Implementing and Improving Pancreas SBRT: One Challenge at a Time

Abstract

The role of stereotactic body radiotherapy (SBRT) is being investigated as an alternative to long-course chemoradiation in select patients with locally advanced pancreatic cancer. Implementation of this technique is complex and fraught with challenges including the proximity of dose sensitive organs, target visibility for image guidance and respiratory motion. Pancreas SBRT was introduced at our institution in 2015 and the technique has undergone multiple iterations since with the goal of improving outcomes. The aim of our current work is to establish optimal respiratory motion management for pancreas SBRT. Initially, all pancreas SBRT cases were treated with a 3 fraction regimen (21-36 Gy) using abdominal compression to minimize respiratory motion. Patients were required to have a biliary stent in situ or fiducial markers implanted prior to simulation for image guidance. Based on our initial local control and toxicity results, institutional policy changed to a 5 fraction regimen (30-50 Gy). Abdominal compression is beneficial for minimizing motion and immobilizing the patient; however, there is a concern that it also causes anatomic deformation to organs at risk (OARs) such as the stomach and duodenum, moving them closer to the target. Thus, an upfront motion assessment was introduced to determine if motion management was necessary. At the time of planning, a free breathing low dose 4D computed tomography (4DCT) scan is acquired to quantify the pancreatic motion using the fiducials. If motion amplitude is <5mm, planning proceeds without abdominal compression, potentially eliminating the problem of moving the OARs into the high dose region. If on free breathing, the motion amplitude is >5mm, either abdominal compression or Active Breathing Coordinator (ABC) is required. For patients where ABC treatment is an option, breath hold and stability assessments will follow to confirm they are suitable candidates. Performing motion assessment prior to CT simulation allows for a more personalized approach to treating patients with pancreas SBRT. Treating patients without the abdominal plate provides greater comfort for patients and has potential dosimetric benefits for OARs. As access to MRI for simulation/planning increases, the free breathing option can readily be built upon as it does not rely on any treatment accessories. Unfortunately, motion evaluation is currently limited to patients with fiducials because stents are not reliable surrogates for motion as they are quite mobile themselves. Interpreting a complex decision tree like this can be confusing for those performing the simulation, making communication and education integral to the success of the program. To date, 18 patients have undergone motion assessment. Five patients were treated free-breathing, 12 patients with abdominal compression, and 1 with ABC. The 12 patients simulated with abdominal compression had an average motion reduction of 9.2 mm to 5.2 mm, confirming the effectiveness of abdominal compression. To quantify intrafraction motion for the free-breathing option, post treatment Cone Beam Computed Tomography (CBCT) images were acquired after every fraction. Upon analysis of the CBCT data, an average overall shift of only 1.5 mm was seen, suggesting there are no issues with stability. The development of 3 different methods for treating SBRT pancreas has resulted in an approach tailored to individual patient’s motion management needs and prepared us for future technological innovations.