Survival Outcomes of Post-mastectomy Breast Cancer Patients Treated with Hypofractionated Radiation Treatment Compared to Conventional Fractionation -a Retrospective Cohort Study.

INTRODUCTION: For post-mastectomy patients, radiation treatment with conventional fractionation with a treatment duration of five weeks was the frequently used regimen, whereas hypofractionated regimens are recently used in the adjuvant treatment, which has a shorter treatment time over three weeks. We determined to estimate the treatment outcome by survival analysis between these two fractionation schedules to determine if any difference exists between these two groups.

METHODS: We retrospectively reviewed the data of 348 breast cancer patients who had received adjuvant radiation treatment to the breast from January 2010 to December 2013. After assessing the eligibility criteria, 317 patients had received post-mastectomy radiation treatment to the chest wall and axilla and followed up till December 2018. The conventional fractionation schedule consisted of 50 Gy in 25 fractions, 2 Gy per fraction over five weeks, whereas the hypofractionated schedule was 42.6 Gy in 16 fractions with 2.66 Gy per fraction, over 3.2 weeks. Survival outcomes using 5- year Overall survival and 5-year Disease-free survival between these two fractionations were estimated and compared between the conventional and hypofractionated radiation treatment.

RESULTS: All patients were females with a median age of 50 [IQR 45 to 58] and a median follow-up of 60 months. Of the 317 patients, 194 (61%) received hypofractionated radiation and 123(39%) conventional fractionation. The Kaplan-Meier estimates of the 5- year survival rate were 81% (95% CI = 74.9 to 87.6%) for the hypofractionated group (n = 194) and 87.8% (95% CI = 81.5 to 94.6%) for the conventional fractionation group (n = 123). The log-rank test revealed no evidence of a difference between the survival rates over time (p= 0.1 ). Restricted mean survival time in the hypofractionated group was 54.5 months, and in the conventional fractionation group was 57 months. Further investigation with cox proportional hazards regression analysis, which controlled for age, N stage, and T stage, showed that patients with conventional fractionation radiotherapy were 0.6 times less likely to die than those with hypofractionated radiation (95% CI for the hazard or risk ratio = 0.31 to 1.21; P = 0.2). However, there is no statistical evidence to say the reduction in mortality is different from null. 5-year disease-free survival for the hypofractionated group (n= 194) was 62.6% (55.7-70.2) whereas that for the conventional fractionation group (n=123) was 67.8% (59.8-76.8). However, there was no evidence to say any difference between the disease-free survival rates on the log-rank test (p=0.39). Restricted mean diseasefree survival time in the hypofractionated group was 45.1 months compared to 46.9 months for the conventional fractionation group.

CONCLUSION: In post-mastectomy breast cancer patients receiving radiation treatment, the survival outcome with conventional and hypofractionated radiation therapy is comparable.