ESTRO 2024 - Abstract Book

S1968

Clinical - Mixed sites, palliation

ESTRO 2024

Alcazar de San Juan, Spain. 8 GenesisCare, Radiation Oncology, Talavera de la Reina, Spain. 9 GenesisCare, RTT, Madrid, Spain

Purpose/Objective:

MRI-linear accelerators have emerged as a promising tool in Radiotherapy, offering enhanced soft-tissue organ delineation and daily adaptation. These features potentially allow for the delivery of high doses of radiation in the Clinical Target Volume (CTV) without compromising mobile organs at risk (OAR). With the acquisition of the first 0.35 T MR-Linac in Spain, we developed a prospective clinical study to report on the workflow, feasibility, acute toxicity, and late toxicity up to 36 months when introducing new technology in our institution. We describe the initial clinical experience at our institution following the implementation of a 0.35 T MR-Linac after 4 months.

Material/Methods:

Prior to clinical implementation, our staff underwent training in MRI safety and workflows supported by the developer's training scheme. All patients were staged, and after evaluation by a multidisciplinary team, the indication for treatment with MR-guided adaptive radiotherapy (MRgRT) or stereotactic MR-guided adaptive radiotherapy (SMART) was validated according to our center's protocols and international guidelines. On the 0.35 T MR-Linac, daily plan adaptation was performed using Adapt to Shape (ATS), which involved contour adaptation and replanning. A TRUFI sequence was primarily used for treatment planning, with T1 or T2 sequences selected at the discretion of the physician based on target localization. Duration of treatment steps, dosimetric data, and treatment failure were recorded. For upper abdominal localization, patients actively assisted in breath-hold beam gating with the help of custom-designed prismatic glasses, allowing sight of a computer monitor mounted on the back wall just behind the MRI system bore. After the completion of each daily treatment, a QA control following the national protocol was conducted. Patients were evaluated and reported acute toxicity (patient-reported and clinician-scored CTCAE 5) was performed. Between July and October 2023, we enrolled 30 patients in this prospective study with 7 tumor sites in 140 total fractions (see Table 1). The median age was 68 (range: 45-84 years). Among tumor sites, the most frequently treated lesions were prostate 57.14 % and pancreas 24,13%. In the clinical scenario of oligometastases, the most frequent lesions were adrenal metastases (7.14 %). The most common diagnosis was adenocarcinoma of the prostate with intermediate favorable risk. A total of 39.28 % patients were treated in breath-hold. The median prescribed dose for SMART was 36.25 Gy (range 24-50 Gy) and the median number of fractions was 5 (ranges 1-5 fractions). 1 patient with prostate cancer was treated with MRgRT in conventional fractionation of 60 Gy in 20 fractions. A gating boundary of 2 mm around a gating region of interest (gROI) following the PTV was commonly used (range: 2-3 mm). Less than 5% of the PTV was allowed outside the gROI with a confidence interval of 75-90%. Daily adaptation to GTV and Oar was performed in all patients. The treatment time for prostate cases ranged from 25 to 45 minutes, while for upper abdominal lesions ranged from 30 to 90 minutes. Acute toxicity for pelvic tumors included: GU with 31.25% and no > grade 3 toxicity. None of the patients presented with GI toxicity. For upper abdominal cases, 1 patient experienced grade 1 nausea. Results:

Conclusion: