2733 - Large Tumor-Bed Boost Size and Cosmesis after Breast-Conserving Surgery and Whole Breast Irradiation: A Post Hoc Analysis of a Phase III Randomized Trial Comparing Hypofractionated and Conventional Fr

Y. Song¹, S. Chen², X. Zhao², H. Jing², H. Fang², Y. Tang³, Y. Song², Y. Liu², S. Qi², G. Sun², Y. LI⁴, and S. Wang²; ¹Department of Radiation Oncology, National Cancer Center/ National Clinical Research Center for Cancer/Cancer Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing, China, ²Department of Radiation Oncology, National Cancer Center/National Clinical Research Center for Cancer/Cancer Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing, China, ³GCP center/Clinical research center, National Cancer Center/National Clinical Research Center for Cancer/Cancer Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing, China, ⁴State Key Laboratory of Molecular Oncology and Department of Radiation Oncology, National Cancer Center/National Clinical Research Center for Cancer/Cancer Hospital, Chinese Academy of Medical Sciences (CAMS) and Peking Union Medical College (PUMC), Beijing, China

Purpose/Objective(s): To evaluate the risk factors associated with the long-term adverse cosmesis, cosmetic deterioration and breast induration in the patients after breast-conserving surgery (BCS) and whole breast irradiation (WBI). Materials/

Methods: This post hoc analysis was performed using data of 614 patients from a phase III, randomized controlled trial comparing hypofractionated radiotherapy (HFRT, WBI at 43.5 Gy in 15 fractions followed by a tumor-bed boost at 8.7 Gy in 3 fractions over 3.5 weeks) with conventional fractionated radiotherapy (CFRT, 50 Gy in 25 fractions followed by 10 Gy in 5 fractions over 6 weeks) after BCS. The patients who had cosmesis results at both baseline and the most recent follow-up were eligible. The endpoints included adverse cosmesis, deterioration of cosmesis and induration. The adverse cosmesis was defined as fair or poor cosmesis based on Harvard 4-scale system. The breast size (CTV volume), boost size and dosimetric parameters of heterogeneity (PTV V_105%, V_107%, V_110% and D_2cc) were collected, and their relative and absolute subjects were both recorded. Logistic regression models were used to assess the relationship between risk factors and these outcomes. The optimal cut-off values of dosimetric parameters were determined using receiver operating characteristic curve (ROC) analyses combined with the optimum Youden index.
Results: The clinicopathologic characteristics and dosimetric parameters were comparable between HFRT (n=311) and CFRT (n=303) groups. All patients received a tumor-bed boost after WBI; 21 (3.4%) received supra/infraclavicular nodal irradiation, and one of them received axillary nodal irradiation. With the median follow-up of 110.1 months (IQR: 97.5-127.1 months), 68 (11.7%) patients developed adverse cosmesis, 65 (10.6%) had cosmetic deterioration and 50 (8.1%) had breast induration. The incidence of breast adverse effects was similar in HFRT and CFRT groups. The variables including BMI, diabetes, hormonal therapy, adjuvant chemotherapy, surgery to radiotherapy interval, adverse baseline cosmesis, CTV volume, relative PTV V_105%, absolute PTV V_107%, relative boost size, and absolute boost size were included in the multivariable analysis. The results showed that the factors associated with adverse cosmesis were adverse baseline cosmesis and relative boost size = 17.2%; the factors associated with cosmetic deterioration was larger relative boost size; and the factors associated with induration was larger absolute boost size. However, the CTV volume, PTV V_105%, PTV V_107% were not independently associated with the adverse effects.
Conclusion: For the patients receiving WBI and tumor-bed boost, the relative or absolute boost size was associated with adverse cosmesis, cosmetic deterioration and breast induration. However, the breast size and dosimetric parameters of dose heterogeneity were not identified to be independently associated with the adverse effects in this study.