Dosimetric Analysis of Patients Treated with Stereotactic Body Radiation Therapy for Malignant Portal Vein Tumor Thrombosis Using MRI-Guided vs. CT-Guided Linear Accelerator

Abstract

The use of MRI-guided linear accelerators (MRI-LINAC) for radiation therapy has allowed for increased treatment precision and real-time tumor tracking. We hypothesized that the use of an MRI-LINAC, recently acquired by our institution, would allow us to treat malignant portal vein tumor thromboses (PVTT) closer to the origin of the portal vein with similar doses to the liver and nearby GI organs and with similar treatment toxicities compared to patients treated using a CT-guided linear accelerator (CT-LINAC). We retrospectively assessed patients who received SBRT for treatment of PVTT within the UVA health system using the electronic medical record. We contoured the origin of the portal vein on the simulation scans (MRI or CT) for each patient. The shortest distance from the GTV to the portal vein origin and to organs at risk (OAR; duodenum, stomach or "other intestine") were measured. The PTV, volume of the liver receiving less than 18 Gy during treatment, and the Dmax to the portal vein origin and OAR were determined. The means for each variable were then compared between CT-LINAC and MRI-LINAC using independent samples t-tests. Treatment toxicities were graded using CTCAE version 5.0 guidelines. All plans met institutional dose constraints for SBRT. A total of 6 patients treated on CT-LINAC and 3 patients treated on MRI-LINAC were identified (mean dose CT 37.7 Gy [range 21-50] in 2-5 fractions; MRI 48.3 Gy [range 45-50] in 5 fractions). The GTV was significantly closer to the portal vein origin in the MRI-LINAC group (mean CT 92.75 mm ± 45.89; MRI 32.00 mm ± 29.46; p = 0.04). The GTV trended towards being closer to OAR in the MRI-LINAC group (mean CT 43.81 mm ± 37.09; MRI 4.36 mm ± 1.85; p = 0.059). The Dmax to both the portal vein origin (mean CT 0.58 Gy ± 0.33; MRI 22.49 Gy ±24.16; p = 0.024) and OAR (mean CT 13.38 Gy ± 5.98; MRI 36.23 Gy ± 5.38; p = <0.001) was significantly higher in the MRI-LINAC group. The PTV (mean CT 152.11 cc ± 169.94; MRI 122.47 cc ± 114.65; p = 0.398) or volume of liver receiving less than 18 Gy during treatment (mean CT 1287.87 cc ± 285.20; MRI 1743.88 cc ±961.94 = 3; p = 0.147) were not significantly different between the two groups. No patients in either the MRI-LINAC or CT-LINAC group reported acute toxicities greater than Grade 1. We found that patients with PVTT treated with MRI-LINAC SBRT at our institution were more likely to have central tumors near dose-limiting OAR. However, due to superior tumor delineation and dynamic tumor tracking, these patients were able to be treated without decreasing treatment dose or increasing acute GI toxicity compared to patients treated with CT-LINAC. A larger sample size and longer follow-up period is warranted to corroborate these initial findings.