ObjectiveTo evaluate the effectiveness of a single fraction of high-dose-rate iridium-192 brachytherapy as a boost after irradiation of the breast in women aged 45 or less. Patients and methodsWe studied 167 consecutive patients with T1-2 tumors, aged between 26 and 45 years undergoing conservative surgery with free margins and radiotherapy (46-50Gy) for invasive breast carcinoma between 1999 and 2008. Parallel metal needles were implanted under local anesthesia and sedation on an outpatient basis. Chemotherapy was used in 85% (66% adjuvant and 19% neoadjuvant) and hormonal therapy in 77%. A single 7Gy-dose of high-dose rate brachytherapy (“fast boost”) was administered. ResultsThe median follow-up was 69 months. In 8 patients, recurrences occurred in the tumor bed or in the margin with an actuarial local control of 95.6% at 10 years (relapse rate 4.4%). There was a relapse in another quadrant and therefore breast control was 94.1%. In patients aged up to 40 years, the breast failure rate was 6.9% at 10 years. Metastases developed in 22 patients. Disease-free survival was 85.5% and overall survival was 92%. Cosmetic results were good or excellent in 97% and acute and late toxicity was minimal. ConclusionsThe fast boost technique with a single fraction is fast, simple, and effective when surgical margins are free. Local control in young women has improved compared with prior reports in the literature. Therefore, this technique can be recommended for breast preservation.