Author: Ali Hosni, Oleksii Semeniuk, Andrea Shessel, Teo Stanescu 👨🔬
Affiliation: Princess Margaret Hospital, Princess Margaret Cancer Centre, Brown University Health 🌍
Purpose: To report on early clinical experience with a two-phase radiotherapy approach for esophageal cancer patients, utilizing CBCT-based conventional C-arm linear accelerator radiotherapy and MR-guided online adaptive radiotherapy on the Unity MR-Linac system. This approach is part of a larger PET-CT and PET-MR study investigating MR-based biomarkers for adaptive radiotherapy.
Methods: A two-phase radiotherapy protocol was implemented, consisting of 40 Gy in 20 fractions delivered to a large CTV using conventional radiotherapy, followed by a 10 Gy boost in 5 fractions to the GTV utilizing online adaptive MRI-guided radiotherapy on a Unity MR-Linac system. Clinical goals for the Unity boost were derived from dose accumulation analysis of additional patients enrolled in the study who received conventional radiotherapy without MRI online adaptation. These conservative goals implicitly provided organ-at-risk (OAR) sparing. A previously developed dose-of-the-day (DOTD) methodology was adapted for esophageal cases to evaluate intra-fraction motion. Deformable image registration (DIR) was performed between pre-treatment adapt-to-shape (ATS) and during beam on treatment delivery (BON) MR images in RayStation treatment planning system. Target and OAR contours were propagated from ATS to BON using DIR, followed by rigid registration of BON contours onto the ATS dose distribution. Changes in dosimetric metrics were then calculated and recorded
Results:
MR image quality was adequate for GTV delineation and monitoring patient setup under motion conditions for all Unity treatment fractions. The DOTD analysis showed that GTV and OAR clinical goals were within ±10 cGy/fraction of the corresponding clinical metrices.
Conclusion: Online adaptive MRI-guided radiotherapy is feasible for esophageal cancer treatment and enables improved sparing of organs at risk.