PURPOSE:

To investigate how radiotherapy (XRT) adds to tumor control using a standardized surgical technique with meticulous control of surgical margins in a randomized trial with 20 years of follow-up.

PATIENTS AND METHODS:

Three hundred eighty-one women with pT1N0 breast cancer were randomly assigned to sector resection with (XRT group) or without (non-XRT group) postoperative radiotherapy to the breast. With follow-up through 2010, we estimated cumulative proportion of recurrence, breast cancer death, and all-cause mortality.

RESULTS:

The cumulative probability of a first breast cancer event of any type after 20 years was 30.9% in the XRT group and 45.1% in the non-XRT group (hazard ratio [HR], 0.58; 95% CI, 0.41 to 0.82). The benefit of radiotherapy was achieved within the first 5 years. After 20 years, 50.4% of the women in the XRT group died compared with 54.0% in the non-XRT group (HR, 0.92; 95% CI, 0.71 to 1.19). The cumulative probability of contralateral cancer or death as a result of cancer other than breast cancer was 27.1% in the XRT group and 24.9% in the non-XRT group (HR, 1.17; 95% CI, 0.77 to 1.77). In an anticipated low-risk group, the cumulative incidence of first breast cancer of any type was 24.8% in the XRT group and 36.1% in the non-XRT group (HR, 0.61; 95% CI, 0.35 to 1.07).

CONCLUSION:

Radiotherapy protects against recurrences during the first 5 years of follow-up, indicating that XRT mainly eradicates undetected cancer foci present at primary treatment. The similar rate of recurrences beyond 5 years in the two groups indicates that late recurrences are new tumors. There are subgroups with clinically relevant differences in risk.