330P Effect of radiotherapy in deep inspiration in patients with left breast cancer: Does the size of the target area affect the dose for the most crucial organs at risk?

After radiotherapy (RT) to the left breast, the long-term risk of developing various cardiovascular diseases may increase. In our study, we present how deep inspiration treatment changes the doses of organs at risk (OAR), especially the heart. Furthermore, we examined if there is a relationship between the size of the irradiated target area and the dose on the heart. From May 2016 to August 2020, 503 patients with left-sided breast cancer underwent RT at our institute. Since May 2018, we have treated our patients in deep breathing (DIBH) using the “voluntary breath-hold” technique. We compared the Dmean and Dmax values for the most critical OAR (left lung, heart, left ventricle, LAD-left anterior descending artery) based on the two treatment methodologies. Subsequently, both groups were then further subdivided into whether the treatment was given by standard fractionation or hypofractionation and created 3-3 groups based on the size of the target area (PTV). In patients treated with normal fractionation in DIBH vs. without, the doses of the OARs were the following: lung Dmean: 9.3 Gy vs. 9.96 Gy (p=0.029), left ventricle Dmean: 3.52 Gy vs. 7.56 Gy (p=2.69E-27), heart Dmean: 2.78 vs. 5.31 (p=1.11E-24), LAD Dmean 12.17 vs. 25.91 Gy (p=4.63E-24) respectively. In the hypofractionated group with vs. without DIBH the values were lung Dmean: 7.43 Gy vs. 8.08 Gy (p=0.17), left ventricle Dmean: 3.48 Gy vs 6.4 Gy (p=2.64E-10), heart Dmean: 2.64 vs 4.62 (p=2.96E-08), LAD Dmean 11.46 vs 18.35 Gy (p=3.37E-06) respectively. In the standard fractionated group, the average doses for each structure decreased mostly in women with large breast sizes after DIBH treatment. Generally, as the breast size increases, the values of the left ventricle, heart, and LAD increase, whilst the values of the lungs decrease. Although, a significant difference could only be detected in a few categories. DIBH RT of the left breast significantly reduces radiation exposure to crucial OAR than free-breathing radiation therapy. At the same time, the coverage of the target area does not change, and our results suggest that this benefit is most pronounced in patients with large breast sizes.