ESTRO 2021 Abstract Book

S1617

ESTRO 2021

Five patients with a DCIS or breast tumor <3cm (cTisN0-cT2N0) underwent 1.5T MR imaging in both prone and supine radiotherapy treatment position before surgery. A 3D T1-weighted spoiled gradient echo scan was acquired in both positions. The GTV, CTV (GTV+2cm), PTV (CTV+5mm) and organs-at-risk (OAR) were delineated on both MRIs. Bulk-density values were assigned to the body (relative electron density (RED) 1.0), lungs (RED 0.33), ipsilateral breast (RED 0.93) and GTV (RED 0.99) for dose calculations. For each scan, we generated an intensity modulated radiotherapy plan with 11 coplanar beams in the presence of a 1.5T magnetic field using the dedicated MR-linac treatment planning system (Monaco, Elekta). The prescribed dose was 26 Gy in 5 fractions. Constraints for the PTV were D98% >95%, D2% <105% and Dmean of 99-101%. The V26Gy of the ipsilateral breast had to be <30%. Constraints for the OAR were: ipsilateral lung: V20Gy <1%, V10Gy <10%, and V5Gy <20%; both lungs: Dmean <5Gy; heart: V20Gy <1%, V10Gy <2%, V5Gy <5%, and Dmean <2 Gy; contralateral breast: Dmean <1 Gy. DVH parameters were compared between the prone and supine position. Results All ten plans (figure 1) met the target coverage and OAR constraints, regardless of position. Median DVH parameters were (prone vs. supine): PTV Dmean: 99.9% vs. 99.4%; PTV D2%: 103% vs. 103%; heart Dmean; 1.07 vs. 0.78 Gy; heart V5Gy: 0.0% vs. 0.0%; ipsilateral lung V5Gy: 1.2% vs. 6.7%; both lungs Dmean: 0.71 vs. 1.15 Gy; contralateral breast Dmean: 0.49 vs. 0.30 Gy. Prone position was favorable for V5Gy to the ipsilateral lung in 3/5 patients and for Dmean of both lungs in 4/5 patients, whereas supine position was favorable for heart Dmean in 3/5 patients (figure 2). Dmean in the contralateral breast was favorable in supine position in 4/5 patients. In medial tumors located close to the chest wall (patient 1 and 2) a lower dose to heart and lungs was observed in supine position. In central tumors that moved away from the chest wall in prone position (patient 3 and 4) prone position was beneficial. In a lateral tumor located close to the chest wall in the left breast, dose to the ipsilateral lung was reduced in the prone position, while dose the heart was lower in the supine position (patient 5).