PRIME II Trial Omission of Adjuvant Radiation Therapy after BCT in Women ≥ 65 years

The phase III PRIME II trial has shown a higher risk of local recurrence with the omission of adjuvant radiotherapy after breast-conserving surgery in patients aged ≥ 65 years with hormone receptor–positive, node-negative disease who were receiving adjuvant endocrine therapy.
No difference in the risk of distant recurrence as the first recurrence event or overall survival was observed.
Study Details: The trial included 1,326 patients from sites in the United Kingdom (n = 1,263), Greece, Australia, and Serbia with T1 or T2 primary breast cancer (with tumors ≤ 3 cm in the largest dimension) who received breast-conserving surgery with clear excision margins and adjuvant endocrine therapy.
Patients were randomly assigned between April 2003 and December 2009 to whole-breast irradiation at 40 to 50 Gy in 20 to 25 fractions (n = 658) or no irradiation (n = 668).
Tamoxifen at 20 mg per day for 5 years was recommended as standard adjuvant endocrine therapy.
The primary endpoint was local breast cancer recurrence.
Key Findings: Median follow-up was 9.1 years. At 10 years, the cumulative incidence of local recurrence was 9.5% (95% confidence interval [CI] = 6.8%–12.3%) in the no-radiotherapy group vs 0.9% (95% CI = 0.1%–1.7%) in the radiotherapy group (hazard ratio = 10.4, 95% CI = 4.1–26.1, P < .001).
The 10-year cumulative incidence of distant recurrence as the first recurrence event was 1.6% (95% CI = 0.4%–2.8%) in the no-radiotherapy group vs 3.0% (95% CI = 1.4%–4.5%) in the radiotherapy group.
For the no-radiotherapy group vs the radiotherapy group, 10-year rates were:

68.9% (95% CI = 64.7%–73.0%) vs 76.3% (95% CI = 72.5%–80.2%) for disease-free survival
80.8% (95% CI = 77.2%–84.3%) vs 80.7% (95% CI = 76.9%–84.3%) for overall survival
97.4% (95% CI = 96.0%–98.8%) vs 97.9% (95% CI = 96.5%–99.2%) for breast cancer–specific survival.
The investigators concluded: Omission of radiotherapy was associated with an increased incidence of local recurrence but had no detrimental effect on distant recurrence as the first event or overall survival among [patients] 65 years of age or older with low-risk, hormone receptor–positive early breast cancer.