# A public rulebook for when an external or synthetic control arm is acceptable

Source: https://onco.cc/ideas/idea-tr1-external-control-rulebook/  
OnCo record `idea-tr1-external-control-rulebook` (Idea). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

Sometimes a trial cannot randomise, so the new drug is compared with past patients' records. Clear published rules on when that is allowed, and how it must be done, would replace case-by-case guesswork.

## Summary

Regulators publish a binding checklist for externally controlled trials: pre-registration of the comparator cohort and analysis before unblinding; target-trial emulation framing; minimum data quality (endpoint ascertainment, line of therapy, index date); quantitative bias analysis and tipping-point reporting; and the settings where it is acceptable (rare disease, effect size large relative to plausible confounding, randomisation infeasible). Submissions meeting the rulebook receive predictable review.

## Fields

- Kind: Idea
- Last checked: 2026-09-08
- Hypothesis: A published rulebook will increase the share of externally controlled oncology submissions that are accepted without a confirmatory-trial demand, while post-approval confirmatory results will not disagree with the external-control estimate more often than they do today for single-arm accelerated approvals.
- Rationale: External controls are already used informally and inconsistently. Cases where the historical comparison misled (later overturned by randomised data) share identifiable features (immortal-time bias, mismatched lines of therapy, endpoint drift) that a rulebook can exclude.
- Proposed test: Retrospectively apply the draft rulebook to past single-arm approvals with later randomised confirmation; check whether rulebook-compliant cases had smaller estimate discrepancies. Then pilot prospectively for two years.
- Maturity: early-clinical
- Actor: regulator

## Sources

- FDA Real-World Evidence programme: https://www.fda.gov/science-research/science-and-research-special-topics/real-world-evidence

## Connected records

- terms: [Accelerated approval](https://onco.cc/terms/accelerated-approval/), [Hazard ratio (HR)](https://onco.cc/terms/hazard-ratio/), [Real-world evidence](https://onco.cc/terms/real-world-evidence/)
- bottlenecks: [Rare and paediatric cancers without markets](https://onco.cc/bottlenecks/b-rare-cancers/), [Trial design, endpoints and cost](https://onco.cc/bottlenecks/b-trial-design/), [Weak real-world evidence and registries](https://onco.cc/bottlenecks/b-real-world-evidence/)
- roadmaps: [Trial modernisation roadmap: the randomised trial → platforms and adaptive designs → decentralised, pragmatic and always-on](https://onco.cc/roadmaps/trial-modernisation-roadmap/)

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