Author: Cheryl Anderson, Lara Best, Amanda Cherpak, Jennifer DeGiobbi, Marc C. LeBlanc, R. Lee MacDonald, Peter R. Martin, Natasha McMaster, Murali Rajaraman, James L. Robar, Lucy Ward, Derek Wilke, Kenny Zhan 👨🔬
Affiliation: Department of Radiation Oncology, QE2 Cancer Centre, Nova Scotia Health 🌍
Purpose: This work investigates the potential benefit of adaptive planning using the Ethos radiotherapy system (Varian Medical Systems) with direct-dose calculation on HyperSight cone beam CT (CBCT) for subjects enrolled in the HyperSight-ARCHER study1. HyperSight CBCT allows for accurate dose calculation relative to planning CT2, however a need remains to evaluate its utility in adaptive radiotherapy.
Methods: Seven patients with head and neck cancer (oropharynx site; prescribed 70 Gy/35 fractions) were imaged on Ethos with HyperSight (iCBCT Acuros reconstruction) on the same day as planning CT acquisition, and again on fraction 21 of radiotherapy treatment. Structures were contoured on all image sets, with target contouring performed by radiation oncologists. Clinically acceptable radiotherapy plans were created in the Ethos treatment planning system for the day 0 and day 21 (Day 21-Adapted) HyperSight CBCTs using the same optimization directives. The day 0 and day 21 CBCTs were rigidly registered, and the day 0 plans were forward-calculated on the day 21 images (Day 21-Original). Planning target volume (PTV) coverage and organ-at-risk (OAR) sparing were compared for the Day 21-Adapted and Day 21-Original plans.
Results: The Day 21-Adapted plans show statistically significantly increased PTV coverage and decreased dose to the parotids and oral cavity when compared with the Day 21-Original plans (p<0.05, Wilcoxon signed rank). Furthermore, clinically unacceptable PTV coverage was observed in 6/7 of the Day 21-Original plans (i.e. D99%<6160cGy or D95%<6650cGy for PTV-high risk; D99%<5000cGy or D95%<5320cGy for PTV-low risk).
Conclusion: Adaptive planning using the Ethos platform with HyperSight allows for significantly improved target coverage and OAR sparing for head and neck radiotherapy patients by day 21 of treatment. These results highlight a potential dosimetric benefit in adaptive radiotherapy, made efficient through direct-dose calculation on CBCT.
1Cherpak et al, COMP ASM 2024
2Sijtsema et al, Radiother Oncol 2025