Pancreatic SBRT on Four Modern Platforms: Dosimetric and Radiobiological Gains with the Ethos Ring-Gantry Linac Versus Truebeam, Halcyon, and Helical Tomotherapy
Abstract
Purpose
To perform the first comprehensive dosimetric and radiobiological comparison of four modern delivery platforms for stereotactic body radiation therapy (SBRT) in pancreatic cancer: a conventional C-arm linac (TrueBeam), two established ring-gantry systems (Halcyon and helical tomotherapy [HT]), and the novel online-adaptive ring-gantry platform (Ethos).
Methods
Eighteen patients with pancreatic cancer were retrospectively planned to 35 Gy in 5 fractions using identical contours and dose constraints on all four platforms. Three-arc VMAT plans were generated for TrueBeam, Halcyon, and Ethos (Acuros XB dose calculation for Ethos; AAA for TrueBeam/Halcyon). HT plans used a 2.5-cm field width helical delivery. Target coverage, conformity index (CI), homogeneity index (HI), gradient metrics (R50, D2cm), organs-at-risk (OARs) doses, and equivalent uniform dose (EUD) with tissue-specific parameters were compared using statistical tests.
Results
All plans met clinical constraints. Coplanar platforms (TrueBeam, Halcyon, Ethos) achieved significantly better CI vs. HT (0.90 ± 0.02 vs. 0.84 ± 0.10) and lower R50 (3.48-3.53 vs. 4.71 ± 0.58) and , and steeper dose fall-off compared with HT. Ethos yielded the lowest mean doses to bilateral kidneys (2.80 ± 0.57 Gy vs. 3.98-4.93 Gy on other platforms) and the most favourable high-dose sparing for bowel, stomach, and duodenum. EUD analysis confirmed Ethos achieved the lowest values in seven of eight OARs, with the largest gains in kidneys (6.4 ± 1.4 Gy vs. 9.1-11.8 Gy) and bowel.
Conclusion
Among contemporary platforms, Ethos provided the better advantageous dosimetric and radiobiological profile for pancreatic SBRT, driven by superior optimisation and dose calculation accuracy.