Partial Breast Radiotherapy for Women with Early Breast Cancer: 10-Year Outcomes from IMPORT LOW (CRUK/06/ 003)

Purpose/Objective(s) IMPORT LOW is a randomized, multi-center phase III trial testing partial breast radiotherapy (RT) using intensity modulated RT in women with early-stage breast cancer at lower than average risk of ipsilateral breast tumor relapse (IBTR). Five-year results concluded non-inferiority for reduced-dose & partial-breast RT with similar normal tissue effects (reduced for breast appearance & hardness as reported by patients; Lancet, 2017). Here we report outcomes after 10 years. Materials/Methods Women age ≥50 who had breast conservation surgery, for invasive breast cancer (excluding classical lobular carcinoma) pT1-2 (≤3cm) N0-1, any grade, with microscopic margins of ≥2 mm, were eligible. Patients were randomized (1:1:1) to 40Gy/15F to whole breast (control); 36Gy/15F to whole breast & 40Gy/15Fr to partial breast (reduced-dose); or 40Gy/15F to partial breast (partial-breast). The primary endpoint is IBTR. 10-year clinician assessed adverse effects were collected. Patient assessments were not recorded at 10 years. Efficacy analysis was intention to treat. Results 2016 patients were recruited from 05/2007 to 09/2010 from 30 UK RT centers (674 control, 673 reduced-dose, 669 partial-breast). Median age was 63 years (IQR 57-68); 42%, 48% & 10% of patients had tumors that were grade 1, 2 & 3 respectively; 3% were node positive. Median follow-up is 121 (IQR 120-124) months. 10-year follow-up forms were received for 518, 520 & 510 whole, reduced & partial groups respectively. 10-year rates of IBTR are 2.8% (95%CI 1.8-4.5), 1.9% (1.1-3.4) & 2.8% (1.7-4.5) in the whole-breast, reduced-dose & partial-breast groups respectively. Absolute treatment differences in IBTR compared with control for reduced-dose is -1.02% (95%CI -1.97, 0.96) & -0.02% (-1.38, 2.58) for partial-breast. 10-year overall survival rates are 87.8% (95%CI 84.9, 90.1), 87.2% (84.3, 89.6) & 90.3% (87.7, 92.4) for control, reduced-dose & partial-breast groups respectively. Clinician assessed adverse effects indicate low rates of moderate/marked events at 10 years (Table 1). Conclusion At 10 years, rates of IBTR & clinician assessed moderate/marked adverse effects remain very low across all treatment groups demonstrating that reduced-dose & partial-breast RT are safe & effective RT techniques. IMPORT LOW is a randomized, multi-center phase III trial testing partial breast radiotherapy (RT) using intensity modulated RT in women with early-stage breast cancer at lower than average risk of ipsilateral breast tumor relapse (IBTR). Five-year results concluded non-inferiority for reduced-dose & partial-breast RT with similar normal tissue effects (reduced for breast appearance & hardness as reported by patients; Lancet, 2017). Here we report outcomes after 10 years. Women age ≥50 who had breast conservation surgery, for invasive breast cancer (excluding classical lobular carcinoma) pT1-2 (≤3cm) N0-1, any grade, with microscopic margins of ≥2 mm, were eligible. Patients were randomized (1:1:1) to 40Gy/15F to whole breast (control); 36Gy/15F to whole breast & 40Gy/15Fr to partial breast (reduced-dose); or 40Gy/15F to partial breast (partial-breast). The primary endpoint is IBTR. 10-year clinician assessed adverse effects were collected. Patient assessments were not recorded at 10 years. Efficacy analysis was intention to treat. 2016 patients were recruited from 05/2007 to 09/2010 from 30 UK RT centers (674 control, 673 reduced-dose, 669 partial-breast). Median age was 63 years (IQR 57-68); 42%, 48% & 10% of patients had tumors that were grade 1, 2 & 3 respectively; 3% were node positive. Median follow-up is 121 (IQR 120-124) months. 10-year follow-up forms were received for 518, 520 & 510 whole, reduced & partial groups respectively. 10-year rates of IBTR are 2.8% (95%CI 1.8-4.5), 1.9% (1.1-3.4) & 2.8% (1.7-4.5) in the whole-breast, reduced-dose & partial-breast groups respectively. Absolute treatment differences in IBTR compared with control for reduced-dose is -1.02% (95%CI -1.97, 0.96) & -0.02% (-1.38, 2.58) for partial-breast. 10-year overall survival rates are 87.8% (95%CI 84.9, 90.1), 87.2% (84.3, 89.6) & 90.3% (87.7, 92.4) for control, reduced-dose & partial-breast groups respectively. Clinician assessed adverse effects indicate low rates of moderate/marked events at 10 years (Table 1). At 10 years, rates of IBTR & clinician assessed moderate/marked adverse effects remain very low across all treatment groups demonstrating that reduced-dose & partial-breast RT are safe & effective RT techniques.