Real-world MMR/MSI testing patterns and factors associated with uptake across metastatic solid tumors: insights from a pan-tumor analysis in US community oncology

Background While current clinical guidelines recommend MMR/MSI testing across several solid tumor types, real-world implementation and timing of this testing remain poorly characterized. This study evaluated real-world MMR/MSI testing patterns and factors associated with testing in patients with metastatic solid tumors in the US. Methods This retrospective cohort study analyzed electronic health record data from adults diagnosed with metastatic colorectal, endometrial, gastroesophageal, or prostate cancer between 1 January 2018 and 30 June 2023, who received systemic treatment for metastatic disease; patients were followed from metastatic diagnosis until last visit or death through 31 December 2023. Patient characteristics and receipt of MMR/MSI testing over time were summarized descriptively, and a logistic regression model assessed factors associated with MMR/MSI testing. Results A total of 30, 177 patients were included (colorectal, n=11, 407; endometrial, n=2, 665; gastroesophageal, n=4, 246; prostate, n=11, 859). Overall MMR/MSI testing prevalence ranged from 37.0% in the prostate cancer cohort to 89.2% in the colorectal cancer cohort. Testing before metastatic diagnosis was relatively uncommon (3.8%–34.8% across cancers), and testing before first-line treatment was similarly infrequent (5.1%–42.8%). Patients in non-colorectal cancer cohorts had significantly lower odds of testing than colorectal patients. Age and current tobacco use were also associated with lower odds of testing, although interpretation of the tobacco association is limited by substantial smoking history missingness. Discussion Testing rates and timing varied substantially. Despite increases over time, MMR/MSI testing remains suboptimal in some cancers and is frequently conducted after key treatment decisions have been made, limiting opportunities to incorporate biomarker information into treatment planning.