@jeffmarc The cancer center my husband goes to also offer Ethos, but he is not a candidate for it because his radiation area is vast anatomical region (prostate bed & pelvic lymph nodes plus the SIB). The Ethos machine is made by Varian. From what I was told by another PC patient in Germany, it is similar to MR Linac systems (such as Elekta Unity or ViewRay MRIdian) in term of both system being advanced radiotherapy technologies, which is designed for Online Adaptive Radiotherapy (ART). It does take longer time to complete each session, comparing to VMAT technique.
https://westernradiationoncology.com/treatment-technology/adaptive-radiation-therapy/