Stage 0 breast cancer (DCIS)
Ductal carcinoma in situ (DCIS), often called "Stage 0" breast cancer, is a cluster of abnormal cells that has not spread beyond the milk ducts, and almost all of it is found by screening mammograms. Roughly one case in five would never have caused harm, and new studies of women who skipped or delayed surgery show far less progression than the older records that made immediate surgery routine.
If a mammogram finds low-risk DCIS, don't read the result as a surgery order: ask whether monitoring is an option for you. It has passed a two-year randomised trial, but it is not yet standard care, and two years is short for a disease measured in decades.
What it is, and how common
DCIS is abnormal cells confined to the lining of the milk ducts. It makes up about 18% of newly diagnosed breast cancers in the US, and almost all of it is found only because of screening.[1] Some DCIS goes on to become invasive cancer; much of it, over a lifetime, does not. The usual response to any DCIS has nonetheless been immediate surgery, often lumpectomy followed by radiation or hormone therapy.
About one in five cases is overdiagnosis
Overdiagnosis means finding a "cancer" that would never have caused symptoms or death in the person's lifetime. A computer model of screening every two years from age 50 estimates that roughly 1 in 5 DCIS cases in a screened population is overdiagnosis: 20%, with a plausible range of 13–30%, and little difference by how abnormal the cells look under the microscope (tumour grade).[2] The share is higher if only screen-detected DCIS is counted. Every overdiagnosed case means treatment that could not help.
What happens without surgery
The older evidence came from historical groups of women whose DCIS went untreated. In those series 36–39% developed invasive cancer in the same breast, though over a median follow-up of more than 30 years.[3] Those figures are what justified routine surgery, radiation and hormone therapy.
A newer observational study followed 1,780 US women whose DCIS was not operated on within six months of diagnosis. Over eight years, invasive cancer appeared in the same breast in 8.5% of the low-risk group and 13.9% of the high-risk group. Eight-year survival free of breast cancer death was 96.4% overall and 98.1% in the low-risk group.[4] This was an observational study, not a trial.
The randomised test: monitoring against surgery
The COMET trial tested the question directly. It enrolled 995 women with low-risk DCIS (hormone-receptor-positive, grade 1 or 2) and analysed 957, randomly assigned to active monitoring every six months or to standard surgery with or without radiation. After two years, invasive cancer had appeared in the same breast in 4.2% of the monitored group and 5.9% of the operated group. The trial had set a margin of 5 percentage points for calling monitoring no worse, and the result sat well inside it: at most, monitoring was about 1 point worse. Quality of life was much the same either way.[5], [6]
What isn't known yet
Two years is short for a disease measured in decades, and the trial cannot say what happens at ten. The result applies to low-risk, hormone-sensitive DCIS, not to higher-grade disease. Monitoring for low-risk DCIS is a strategy that has passed a two-year non-inferiority test, not a settled standard of care. The wider picture of how screening creates diagnoses like this one is in Cancer screening.
What to do
- Ask what kind of DCIS it is. Grade and hormone-receptor status decide whether monitoring is even on the table.
- If it is low-risk, ask whether monitoring is an option, and what the schedule would be (COMET used checks every six months).
- Keep your regular mammograms either way; the general schedule is on Cancer screening.