I agree with Pavel_Roshanovon the site characteristics as a predictor of recruitment outcome. I would also add to do a focus group analysis with some potential investigators whether the design is feasible for recruitment.
Pharmaceutical companies generally conduct workshops with a sample of target investigators so they can design the study based on realistic accrual expectations but i don’t think this is a common practice in studies designed by academic centers.
I have extensive experience running both NIH and pharmaceutical company sponsored RCTs in community oncology setting. Some studies end up accruing really poorly because the inclusion criteria were too restrictive or impractical. If a requirement is not aligned with the site’s current practice, it may not enroll even one patient. The unrealistic follow-up requirements might also discourage enrollment.
Sometimes, even a pilot may not be informative about the success of recruitment if the pilot site is not a representative of the rest of the sites, but still will have benefits that Pavel_Roshanov mentioned.
Good luck.