Having no rectum would require a change to how brachytherapy placement is done, but I did read of someone having it done a long time ago. I think it can be done using real-time MRI guided placement in specialist centres with the right kit..
Being T3b, I presume we're talking about high dose rate brachytherapy here? HDR brachy does have an extra dose adjustment fine tuning which LDR doesn't and could be useful where placement is more challenging.
Is this just brachytherappy, or will it include a reduced dose of external beam too?