ESTRO 2023 - Abstract Book

S1117

Digital Posters

ESTRO 2023

PO-1387 1.5-Tesla Magnetic Resonance-Guided Adaptive Stereotactic Body Radiotherapy for Liver Malignancies

C. Onal 1 , G. Yavas 1 , C. Yavas 1 , G. Arslan 1 , E. Efe 1 , M. Haberal 2

1 Baskent University, Department of Radiation Oncology, Ankara, Turkey; 2 Baskent University, Department of General Surgery, Ankara, Turkey Purpose or Objective Magnetic resonance imaging-guided adaptive radiotherapy (MRgRT) is one of the most promising technological advances in radiation oncology in recent years. MRI-g-ART has a high potential for further quality improvement, permitting higher ablative radiation doses while sparing adjacent healthy tissue in the treatment of primary liver tumors and hepatic metastases. We sought to evaluate the preliminary efficacy and toxicity of 1.5T-MR-LINAC in patients with liver tumors or hepatic metastases. Materials and Methods Fourteen patients with 21 liver lesions treated with 1.5T MRgRT (Unity® MR Linac System, Elekta AB, Stockholm, Sweden) retrospectively evaluated. Five patients (36%) had Klatskin tumor and 9 patients (64%) had liver metastasis (five patients with colon cancer, two with pancreas cancer, one with gastric cancer and one with breast cancer). The treatment details and early treatment results as well as toxicity outcomes were reported. Results Median age for entire cohort was 63 years (range, 51–76 years). There were four female patients (29%) and ten male patients (71%). The median follow-up time was 9 months (range 3-24 months). The median prescribed dose was 33 Gy (range, 21–54 Gy) administered in median 5 fractions (range, 3–5). The median duration of treatment including patient preparation, adaptive planning, imaging and treatment delivery was 50 minutes (range, 27–70 minutes). Eight lesions showed complete response, while three were considered stable, three had partial response, and seven were progressed. The response rate was 67%. Although 11 of the 21 lesions were controlled in the treatment field, new lesions developed outside the treatment field. Four people died as a result of disease progression. There was no acute toxicity of grade 2 or higher observed during or after treatment. Conclusion MRgRT is a safe and effective treatment modality for liver cancer and metastases. Our promising preliminary results include high levels of local control and only grade 1 mild acute toxicity. Prospective studies with more patients, on the other hand, are required. 1 Department of Radiation Oncology (Maastro), GROW School for Oncology and Reproduction, Maastricht University Medical Centre+, Maastricht, The Netherlands Purpose or Objective Breathing motion may result in large PTVs and poor CBCT image quality for patients treated with liver SBRT. To overcome this, we developed and implemented a markerless breath-hold liver SBRT technique using non-invasive nasal high flow therapy (NHFT) for breath-hold prolonging and surface-guided radiotherapy (SGRT) for breath-hold monitoring. In this study, we evaluate the first two years of clinical experience. Materials and Methods Heated and humidified air was administered via a nasal cannula (40 L/min, 80% oxygen, 34°). Patients performed voluntary inspiration breath-holds supported by visual feedback within a 2 mm gating window. After a training session, 4 or 5 contrast enhanced breath-hold CT scans were acquired to enable delineation of an ITV to account for inter- and intra-breath-hold variations. Patients were treated in 3–8 fractions using VMAT with 6 or 10 MV FFF beams using SGRT-controlled beam-hold. Patient setup was performed using SGRT and 4-DoF CBCT-CT matching based on the liver. After treatment, a CBCT scan was acquired for evaluation purposes. Results Between June 2020–June 2022, 17 liver SBRT patients started with the training session: 10 completed treatment in breath hold, 5 continued in free-breathing (reasons: inability to understand procedure or position the arm above the head for a longer time and unreproducible or unstable breath-holds), 2 were not treated due to an increase in metastases. The average maximum breath-hold duration during treatment ranged per patient from 47–108 s. CBCT imaging took 60 seconds and half of all 100 scans were acquired during a single breath-hold. All patients could hold their breath for at least 60 seconds during one or more treatment fractions. For the 49 treatment fractions, the average number of breath-holds required for treatment was 6.6±4.1 (range: 2–19). The average time from CBCT before treatment to CBCT after treatment was 19±11 minutes (range: 9–51 min). The difference in centroid position of the GTVs on the multiple planning CT scans was on average 3±3 mm for all directions (range: 0–10 mm). However, this difference was caused by a combination of breath-hold and delineation variations (volume differences ranged from 0.5–14.4 cm ³ ). For all patients with a GTV difference >4 cm ³ , tumor visibility decreased over time due to the washout of contrast, resulting in smaller volumes on subsequent CT scans. The ITV was on average 6.3 cm ³ /28% larger (range: 1.1–23.9 cm ³ /5–72%) than the largest GTV. The 3D displacement vector for the 43 CBCT scans acquired after treatment was on average 5.0 mm (range: 0.7–12.9 mm; Figure 1) and for all but 2 scans less than our 1 cm PTV margin. PO-1388 Clinical implementation and evaluation of liver SBRT in breath-hold using nasal high flow therapy C. Hazelaar 1 , R. Canters 1 , I. Lubken 1 , K. Kremer 1 , F. Vaassen 1 , J. Buijsen 1 , M. Berbée 1 , W. van Elmpt 1