What are the risks of stopping anastrozole at 2 years for ER+ tumors?

The short version: the evidence you are looking for does not exist, and the closest trial tested a different question.

No trial has ever randomly assigned women to stop anastrozole at 2 years versus continue to 5 years. The ATAC trial compared 5 years of anastrozole to 5 years of tamoxifen, not a test of stopping early [1, 2]. In the hormone-receptor-positive group, anastrozole reduced recurrence by about 17% over the full 5 years compared to tamoxifen (hazard ratio 0.83) [1, 2]. But that is a comparison of two different drugs, not a test of duration.

What the ATAC trial does show is that the benefit of anastrozole over tamoxifen grew over time, not that it was concentrated in the first two years [2]. The 100-month analysis found a hazard ratio of 0.81 for time to recurrence, meaning the protection continued well beyond year 2 [2]. The annual recurrence rate in years 5-10 was actually higher than in years 0-5 in one ATAC sub-study: about 2.80% per year versus 1.82% [9]. That means stopping at 2 years removes protection during a period when the risk of recurrence is still substantial.

The real-world data on discontinuation is sobering. One study found that 32% of women stopped their initial endocrine therapy within the first year, and 81% had stopped by 5 years [16]. The median time on the first drug was only 2.2 years [16]. That means many women are stopping early in practice, but the studies that track this do not report their recurrence outcomes. The question you are asking has never been answered with a randomized trial.

My call: stopping at 2 years instead of 5 means giving up a proven reduction in recurrence that continues beyond year 2, but the exact percentage increase in your personal risk is unknown because no trial tested this question. Confidence: moderate, because the indirect evidence is consistent but the direct trial is missing.