ESTRO 2021 Abstract Book

S324

ESTRO 2021

Conclusion The SSI is a QA criterion for DIR with higher and more consistent correlations with the endpoint error than established QA criteria. Due to its higher specificity and sensitivity it is more suited to make decisions on accepting registrations for clinical use. Additionally, the SSI metric does not require operator input and is therefore well suitable in clinical work-flows aiming to reduce operator burden. This also enables fully automated QA for DIR for adaptive radiotherapy treatments. OC-0425 Dosimetric effects of national guidelines in breast cancer radiotherapy 2008-2016 (DBCG RT- Nation) L. Refsgaard 1,2 , T. Ravkilde 3 , E. Riis Skarsø 4 , E. Svitzer Yates 3 , L. Bech Jellesmark Thorsen 1,3 , B. Vrou Offersen 1,3,4 , S.S. Korreman 3,4,2 1 Aarhus University Hospital, Department of Experimental Clinical Oncology, Aarhus, Denmark; 2 Aarhus University, Department of Clinical Medicine, Aarhus, Denmark; 3 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark; 4 Aarhus University Hospital, Danish Center for Particle Therapy, Aarhus, Denmark Purpose or Objective This abstract reports on the volume and planned radiotherapy (RT) dose of the heart and internal mammary lymph nodes (IMN) for all high-risk breast cancer (HRBC) patients treated in one institution in 2008-2016. We investigate the effect of three events involving changes to national guidelines. Materials and Methods Patients were identified from the Danish Breast Cancer (DBCG) database and selected on the following criteria: early HRBC, no prior cancer, minimum age of 18 years and indication for loco-regional RT according to DBCG guidelines. In total, 1826 patients of whom 1518 were included in the analysis (no RT: 49, other RT: 10, missing data: 130, bilateral RT: 33, in ongoing protocol: 135). In 2008, 88 patients were treated with 48Gy in 24 fractions and 8 patients were treated with 50Gy in 25 fractions. From 2009 all patients were treated with 50Gy in 25 fractions. The laterality of RT was determined by assessing the ratio of integral dose on the left and the right half of the dose grid. Ambiguous cases were manually checked. If the IMN structure was present, it was interpreted as an intention to treat it. The effect of three events (E) on the volume and RT dose of the heart and IMN was quantified. E1: Between 2010 and 2012, national workshops were organized to strengthen the delineation consensus within the Danish Breast Cancer Group (DBCG). E2: In June 2014, the DBCG changed guidelines to include the IMN in the RT target for all HRBC patients. E3: In 2015, the ESTRO consensus guidelines on delineation of clinical target volumes (CTV) for BC RT were published. Results All right sided patients (RSP), except two (intentionally treated as lymph node negative) had IMN delineated. All left sided patients (LSP) planned after E2 had IMN delineated. Figure 1 shows the average volume and the median V90% CTV IMN for each year. Figure 2 shows the same for the heart volume and mean heart dose (MHD). The average volume of both heart and IMN stabilized around 2011, which can be correlated to E1. In 2015, the average volume for IMN decreased, indicating a change in delineating practice, caused by E3. The MHD for LSP decreased from 2008 to 2013 because of the increase in heart volume and the introduction of gating in 2012. However, in 2014 the intention to treat IMN for the LSP (E2) lead to an increase in MHD. Overall, the LSP had a lower IMN V90% than the RSP, because of constraints to the heart dose. The median value of V90% for CTV IMN for 2014 - 2017 was 93.6% (Q 1 =79.3%, Q 3 = 98.7%) for LSP and 99.1% (Q 1 =96.2%, Q 3 =99.9%) for RSP. For the same period, the MHD was 1.79Gy (95% CI: 1.66Gy – 1.91Gy) for LSP and 0.96Gy (95% CI: 0.92Gy – 1.00Gy) for RSP.