Background and Purpose: Stereotactic body radiation therapy delivered using MR-guided radiotherapy (MRgRT) and automatic breathold gating has shown to improve overall survival for locally advanced pancreatic cancer (LAPC) patients. The goal of our study was to evaluate feasibility of treating LAPC patients using abdominal compression (AC) and impact of potential intrafraction motion on planned dose on a 1.5T MR-linac. Methods & Materials: Ten LAPC patients were treated with MRgRT to 50 Gy in 5 fractions with daily online plan adaptation and AC. Three orthogonal plane cine MRI were acquired to assess stability of AC pressure in minimizing tumor motion. Three sets of T2w MR scans, pre-treatment (MRI pre ), verification (MRI ver ) and posttreatment (MRI post ) MRI, were acquired for every fraction. A total of 150 MRIs and doses were evaluated. Impact of intrafraction organ motion was evaluated by propagating pre-treatment plan and structures to MRI ver and MRI post , editing contours and recalculating doses. Gross tumor volume (GTV) coverage and organs-at-risk (OARs) doses were evaluated on MRI ver and MRI post . Results: Median total treatment time was 75.5 (49-132) minutes. Median tumor motion in AC for all fractions was 1.7 (0.7-7), 2.1 (0.6-6.3) and 4.1 (1.4-10.0) mm in anterior-posterior, left-right and superior-inferior direction. Median GTV V50Gy was 78.7%. Median D5cm 3 stomach_duodenum was 24.2 (18.4-29.3) Gy on MRI ver and 24.2 (18.3-30.5) Gy on MRI post . Median D5cm 3 small bowel was 24.3 (18.2-32.8) Gy on MRI ver and 24.4 (16.0-33.6) Gy on MRI post . Conclusion: Dose-volume constraints for OARs were exceeded for some fractions on MRI ver and MRI post . Longer follow up is needed to see the dosimetric impact of intrafraction motion on gastrointestinal toxicity.
Purpose: To investigate the dosimetric impact of magnetic (B) field on varying air cavities in rectum patients treated on the hybrid 1.5 T MR-linac. Methods: Artificial air cavities of varying diameters (0.0, 1.0, 1.5, 2.0, 2.5, 3.0, and 5.0 cm) were created for four rectum patients (two prone and two supine). A total of 56 plans using a 7 MV flattening filter-free beam were generated with and without B-field. Reference intensity-modulated radiation therapy treatment plans without air cavity in the presence and absence of B-field were generated to a total dose of 45/50 Gy. The reference plans were copied and recalculated for the varying air cavities. D 95 (PTV 45-PTV 50), D 95 (PTV 50-aircavity), V 50 (PTV 50-aircavity), D max (PTV 50aircavity), and V 110% (PTV 50-aircavity) were extracted for each patient. Annulus rings of 1-mm-diameter step size were generated for one of the air cavity plans (3.0 cm) for all four patients to determine D max (%) and V 110% (cc) within each annulus. Results: In the presence of B-field, hot spots at the cavity interface start to become visible at~1 cm air cavity in both supine and prone positioning due to electron return effect (ERE). In the presence of B-field D max and V 110% varied from 5523 ± 49 cGy and 0.09 ± 0.16 cc for 0 cm air cavity size to 6050 ± 109 cGy and 11.6 ± 6.7 cc for 5 cm air cavity size. The hot spots were located within 3 mm inside the rectal-air interface, where D max increased from 110.4 ± 0.5% without Bfield to 119.2 ± 0.8 % with B-field. Conclusions: Air cavities inside rectum affects rectum plan dosimetry due ERE. Location and magnitude of hot spots are dependent on the size of the air cavity. K E Y W O R D S 1.5 T magnetic field, electron return effect (ERE), MR-guided radiation therapy, rectal cancer 1 | INTRODUCTION Magnetic resonance (MR)-guided radiation therapy using a hybrid MR-linear accelerator (linac) system has become clinically available using commercial systems in recent years. 1,2 These systems have enabled methods of increased treatment precision under real-time guidance and online plan adaptation. One such system is the Unity 1.5 Tesla (T) hybrid MR-linac system with a 7 MV Elekta linac and a high-field strength 1.5 T Philips MR magnet. 3 Higher-field strength hybrid systems provide superior MR image quality for daily adaptation and therapy response assessment, but pose multiple dosimetric challenges including electron return effect.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.