Terminology is confused here, because in the context of prostate cancer treatment, we usually use the wrong term.
Whole pelvis here or as used by a prostate oncologist usually refers to to prostate, seminal vesicles, and pelvic lymph nodes, and is quite specific to just those. A radiotherapist/radiographer would call this prostate and nodes which is the correct term. It's an accurately targeted dose, and not just blasting the whole pelvic area. When I elected to have it, my lymph nodes were treated at 46Gy prophylactically. I asked about the risk of lymphedema, and my oncologist said it was rare anyway with radiotherapy, and he'd never had a case at the lower prophylactic dose, which is why I went ahead with it. A higher dose is used when there is known cancer in them.
Whole pelvis to a radiotherapist/radiographer (or anyone not talking about prostate cancer) is something completely different - it's a palliative treatment to the whole pelvic bone (and not soft tissues), and is very rarely done.
Treating lymph nodes can leave them working normally, so I imagine it can avoid lymphedema, but there's probably a risk too - I don't know how the two balance out. If a surgeon thinks significant pelvic lymph node dissection (PLD) is going to be required with a prostatectomy, that would certainly be a very significant factor swaying towards radiotherapy in my view. Lymphedema can be a significant disability, and it doesn't have any good treatments readily available. Lymph node transplants are being tried at in the US.