ESTRO 2021 Abstract Book

S941

ESTRO 2021

(CTV volume, left lung volume and expansion, heart movement) and dose reductions to organs at risk (OARs) were analyzed. Materials and Methods In this retrospective study, 30 patients with left-sided breast cancer who received adjuvant radiotherapy after breast conserving surgery were selected. Simulation scans of both DIBH with active breathing control device and deep breathing 4D-CT were acquired for each patient. Three treatment plans using tangential angle Intensity modulated radiation therapy were generated for each patient with a total dose of 50 Gy in 25 fractions. One plan was based on the DIBH images, and the other two plans were based on the MLE and SLE images from deep breathing 4D-CT. Dosimetric comparison and normal tissue complication probability (NTCP) of different respiratory phases were analysed. Displacement magnitudes for the heart and left lung were assessed between SLE and DIBH using 3D slicer software. Pearson correlation coefficient and linear regression were used for correlation evaluation between anatomical factors and dose reductions to OARs.

Results The mean heart dose (Dmean_Heart) was 3.97 Gy in SLE, 3.42 Gy in MLE, and 2.64 Gy in DIBH (P<0.001). DIBH plans also showed significantly lower mean doses to left ventricle (LV) (Dmean_LV), left anterior descending coronary artery (LAD) (Dmean_LAD), and left lung (Dmean_Lt lung) compared with either MLE or SLE plans (P<0.001) (Table 1). DIBH plans could reduce the NTCP for long term cardiac mortality by 27.7% and 32.2% compared with MLE and SLE pans, respectively. From SLE to DIBH, left lung and heart showed obvious displacement magnitudes: 6.97 mm of anterior expansion and 13.74 mm of inferior expansion on average for left lung; 4.08 mm of right movement and 9.71 mm of inferior movement on average for heart. Pearson correlation coefficient showed that in DIBH plans, lung anterior expansion correlated to lower Dmean_Heart