Abstract P4-11-03: The Impact of Fractionation on Local Relapse for Patients with Grade 3 Breast Cancer

Abstract Background: Several randomized trials have demonstrated that hypofractionated (HF) and conventionally fractionated (CF) radiotherapy (RT) provide equivalent local control following breast conserving surgery (BCS). However, an update of the Canadian trial suggested that patients with grade 3 disease had an increased risk of local relapse after HF. The risk of local relapse following HF or CF according to grade was investigated among a population-based cohort from British Columbia, Canada. Materials and methods: Female patients diagnosed between 1990 and 2000 with T1-T2N0M0 breast cancer treated with lumpectomy, axillary dissection and RT with at least 6 nodes removed and RT delivered to the breast were identified. Whole breast RT prescriptions were distributed in two groups : HF (typically 42.5 to 44 Gy in 16 fractions), and CF (45Gy to 50 Gy in 25 fractions). The 45 Gy prescription was followed by a boost to the biopsy cavity regardless of the margin status. Patients with close or positive margins received a boost (typically 7.5 to 10Gy in 3 to 4 fractions, or 10 to 20Gy in 5 to 10 fractions). Baseline demographic (age, year of diagnosis), tumour (grade, histology, size, lymphatic vascular space invasion (LVI), presence of extensive DCIS) and treatment factors (margin status, hormonal or chemotherapy use, RT fractionation group, and RT boost use) were abstracted. Cumulative rates of local relapse were estimated using a competing risk approach (distant relapses or death were competing risks) and compared across groups using Gray's test. Factors significant on univariate analysis were included with fractionation group in a multivariate (Fine and Gray) model among grade 3 patients. Results: The cohort consisted of 1,335 patients diagnosed with grade 3 breast cancers: 252 received CF and 1083 patients received HF. The fractionation groups were well balanced for most of the aforementioned factors except median age (56 years for CF vs 52 years for HF (P<0.01), and use of systemic therapy (hormones alone: 26% vs 19%; chemotherapy alone: 27%vs 33%; and chemo+hormone therapy: 8% vs 10% (p=0.04) for HF compared to CF). The 10-year cumulative incidence rate of local relapse in patients with grade 3 breast cancers was 6.9% for the HF group and 6.2 % for the CF group (p=0.99). A Fine and Gray multivariate competing risk model showed that age under 40 years (p=0.02), positive margins (p=0.05) and negative ER status (p=0.01) were associated with an increased risk of local relapse, but fractionation group was not (Hazard ratio=0.95, p=0.88). Conclusions: There was no evidence that hypofractionation was inferior to conventional fractionation for breast conserving therapy in patients with T1-T2 N0, grade 3 breast cancer in a population-based series. Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P4-11-03.

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