Why do cancer-risk timelines after quitting differ by cancer site?

01

Even in the same cohort, the cancer-site curves were not identical

Park and colleagues' 2024 Korean study linked repeated smoking questionnaires from health examinations to cancer-registry records. Among men, those reporting sustained cessation had lower adjusted incidence of lung, liver, stomach and colorectal cancers than continuous smokers. The modelled time-course for lung cancer declined earlier than the curves for the other named sites. An all-cancer average therefore cannot supply each cancer's timetable.

These were selected health-screening participants, not a random assignment to quit or continue. Smoking was reported in two-year cycles; exact quit dates were unavailable and some missing reports were imputed. The duration analyses were in men because there were too few women smokers with events for the corresponding analyses. Early increases in the main curves were absent in a sensitivity analysis using people who had already quit before entry. The authors discuss illness-related quitting as one possible explanation. The early curve is not evidence that quitting causes cancer, or that it has no value until a particular year.

Past exposure and age continue to matter after new smoke exposure stops. Different cancers also have different background causes and patterns of detection. A study's date is an observation or modelling result, not a shared deadline at which every previous effect has disappeared.

[1][4]

02

Two actual numbers that cannot be turned into a race between lungs and bladder

The Korean study's lung-cancer estimate for men reporting sustained cessation was a hazard ratio of 0.58 compared with continuous smokers. In a different pooled analysis of ten Japanese cohorts, men who had quit for at least ten years at baseline had a bladder-cancer hazard ratio of 1.26 compared with never smokers, with a 95% confidence interval from 0.97 to 1.63. Both outcomes concern new cancers, but the countries, smoking histories, time categories and reference groups differ.

The first number is below 1 and the second above 1. That does not establish that lungs ‘recover faster’ than the bladder: 1 means a different reference group in each analysis. Nor does the bladder interval crossing 1 prove exact equivalence to never smoking. The Japanese study used baseline smoking information; its cessation-duration analyses were limited to men, and adjustment for past cumulative exposure made the trend less clear. Occupational exposure was not accounted for. There is a favourable cessation-duration pattern, but no universal ten-year normalization rule.

[1][2]

03

A cancer-incidence curve is not a cancer-death curve

The lung and bladder studies counted newly diagnosed cancers. A U.S. analysis published in the 2024 issue of JAMA Internal Medicine studied cancer mortality as a broad cause-of-death category instead. It found lower mortality with longer time since stopping, while excess cancer mortality compared with never smokers remained in some long-quit groups. That is not a site-by-site incidence timetable.

For a concrete distinction, ‘fewer new lung cancers than among continuing smokers’ does not tell us the same numerical reduction in deaths from bladder cancer. A death outcome includes a different sequence after disease occurs. Outcomes in people already diagnosed, including recurrence or survival, need studies of that population; a prevention cohort cannot provide their prognosis. The U.S. mortality analysis also excluded people reporting major relevant diseases at entry and those who had quit very recently, which limits direct comparison.

[1][2][3]

04

Use a timeline for perspective, not to cancel care

A useful reading of a timeline is ‘stopping continued exposure has benefits, and the outcome named here matters,’ not ‘all cancers are ruled out after this anniversary.’ Questions about a persistent or worrying symptom, an existing diagnosis or screening belong with a qualified clinician. Eligibility and follow-up cannot be decided from one population risk ratio; do not cancel an arranged examination because a quit anniversary has passed.

In England, NHS local stop-smoking services provide a separate route for help with quitting. They do not calculate personal cancer risk or replace cancer assessment. This page does not collect smoking history, determine screening eligibility or provide cancer treatment advice.

[4][5]

05

What to keep in mind

• Different cancer sites do not share one reset date.
• A number below 1 versus smokers cannot be ranked against one versus never smokers.
• Incidence, mortality and postdiagnosis outcomes are different questions.