Interval cancers explained in the NHS Breast Screening Programme
Updated 5 October 2020
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Of 1,000 women screened for breast cancer:
- 8 are diagnosed with cancer
- 992 have a normal result, of whom around 3 will develop an interval cancer
An interval cancer is a breast cancer found during the 3 years after a normal result and before the next screening appointment. In England, each year approximately 6,000 women will develop an interval cancer.
The illustration below shows why interval cancers can occur. It also highlights how professionals and organisations can be open and transparent.
If a woman develops an interval cancer the screening programme should:
- review the previous screening mammograms
- compare the previous mammograms to those taken at the time of diagnosis
- give information to the woman (if she wishes) to explain the findings
- use the experience to learn and improve
Of the 6,000 women with interval cancers each year in England:
- around 4,800 (80%) will have developed cancer between screening appointments — this means there was no sign of cancer at the previous screen
- around 1,200 (20%) had a cancer which was not picked up at their previous screen (false negatives)
When a cancer was not picked up at the previous screen (false negative) this is usually because the cancer changes were hard to see and would only be seen with hindsight.
In a small number of cases, other screeners think the changes should have been picked up. These are notifiable safety incidents. Services should follow the statutory requirements of duty of candour. Any such incident should be investigated so lessons can be learnt and a formal apology offered to the woman.
Review teams in breast screening units currently classify interval cancers into categories 1, 2 or 3 to support learning and development. However, a national survey showed that screening units vary in the way they do this. In some units this classification may have significant overlap with the decision about which interval cancers should fall within the remits of a notifiable safety incident, but this is not the case for all units.
Local review teams should therefore reach a clearly documented decision as to which cases should be considered notifiable safety incidents — for instance, when review shows there has been a general service failure or inadequate assessment processes.
Further work is being done to ensure a clearer and more consistent approach to classification of interval cancers.
Breast screening: interval cancers explained
An interval cancer is an invasive cancer diagnosed within the 3-year period after a normal result. Interval cancers occur in around 3 of every 1,000 women screened. Of 100 interval cancers that are reviewed, 80 women had a normal screening mammogram, 13 women have cancers that most screeners would not have detected and 7 women have cancers that most screeners would have detected.
All interval cancers should be reviewed or audited.
