ESTRO 2020 Abstract book

S542 ESTRO 2020

setting, the adoption of IMRT seems advantageous when compared to 3DCRT. PO-0930 Wide tangents versus volumetric arc therapy to treat the internal mammary chain using breath hold D. Gujral 1 , S. Nazir 1 , B. Hunter 1 , P. McNaught 1 , L. Williams 1 , S. Porter 1 , S. Coughlan 1 , S. Cleator 1 1 Imperial College Healthcare NHS Trust, Department of Clinical Oncology, London, United Kingdom Purpose or Objective Inclusion of the internal mammary chain (IMC) radiotherapy (RT) for early breast cancer is increasingly more common to improve survival and, consequently, doses to the heart and lungs are increased. This study compared conformal radiotherapy using wide tangents (WT) versus volumetric arc therapy (VMAT) with and without a breath hold technique. Material and Methods Datasets from 15 patients were used to compare WT to treat the breast/chest wall and IMC with VMAT in free breathing (FB) and voluntary deep inspiratory breath hold (vDIBH). Each technique was tested for right-sided RT and vDIBH only was used for left-sided RT). ESTRO guidelines were used for outlining left and right IMC and level 3 and 4 lymph nodes for all cases. We compared target volume coverage and OAR doses for each technique using the Mann-Whitney test, and p-values of <0.05 were statistically significant. Results Constraints for breast planning target volume (PTV) coverage (V95%>95%) were met and were similar for each technique. Coverage of the IMC PTV was better with VMAT than WT (>98% vs <95%). SCF PTV coverage was also better with VMAT vs WT (99.5% vs 97.1%). For right-sided treatment, mean heart dose (MHD) was significantly higher with VMAT vs WT both in vDIBH (2.3 Gy vs 0.8 Gy, respectively (resp.), p<0.0001) and in FB (2.8 Gy vs 1.1 Gy, resp., p<0.0001). There was no difference in right MHD with vDIBH using VMAT (p=0.07), but vDIBH significantly reduces right MHD compared to FB when treating with WT (0.8 Gy vs 1.1 Gy, resp, p=0.004). For left-sided treatment, there was no difference in MHD with VMAT vs WT in vDIBH (p>1), but heart V17 Gy was significantly lower with VMAT (0.6 Gy vs 5.0 Gy, resp., p=0.0008). Ipsilateral lung constraints were consistently lower with VMAT, apart from the V5 Gy<50%. For right- sided RT, the contralateral (C/L) lung mean dose was higher with VMAT vs WT, regardless of whether breath hold was used (3.2 Gy vs 0.4 Gy, resp, p<0.0001) or not (3.0 Gy vs 0.4 Gy, resp., p<0.0001). For left-sided RT in vDIBH, the C/L lung mean dose was higher with VMAT vs WT (3.4 Gy vs 0.4 Gy, resp., p<0.0001). The C/L breast maximum dose was significantly lower with VMAT in vDIBH vs WT in vDIBH on the right (50.6% vs 99.8%, resp., p=0.0002) and left (49.4% vs 96%, resp., p=0.0001), as well as right VMAT in FB vs WT in FB (41.2% vs 102.3%, resp., p=0.0006). The C/L mean breast dose was consistently significantly higher in VMAT than WT, but similar in right-sided VMAT in FB vs VMAT in vDIBH (4.4 Gy vs 4.4 Gy, resp., p=0.49). Conclusion There is a dosimetric advantage to using vDIBH for right- sided RT that includes the IMC in terms of PTV coverage and reduction of MHD, particularly if using WT. VMAT allows for reduced lung doses for both left- and right-sided RT. The significance of contralateral mean breast dose needs to be studied further in terms of late effects risk. PO-0931 Lymphatic function and morphology in the arms of breast cancer treated women A. Johannessen 1 , B. Offersen 2 , M. Alstrup 3 , S. Mohanakumar 3 , V. Hjortdal 4 1 Department of Clinical Medicine, Department of Experimental Clinical Oncology- Aarhus University Hospital, Aarhus, Denmark ; 2 Department of Clinical

4 Heinrich Heine University- Medical faculty, Department of Radiation Oncology, Düsseldorf, Germany Purpose or Objective Aim of the present study was to comparatively estimate the risks of radiation-induced secondary lung and breast cancer and ischemic heart disease for different radiotherapy techniques (IMRT versus 3DCRT) in women with node-positive left-sided breast cancer, candidates for regional node irradiation (RNI). A special focus of the study was on the impact of the addition of internal mammary chain (IMC) irradiation on risk estimates. Material and Methods For this risk modelling study, RNI contouring and re- planning was performed with a dose of 50Gy in 25 fractions. For each technique (3D-CRT and IMRT), two treatment plans were created, based on the inclusion of IMC (n=40). We calculated estimates of excess relative risk (ERR) and excess absolute risk (EAR) for radiation-induced lung and breast cancer and major coronary events using linear, linear-exponential and plateau models. Statistical analyses were conducted using Wilcoxon signed-rank tests to estimate statistical significance. Results The addition of IMC irradiation to RNI significantly increased the dose exposure of the heart, lung and contralateral breast in both, 3DCRT and IMRT plans (p=0.002, respectively). This correlated with an increase of the ERR for secondary lung cancer (58% vs 44%), secondary contralateral breast cancer (49% vs 31%) and ischemic heart disease (41% vs 27%) in IMRT plans, if the IMC was added as a target volume. However, the use of IMRT significantly reduced the mean cardiac dose as compared to 3DCRT and resulted in decreased ERR (64% 3DCRT vs 41% IMRT, p=0.002) and 10-year EAR for major coronary events in IMC irradiation (see figure 1). Nevertheless, even if using the IMRT technique, the estimated additional absolute 10-year cardiac risk from IMC irradiation was large (up to 4%), thus compromising the benefit of IMC irradiation for patients with high cardiovascular baseline risks. The application of IMRT significantly reduced mean lung dose exposure, resulting in a significant reduction of ERR for secondary lung cancer if the IMC was included (75% vs 58%, p=0.004). Regarding the ERR for secondary contralateral breast cancer, there was a significant increase in ERR through the addition of the IMC (p=0.002), without any influence of the RT technique.

Conclusion Although IMC irradiation has been shown to increase survival rates in node positive BC patients, it increased dose exposure of organs at risk in left-sided BC, resulting in significantly increased risks for secondary lung and contralateral BC and ischemic heart disease. In this