Abstract GS4-06: Accelerated partial breast or whole breast irradiation after breast conservation surgery for patients with early breast cancer: 10-year follow up results of the APBI IMRT Florence randomized phase 3 trial

Background. Partial breast irradiation (PBI) is a reasonable alternative for a whole breast irradiation (WBI) in selected early stage breast cancer patients. The 5-year analysis of the APBI IMRT Florence phase 3 trial showed no significant difference in terms of ipsilateral tumor recurrence (IBTR) and survival rates between the two arms. We present the 10-year follow up data. Methods. From March 2005 to June 2013, women aged more than 40 years affected by early BC, with a maximum pathological tumor size of 25 mm, were randomly assigned in a 1:1 ratio to receive either WBI using three-dimensional conformal radiotherapy (3DCRT) or accelerated PBI (APBI) using intensity-modulated radiotherapy (IMRT) technique. Patients in the APBI arm received a total dose of 30 Gy to the tumor bed in five daily fractions. The WBI arm received 50 Gy in 25 fractions, followed by a boost on the tumor bed of 10 Gy in five fractions. The primary end-point, IBTR rate is now assessed at 10 years in the ITT population, as well as secondary endpoints: overall survival (OS), breast cancer specific survival (BCSS), distant metastasis free survival (DMFS), contralateral breast cancer (CBC), and locoregional recurrences (LRR). This trial is registered with ClinicalTrials.gov, number NCT02104895. Results. Of the 520 patients, 260 were enrolled in the APBI-arm and 260 in the WBI-arm; median age was 62.8 years (63.6 in the APBI and 61.6 in the WBI arm, p=0.20). Both treatment-arms achieved a median 10-year follow-up (10.1 in APBI-arm and 10.4 in WBI-arm, p=0.39) and were comparable regarding age, tumor size, grade, tumor type, and adjuvant endocrine treatment. No significant difference in terms of IBTR rates between treatment arms was shown (log rank test p=0.58). In the group who received APBI, the 5-year IBTR was 1.96% (5 events; 95% CI: 0.3;3.7) and the 10-year IBTR was 3.74% (9 events; 95% CI: 1.5;6.3). In the group who received WBI, the 5-year IBTR rate was 1.2% (3 events; 95% CI: 0;2.5) and the 10-year IBTR was 2.5% (6 events; 95% CI: 0.7;4.5). HR for APBI patients compared with WBI patients was 1.33 (p=0.58; 95% CI: 0.49;3.56). There were no significant differences between treatment arms regarding survival outcomes. OS (log rank test p=0.33): APBI 95.4% (95% CI: 93.6;97.2) versus WBI 94.3% (95% CI: 92.3;96.3); HR for APBI patients 0.66 (p=0.33; 95% CI: 0.29;1.53). BCSS (log rank test p=0.55): APBI 98% (95% CI: 96.8;99.2) versus WBI 97.5% (95% CI: 96.2;98.8); HR for APBI patients 0.68 (P=0.55; 95% CI: 0.19;2.42). DMFS (log rank test p=0.45): APBI 97.4% (95% CI: 96.0;98.8) versus WBI 96.1% (95% CI: 94.4;97.8); HR for APBI patients 0.67 (p=0.45; 95% CI: 0.24;1.89). Cumulative incidence estimates of 10-year LRR were 3.9% (95% CI: 2.24;5.56) for APBI versus 3.0% (95% CI: 1.53;4.47) for WBI (log rank test p=0.59). CBC observed after APBI were 5 (1.9%) as compared to 9 (3.5%) after WBI (p=0.42). Conclusion. IBTR rate after 10 years in patients with early breast cancer who were treated with APBI using IMRT technique in 5 fractions is rare and not significantly different from patients treated with WBI. OS, BCSS, DMFS, and LRR control are also comparable. Thus, APBI should be considered a reasonable alternative for a WBI in early breast cancer patients. Citation Format: Icro Meattini, Calogero Saieva, Sara Lucidi, Monica lo Russo, Vieri Scotti, Isacco Desideri, Livia Marrazzo, Gabriele Simontacchi, Monica Mangoni, Carlotta Becherini, Luca Visani, Lisa Paoletti, Erica Moretti, Luca Triggiani, Marco Bernini, Lorenzo Orzalesi, Luis Sanchez, Jacopo Nori, Stefania Pallotta, Simonetta Bianchi, Lorenzo Livi. Accelerated partial breast or whole breast irradiation after breast conservation surgery for patients with early breast cancer: 10-year follow up results of the APBI IMRT Florence randomized phase 3 trial [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr GS4-06.