Theory 044/Medicine & Health /Screening
Screening Intervals Follow Doubling Time
Intervals are set by convention. They should be set by how fast the thing grows.
The optimal screening interval scales with tumour doubling time rather than being a fixed number of years, so fast-growing cancers need shorter intervals and slow-growing ones are over-screened by current schedules.
01The evidence
Screening intervals differ across programmes for the same organ and the same test, which suggests convention rather than derivation.
02The tension
Intervals are chosen by trial and error and then applied uniformly, because doubling time is unknown for an individual before diagnosis.
03The mechanism
A cancer is detectable for a window set by its growth rate. To catch it while treatment is more effective, the interval must be short relative to the time it spends in the detectable preclinical phase.
That makes interval a function of doubling time, and predicts that a fixed interval will over-screen slow cancers and under-screen fast ones, with the balance set by the distribution of doubling times.
04What this is not
Fixed intervals treat all cancers as equally fast. This scales the interval to growth, which reallocates screening effort.
05Predictions
- P1Benefit should improve when intervals are scaled to growth rate.
- P2Fast cancers should be missed by long intervals.
- P3Slow cancers should be over-detected by short ones.
06Refutation
Fixed intervals performing as well as scaled ones would leave convention adequate.
07What it changes
It would personalise screening schedules.