For prostate cancer patients with N+ disease, is there evidence demonstrating improved outcomes with the addition of nodal RT, as opposed to prostate RT alone plus ADT?
There is no direct evidence. In the STAMPEDE trial, which showed benefit of RT in node positive patients, RT involved prostate and nodes in a majority of patients (>80%).
I agree with @Dr. First Last that the evidence is indirect. Parker et al. STAMPEDE M1|R0 stratified by N0/N1, noting that those patients also all had metastatic disease.
We 'interpolate' from the evidence for T3-T4N0M0 and Tany Nany M1 that RT to the prostate is worthwhile for N1M0 patients. And we extrapolate that treating regional N1 disease probably makes sense for M0 patients.
Parker et al. nicely summarize in their Discussion:
"It is well known that prostate radiotherapy improves survival for men with locally advanced (T3–4 N0 M0) prostate cancer.22 We have now found that prostate radiotherapy also improves survival for men with a low metastatic burden (Tany Nany M1) prostate cancer. It therefore seems safe to conclude that radiotherapy would also improve survival for men with pelvic node-positive prostate cancer (Tany N1 M0). This is important, because it is not feasible to do randomised trials specifically in men with non-metastatic node-positive prostate cancer and because such men often receive systemic treatment alone. In the current study, roughly 60% of patients were N1 in both the high and low metastatic burden subgroups. The benefit we have shown for prostate radiotherapy in prostate cancer with a low metastatic burden also raises another question: would there be further benefit from additional radiotherapy to the oligometastases themselves?"
The STAMPEDE protocol does also address N1 circumstances for SOC RT. I read this to mean in patients not on the M1 study of prostate-only RT. At any rate, they agree with NCCN. Copying from the protocol below:
6.1.2.B N+M0 Participants
For participants with node-positive, M0 disease there are no randomised data on whether radiotherapy is indicated or not. However the NCIC PR.3 / MRC PR07 trial included participants with unknown nodal status who received whole pelvic radiotherapy [Warde et al. 2011] and demonstrated a large overall benefit. Additionally, non-randomised data from the STAMPEDE control arm suggests that the benefit observed in participants with N0M0 disease can be extended to those with pelvic nodal involvement. Therefore the STAMPEDE TMG recommends that pelvic nodal radiotherapy be considered for participants with node-positive, M0 disease at the discretion of the treating clinician [James et al. 2015].