2742 - Simultaneous Integrated Tumor Bed Boost vs. Sequential Boost during Conventional Fractionated Whole-Breast Irradiation after Breast-Conserving Surgery

D. Q. Wang¹, H. Jing¹, H. Fang¹, T. Yu¹, Y. W. Song¹, Y. Liu¹, J. Jin², S. Qi¹, Y. Tang³, N. Lu¹, B. Chen¹, N. Li⁴, Y. Zhai¹, W. Zhang¹, Z. Qiu¹, M. Wang¹, Z. H. Zhang¹, Y. X. Li¹, and S. Wang¹; 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, ²1State 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, ³State 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 and Peking Union Medical College (PUMC), Beijing, China, ⁴Department of Radiation Oncology, Shanxi Province Cancer Hospital/ Shanxi Hospital Affiliated to Cancer Hospital, Chinese Academy of Medical Sciences/Cancer Hospital Affiliated to Shanxi Medical University, Taiyuan, China

Purpose/Objective(s): Tumor bed boost during whole-breast irradiation (WBI) was proved to improve the local control, while it is unknow whether simultaneous integrated boost (SIB) or sequential boost (SeB) is preferable. This study aimed to compare the safety and efficacy of SIB and SEB during conventional fractionated whole-breast radiotherapy after breast-conserving surgery. Materials/

Methods: Retrospectively reviewed 272 patients treated with SIB between 2015 and 2018 and 362 patients treated with SeB between 2010 and 2015. The prescribed dose was 50 Gy in 25 fractions to whole breast and simultaneously 60 Gy in 25 fractions (SIB) or sequentially 10 Gy in 5 fractions (SeB) to tumor bed. Toxicities were evaluated according to Common Toxicity Criteria for Adverse Events v3.0. Breast cosmetic evaluation based on Havard system. Local control (LC), locoregional control (LRC), disease-free survival (DFS), overall survival (OS) and breast cancer-specific survival (BCS) were calculated by Kaplan-Meier method. The impact of SIB and SeB on survival outcomes was assessed by inverse probability of treatment weighting (IPTW) by adjusting for age, body mass index, TNM stage, histological grade, lymphovascular invasion (LVI), molecular subtype, administration of chemotherapy, chemotherapy cycles, endocrine therapy and anti-HER2 targeted therapy. Survival rates before or after weighting were compared with the log rank test. Chi-square test was preformed to compare the characteristics and grade 2 or higher toxicities between groups.
Results: Median follow-up was 80 months. Patients with SIB have more T2-3 stage, N1-3 stage and administration of chemotherapy than patients with SeB. Regional nodal was irradiated in 39 (14.3%) patients with SIB and 16 (4.4%) patients with SeB. Intensity-modulated radiotherapy was used in 100% of patients with SIB and 100% WBI and 6.1% SeB, the remaining 93.9% of patients with SeB were irradiated with electrons. The incidence of grade 2 or higher skin toxicity (3.3% vs 7.1%), pneumonitis (0.7% vs 3.3%) and breast swelling (0.4% vs 2.5%) was significantly lower in patients with SIB than SeB. Grade 2 or higher breast pain, induration, lymphedema, shoulder mobility and fair or poor cosmetic result were comparable between groups. The 5-year LC, LRC, DFS, OS and BCS rates of SIB and SeB were 95.9% vs 98.0% (p = 0.1), 95.6% vs 97.1% (p = 0.3), 94.1% vs 94.3% (p = 0.9), 98.9% vs 98.0% (p = 0.2) and 99.3% vs 98.6% (p = 0.4), respectively. After IPTW, there is no significant difference between SIB and SeB in 5-year LC (96.0% vs 97.2%, p = 0.4), LRC (95.5% vs 96.4%, p = 0.6), DFS (94.1% vs 93.5%, p = 0.4), OS (99.1% vs 98.1%, p = 0.1) and BCS (99.4% vs 98.6%, p = 0.4).
Conclusion: In contrast to SeB, SIB is alternative approach since it reduced acute toxicities with comparable late toxicities and survival outcomes but shorten treatment. Further follow-up is needed to assess long-term outcomes.