ESTRO 2023 - Abstract Book

S100

Saturday 13 May

ESTRO 2023

(PBI) were presented in women who underwent breast conserving surgery (BCS). Multilevel logistic regression analyses were applied to identify variables associated with (1) post-BCS WBI with boost versus without, and (2) regional RT versus axillary lymph node dissection (ALND) in women with N+ disease. Results During 2008-2019, 176,292 women were diagnosed with M0 breast cancer in the Netherlands. An increasing percentage received RT in 2008 (61%)–2016 (70%), caused by a shift from mastectomy to breast conserving therapy and increased post- mastectomy RT use (Fig 1A). After 2016, RT use slightly decreased to 67%, as women aged ≥ 70yrs more frequently received BCS without RT (Fig 1B/C). RT de-escalation was also observed by increased PBI use and decreased boost RT use following WBI (Fig 2). Older women (>75vs<50yrs OR:0.04, 95%CI:0.03-0.05) were less likely to receive boost RT, while women with N+ or higher grade disease (pN+ vs pN0 OR:1.46 95%CI:1.32-1.60 / grade III vs I OR:11.46, 95%CI:9.90-13.26) or residual tumour (focal R1 vs R0 resection OR:28.08, 95%CI:23.07-34.17) had an increased probability of receiving boost RT. Variation in boost RT was observed between regions of residence but could not be explained by hospital characteristics. In women with N+ disease, increased regional RT use (32% in 2011-61% in 2019) coincided decreased ALND use (76% in 2011-24% in 2019). Women aged >75yrs least often received regional treatment and were less likely to receive regional RT instead of ALND (>75vs<50yrs OR:0.68, 95%CI:0.95-0.78). Also, women with poorer WHO performance status, higher T-stage or grade, or >30 minutes travel time for RT less likely received regional RT instead of ALND, while women with lymph node micrometastases more likely received regional RT. N+ treatment varied between regions of residence (Southwest vs North OR:0.55, 95%CI:0.37-0.80) but not for hospital characteristics.