Pancreatic SBRT on four modern platforms: dosimetric and radiobiological gains with the Ethos ring-gantry linac versus TrueBeam, Halcyon, and helical tomotherapy.

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). MATERIALS AND

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). For HT plans, a dynamic field width of 2.5 cm was used, and the collapsed cone convolution algorithm was utilized. 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 a more favorable planning performance under these standardized conditions. These findings are limited to standard-dose (35 Gy/5F) SBRT planning and do not imply clinical superiority at escalated dose levels. CLINICAL TRIAL NUMBER: Not applicable.