What is the best IGRT strategy in treating the prostate bed (adjuvant or salvage): Daily anatomy KV images vs. fiducial seed kv images vs. CBCTs?
What is your IGRT strategy for post-op prostate radiation therapy?
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For our post-prostatectomy men, we use daily kV and line up to surgical clips if available (contouring out a clip each quadrant of the prostate bed if possible) with a PTV expansion of 5 mm. If clips are not present, then we use CBCT to evaluate soft tissue alignment and use a slightly larger PTV of 6-9 mm. The tighter PTV with surgical clips can help improve your normal tissue DVH (ref below).
When using CBCT, we start off with daily imaging, but if patients demonstrate good setup we will relax to twice weekly and just use kV daily for bony anatomy as a reliable surrogate for the prostate bed. In roughly 15% of cases we will see mismatched bladder or rectal filling that will prompt either daily use of CBCT to align to the posterior aspect of the bladder, or force consideration of replanning. We have not implanted fiducials in the postop setting, although I have heard of some practices that do. A potential issue is that the markers may represent the bottom aspect of the CTV more adequately than the top, and I would also expect these to be at greater risk for seed migration than in an intact gland.
I think setting up to bony anatomy only without evaluating soft tissue presents a risk to miss the prostate bed for men whose rectal or bladder filling does not set up reproducibly, especially if PTV margins are too tight.