Calculator
Recurrence hazard is highest in the first 1–2 years, then falls.
Recurrence + second primary, in the early (≈ first 2 years) period.
Share of events found by a scheduled scan before symptoms.
Detected events leading to curative or life-prolonging treatment.
Results
Effective annual hazard
–
Peak risk × time-since-treatment factor.
Interval event risk
–
Probability of an event during one interval.
Number needed to scan — any event
–
Scans per recurrence or new cancer detected.
NNS — CT-detected event
–
Includes the CT detection fraction.
NNS — actionable event
–
The most defensible yield metric.
Yield: actionable events / 100 scans
–
The same number, the intuitive way round.
Enter values and calculate.
What this does — and doesn't — tell you
- It is a yield calculator, not a benefit calculator. A low "number needed to scan" means recurrences are found efficiently — it does not mean that finding them earlier improves survival. That requires trial evidence, which for routine intensive CT follow-up is, at best, mixed (IFCT-0302).
- "Number needed to scan" (NNS) is a detection-yield metric. It is deliberately not called NNT (number needed to treat), which is an effectiveness measure from randomised trials. Do not interpret NNS as a treatment effect.
- The hazard is time-dependent. Recurrence risk peaks within the first two years and declines thereafter, so the tool scales the peak-phase risk by a months-since-treatment factor. The factor is an illustrative profile, not a validated curve.
- The preset numbers are illustrative placeholders. Replace them with your own registry / cohort data. Risk varies by stage, nodal status, margins, histology, Ki-67 and prior therapy.
- Interval cancers exist. The CT detection fraction (< 100%) accounts for events that present symptomatically between scans.
Formulae
| Quantity | Formula |
|---|---|
| Effective annual hazard | h = peak_annual_risk × time_factor |
| Interval event risk | p = 1 − exp(−h × interval_months / 12) |
| NNS, any event | 1 / p |
| NNS, CT-detected event | 1 / (p × CT_detection_fraction) |
| NNS, actionable event | 1 / (p × CT_detection_fraction × actionable_fraction) |
| Yield (actionable / 100 scans) | 100 × p × CT_detection_fraction × actionable_fraction |
Time-since-treatment factor (illustrative)
| Period | Factor on peak hazard | Rationale |
|---|---|---|
| 0–24 months | 1.0 | Early peak of the recurrence hazard. |
| 25–48 months | 0.6 | Hazard declining after the early peak. |
| 49–60 months | 0.35 | Lower late hazard; second primaries persist. |
| Over 60 months | 0.2 | Mostly second primary risk; consider stopping or spacing imaging. |
Preset assumptions (illustrative — replace with local data)
| Group | Peak-phase annual event risk | Notes |
|---|---|---|
| Low-risk NSCLC | 4% | Approximate stage IA-like, R0, no adverse features. |
| Intermediate-risk NSCLC | 10% | Higher local stage or adverse pathological features. |
| High-risk NSCLC | 20% | N+, stage III, close margin, post-CRT, or otherwise high relapse risk. |
| SCLC after response | 45% | Very high early recurrence risk. |
| Typical carcinoid, R0 N0 | 1% | Low annualised risk; late recurrence possible — long horizon. |
| Atypical or N+ carcinoid | 6% | Varies by nodal status, Ki-67, necrosis and margins. |
Key reference. Westeel V, et al. Results of the IFCT-0302 phase III trial assessing minimal versus CT-scan-based follow-up for completely resected non-small-cell lung cancer. Overall survival was not significantly improved by CT-based surveillance. Recurrence-hazard timing (early peak within ~2 years) is well described in resected NSCLC cohorts.
Clinical framing & assumptions: Nordic Thoracic Oncology Group (NTOG). Interactive engine: powered by Vahtian. Runs entirely in your browser; no data is stored or transmitted. This is an educational decision-support tool. It is not a medical device, does not recommend a surveillance schedule, and does not replace national guidelines, MDT review, or clinical judgement.