Are cold spots on an external beam plan due to prostate fiducials concerning?

The dosimetry around fiducials is complex, with both some minimal shielding and increased dose due to scatter coming into play. Honestly, I'm not sure how accurate the planning system algorithms are in determining dose in the immediate vicinity around the typical tiny fiducial. On balance, I think the beneficial effects gained from improved accuracy of treatment delivery outweigh any theoretical concerns about dose inhomogeneity. Also, I have not seen any reports of isolated failures in the prostate at the site of a fiducial. Thus, I have not changed my planning technique to account for the presence of fiducials. If you're really concerned, you could use a simultaneous integrated boost to deliver a slightly higher dose to the region.

We recently looked into this with our physics team. I fully agree with Dr. @Dr. First Last about the difficulties in knowing how accurate the planning systems are around the tiny fiducials.

When using Acuros in Eclipse, we noticed that the optimizer appears to compensate for the cold spots by forcing additional dose at the fiducials. This, in turn, appears to create a pattern of a hot outer shell, cold inner shell (immediately adjacent to the fiducials), and then hotter center within the fiducial. If the TPS can be believed, we are still able to keep the cold spots within at least ~97% of the prescription dose.