The way I look at this is that both the EBRT and Brachytherapy comprise the primary treatment. In my case, I started HT one month before radiation (IMRT and HDR BT). I understand this was all part of my primary treatment and that the HT would technically be neo-adjuvant as it preceded the planned radiation treatment.
Referring to the "boost" I think that may be more of a describer than a technical term. Dr. Frank Critz (Prostrcsion) pioneered the combined use of EBRT and LDR BT and refers to the EBRT as boosting the brachytherapy effectiveness when given together. So one calls the BT a boost and the other calls the EBRT a boost . Regardless of the description, the point is that together they work better for many men than either does as mono-therapy.