What duration of ADT + abiraterone do you recommend prior to initiation of HDR brachytherapy followed by EBRT for very high risk localized cT3bN0 prostate cancer?

This is a short answer (for me) given the lack of data to support brachy boost in T3b disease in a randomized trial.

The only randomized multicenter trial comparing dose-escalated EBRT to brachy boost is ASCENDE-RT. They specifically excluded T3b patients (see supplementary content for all exclusion criteria).

Hoskin trial tested HDR boost in a small single center randomized trial to very low dose EBRT and does not even break down if T3b patients were included, but given they gave full dose only to the proximal SVs, one can deduce likely few if any were included.

The STAMPEDE trial that tested the benefit of abiraterone, but did not allow brachytherapy boost. Was solely EBRT.

So in summary, there is no randomized data supporting brachyboost in ...