Yes, that's what I was afraid of. Understand that cone beam CT (or stereoscopic X-rays) are
used with fiducials or transponders by many ROs doing SRT. They place them in the walls of the prostate bed. That is no guarantee that toxicity will be reduced (ask 142!), but I think it is prudent (just my non-professional opinion), especially because the prostate bed and LNs move separately.
"Total positioning errors >5 mm occurred in 14.1%, 38.7%, and 28.2% of all fractions in the LR, SI, and AP axes, respectively. "www.redjournal.org/article/S0360-3016(06)03226-3/abstract
To what extent those positioning errors affect toxicity is anyone's guess.
In the following, read the section "Image-guided radiotherapy."/www.ncbi.nlm.nih.gov/pmc/articles/PMC4646477/
This gets more complex when the pelvic LN area is simultaneously treated with the prostate bed. I know this is controversial and many believe that soft-tissue alignment is adequate. Maybe worth hearing from a few different ROs.