Princess Margaret Cancer Center, University Health Network Toronto, ON
A. M. Glynn1,2, S. Saha1,2, M. Velec1,2, T. Stanescu1,2, J. D. Martinez3, P. M. Schuffenegger4, A. Hosni1,2, A. Mesci1,2, C. Rocca1, A. Shessel1, T. Kyoung Kim5, and L. A. Dawson1,2; 1Radiation Medicine Program, Princess Margaret Cancer Centre, Toronto, ON, Canada, 2Department of Radiation Oncology, University of Toronto, Toronto, ON, Canada, 3Biostatistics, University of Toronto, Toronto, ON, Canada, 4Radiation Oncology Unit, Department of Hematology – Oncology, Faculty of Medicine, Pontificia Universidad Catolica de Chile, Santiago, Chile, 5Department of Medical Imaging, University of Toronto, Toronto, ON, Canada
Purpose/Objective(s): The MR-Linac (MRL) provides a platform for adaptive stereotactic body radiation therapy (SBRT). This study aims to 1) compare liver tumor volumes on multiphasic liver CT and MR simulation imaging to tumor volumes on MRL images and 2) measure changes in liver tumor volume during SBRT. Materials/
Methods: A single-center prospective MR imaging study was developed before the implementation of MRL liver cancer SBRT. Patients with a diagnosis of hepatocellular carcinoma (HCC), cholangiocarcinoma, or liver metastases treated with SBRT using MRL or cone beam CT (CBCT) were eligible. At simulation, consenting patients underwent T2w and T1w multiphasic CT and multiphasic 3T MR liver imaging in exhale breath hold (standard of care ‘gold standard’), as well as 1.5T MRL T2w-exhale navigator-triggered images (MRL0). Repeat MRL T2w images were acquired at the time of each SBRT fraction (up to 5). Liver tumors were contoured twice on multiphasic simulation CT/MR imaging and on each MRL T2w image, by an GI radiation oncology fellow, with review from an experienced GI radiation oncologist. Liver tumor volumes on simulation imaging were compared to those from baseline MRL imaging (MRL0). Changes in liver volume from MRL0 images to MRL T2w images during SBRT were measured and analyzed using mixed model statistical program. Results: From August 2019 to January 2023, liver tumors from 23 patients with 29 tumors, treated with MRL (22) or CBCT (7) guided SBRT were contoured on 23 multiphasic CT/MR images and 104 MRL images. Diagnoses included HCC without vascular invasion (6), HCC with vascular invasion (4), cholangiocarcinoma (2), mixed cholangiocarcinoma/HCC (1), liver metastases (11), from sarcoma (3), neuroendocrine (3), prostate (2), lung (1), colorectal (1) and parotid (1) cancers. Three patients had multiple tumors. The median gold standard tumor volume was 39.3cc (range 2.8-346.1), and median MRL0 volume was 29.0cc (1.7-325.2). The mean time from MRL0 to MRL1 was 15 days (range 6-30). Tumors were visible on all MRL images. Tumor volumes on MRL0 images were smaller than volumes contoured on simulation MR/CT images obtained on the same day, by an average of 9.5 cc (p=0.024). There was no statistically significant intra-observer variability in contoured volumes (p=1.08). On average, volumes on MRL0 were 4.9cc smaller than those on MRL1-5. There were no predictable volume changes over SBRT (p=0.602). In the majority of tumors, volumes remained stable or increased over SBRT (by 40% in one patient). Less commonly, tumor shrinkage was seen (by 50% in one patient). Conclusion: Liver tumor volumes based on T2wMRL images were smaller than volumes based on multiphasic CT/MR images. This systematic difference needs to be considered when adapting to tumor changes during SBRT. There were no predictable trends in tumor volumes over the course of SBRT, although most tumor volumes remained stable or increased a little over time. Unexpected large tumor growth and shrinkage were seen uncommonly.
