ESTRO 2024 - Abstract Book

S527

Clinical - Breast

ESTRO 2024

visualize the breast tumor on cone-beam CT scans. Additionally, the convenience of a single-dose regimen reduces treatment burden for patients compared to multiple-fractionated postoperative radiotherapy. In this study, we report the first clinical experience with single-dose stereotactic MR-guided adaptive radiotherapy for preoperative PBI in low-risk breast cancer patients.

Material/Methods:

The ABLATIVE-2 trial (NCT05350722) is a multicenter prospective single-arm trial. Eligible participants include women ≥50 years, with a unifocal non - lobular invasive breast tumor ≤2 cm, Bloom and Richardson grade 1 or 2, estrogen receptor-positive, HER2-negative, and tumor-negative sentinel node. We introduced a comprehensive workflow for MR-guided adaptive radiotherapy. MRI- and CT-based simulations were performed for treatment planning using breath-hold technique. Definitive treatment position was determined by comparing the distance between the heart and breast tumor in prone and supine position. The gross tumor volume (GTV) was defined as the enhancing breast tumor on diagnostic 3TMRI (in prone position), using the T1-weighted acquisitions (with and without gadolinium contrast and subtracted images) and delineated on the balanced steady-state free precession (TRUFI) on the 0.35T MR-linac. The GTV was expanded by a uniform 2 cm margin to create the clinical target volume (CTV), whilst excluding the adjacent chest wall and the first 5 mm beneath the skin. Both GTV and CTV were expanded by 3 mm to create the planning target volume (PTV). The prescribed doses to the PTVGTV and PTVCTV were 20 Gy and 15 Gy, respectively. MR-guided adaptive radiotherapy for preoperative single-dose PBI was delivered using intensity modulated radiotherapy (IMRT) involving a range of 17-21 beams and 51-63 segments per beam aiming to generate a homogeneous treatment plan. During the online adaptive treatment procedure, the radiation oncologist and radiation therapists adapted the GTV and organs at risk where needed, followed by replanning. Detailed dosimetric and treatment delivery time data were retrieved from the MR-linac database. Patient satisfaction during treatment delivery was evaluated using a questionnaire. Acute toxicity was assessed according to the Common Toxicity Criteria Adverse Events version 5.0 prior to PBI and after one week. To date, two patients with breast cancer, clinical stage T1cN0(sn) were treated with a single fraction of 20 Gy using MR-guided adaptive radiotherapy. Both patients were treated in supine position due to the greater distance from the target volume to the heart and lungs compared with the MRI in prone position. The MRI images in prone and supine position of both patients are shown in Figure 1. All dosimetric parameters are shown in Table 1. A boundary structure identical to PTVGTV was used for gated delivery. The duration of treatment delivery, i.e. from entry into the treatment room to treatment completion were 95 and 80 minutes, respectively. Both patients were able to maintain the breath-hold with minimal challenges and reported a positive experience in actively participating during treatment delivery besides experiencing some cold and noise. One week after radiotherapy, grade 1 fatigue (n=2), breast pain (n=1), and breast edema (n=1) were observed. To date, one patient had the three-month follow-up, and experienced grade 1 hyperpigmentation. No other toxicity and no grade 2 or higher toxicity was observed. Results: