# Agree in advance how to borrow evidence between similar rare cancers

Source: https://onco.cc/ideas/idea-bio2-bayesian-borrowing-acceptance/  
OnCo record `idea-bio2-bayesian-borrowing-acceptance` (Idea). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

Statistical methods can combine information across similar rare cancers to reach an answer with fewer patients. Regulators need to say in advance when that is acceptable.

## Summary

Hierarchical Bayesian models with borrowing across histologies or across related rare diseases can substantially reduce required sample size, and are used in basket trial analyses. Sponsors avoid them because acceptance is uncertain at review. A published position on acceptable borrowing structures, prior specification and pre-registration requirements would let sponsors design smaller trials with confidence.

## Fields

- Kind: Idea
- Last checked: 2026-09-08
- Hypothesis: Regulatory clarity on borrowing methods reduces the median sample size of rare cancer registrational trials by a third without increasing the rate of subsequent effect reversals in confirmatory data.
- Rationale: Similar clarity on adaptive designs and on external control arms rapidly changed practice once guidance existed. The statistical machinery is mature; the missing element is a pre-agreed acceptability boundary.
- Proposed test: Simulation study across historical rare cancer datasets to quantify type I error and bias under candidate borrowing structures, submitted jointly by academic statisticians and regulators as the basis for guidance.
- Maturity: speculative
- Actor: regulator

## Sources

- Bottleneck evidence (Rare and paediatric cancers without markets): Gatta et al., Rare cancers are not so rare: the rare cancer burden in Europe (EJC 2011): https://doi.org/10.1016/j.ejca.2011.08.008

## Connected records

- institutions: [American Society of Clinical Oncology (ASCO)](https://onco.cc/institutions/asco/), [European Society for Medical Oncology (ESMO)](https://onco.cc/institutions/esmo/)
- terms: [Basket, umbrella, and platform trials](https://onco.cc/terms/basket-umbrella-platform/), [Hazard ratio (HR)](https://onco.cc/terms/hazard-ratio/)
- bottlenecks: [Rare and paediatric cancers without markets](https://onco.cc/bottlenecks/b-rare-cancers/), [Regulatory divergence between regions](https://onco.cc/bottlenecks/b-regulatory-fragmentation/), [Trial design, endpoints and cost](https://onco.cc/bottlenecks/b-trial-design/)
- key papers: [Rare cancers are not so rare: the rare cancer burden in Europe](https://onco.cc/key-papers/paper-gatta-eur-j-cancer/)

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