Dosimetric impact of intrafraction motion under abdominal compression during MR-guided SBRT for (Peri-) pancreatic tumors

Abstract

Objective – Intrafraction motion is a major concern for the safety and effectiveness of high dose stereotactic body radiotherapy (SBRT) in the upper abdomen. In this study, the impact of the intrafraction motion on the delivered dose was assessed in a patient group that underwent MR-guided radiotherapy for upper abdominal malignancies with an abdominal corset. Approach – Fast online 2D cine MRI was used to extract tumor motion during beam-on time. These tumor motion profiles were combined with linac log files to reconstruct the delivered dose in 89 fractions of MR-guided SBRT in twenty patients. Aside from the measured tumor motion, motion profiles were also simulated for a wide range of respiratory amplitudes and drifts, and their subsequent dosimetric impact was calculated in every fraction. Main results – The average (SD) D99% of the gross tumor volume (GTV), relative to the planned D99%, was 0.98 (0.03). The average (SD) relative D0.5cc of the duodenum, small bowel and stomach was 0.99 (0.03), 1.00 (0.03), and 0.97 (0.05), respectively. No correlation of respiratory amplitude with dosimetric impact was observed. Fractions with larger baseline drifts generally led to a larger uncertainty of dosimetric impact on the GTV and organs at risk (OAR). The simulations yielded that the delivered dose is highly dependent on the direction of the baseline drift. Especially in anatomies where the OARs are closely abutting the GTV, even modest LR or AP drifts can lead to substantial deviations from the planned dose. Significance – The vast majority of the fractions were only modestly impacted by intrafraction motion, increasing our confidence that MR-guided SBRT with abdominal compression can be safely executed for patients with abdominal tumors, without the use of gating or tracking strategies.

Feasibility and Acute Toxicity of Hypo-Fractionated Radiotherapy on 0.35T MR-LINAC: The First Prospective Study in Spain

“…They concluded that there was no correlation between respiratory movement and dosimetry impact. Still, they advised that this represents a limited view of the respiratory cycle and does not consider larger respiratory cycles [27]. Therefore, we still perform DIBH in the thorax and upper abdominal treatments, and physicists design a robust modulation plan with a time limit of less than 15 min on Beam ON.…”

Initial clinical experience with using a 1.5 Tesla MR-Linac for stereotactic body radiotherapy for cancer of the upper abdomen

“…Stereotactic body radiotherapy (SBRT) has been used successfully for the treatment of unresectable upper abdominal malignancies (1)(2)(3)(4)(5)(6). Compared with conventional 3-dimensional conformal radiotherapy or intensity-modulated radiotherapy, SBRT offers highly precise and accurate treatment through the delivery of high radiation doses to tightly de ned target areas in small numbers of fractions.…”

Advancing the frontiers of MR-guided radiotherapy for pancreatic cancer

“…This motion trace was characterized by its respiratory motion amplitude and maximum drift in the same manner as in our previous studies. 61,107 The respiratory amplitude was calculated by high-pass filtering the motion trace and calculating the peak-to-peak distance, and the drift magnitude was calculated by low-pass filtering and extracting the maximum absolute value of the drift motion. The overall respiratory amplitude and drift were calculated as the 2-norm of their values in the three cardinal directions: cranio-caudal (CC), anterior-posterior (AP ), and left-right (LR).…”

“…Mitigating respiratory motion with passive motion management, like abdominal compression, is a wellestablished method in the upper abdomen, 33, 61, 66, 92-95 but residual motion and small drifts can still have a significant dosimetric impact on target coverage and OAR dose. 83,[105][106][107] Online motion monitoring in the upper abdomen is challenging due to the limitations of contemporary MRI techniques. In order to capture detrimental breathing irregularities and tumor drifts, imaging ideally needs to be conducted in a time-resolved fashion, at a frequency fast enough to capture respiratory motion.…”

“…In current practice, this is mainly achieved with 2D cine MRI, which was employed in a previous study to perform delivered dose reconstruction in the upper abdomen. 107 However, this technique suffers from limited field-of-view (FOV), being unable to capture tissue motion outside the 2D plane(s), and is unsuitable for assessing out-of-plane motion. Attempts to design time-resolved volumetric acquisitions were often focused on reconstructing 3D motion from 2D cine MRI.…”