) and CA pts (p=0.01) saw FSP as more convenient (p=0.003) than younger and unaffected pts. CA pts reported higher reassurance than unaffected (p=0.04), while those >50 were more likely to participate in an FSPVAX study (p=0.04). US pts perceived more side effect risk of FSPVAX than non-US (p=0.003). Conclusions: Concerns about ASA CP and FSPVAX convenience, side effects, and efficacy are common among US and international patients with LS.

*All comparison for FSPVAX to COLO and FSPVAX to ASA CP p<0.001.