More or less.

Top Line: Are there particular sub-volumes of the prostate or rectum where radiation dose variation is associated with a higher risk of PSA progression or toxicity? 
The Studies: In two separate publications, the TROG group has analyzed the risk of PSA progression and rectal toxicity in the RADAR trial using voxel-based dosimetric analysis. In addition, they validated the findings using data from the RT01 and CHHiP trials. The idea here is to go beyond the traditional DVH analysis and pinpoint the actual voxels where dose variation are most meaningful. In total, over 1200 treatment plans were analyzed to compare the average dose within each voxel of the treatment plan between groups of patients with and without the event of interest. In the PSA study, patients with PSA progression had a significantly lower average dose along the posterior border of the planning target volume. On the other hand, patients who developed rectal bleeding had significantly higher dose on the anterior rectal wall adjacent to the target. This isn’t really a big surprise. Most prostate cancers are located in the peripheral zone and virtually all decisions regarding dose coverage and toxicity boil down to trade-offs between higher dose to posterior target versus lower dose to rectum. Importantly, these findings mainly come from patients treated with 3D-CRT and without IGRT. The findings weren’t as significant for the CHHiP treatment plans where mostly IMRT was used.
TBL: Dosimetric trade-offs at the prostate/rectum interface significantly influence both PSA progression and rectal toxicity, though the optimal balance remains TBD. | Marcello, Radiat Oncol 2020 and Int J Radiat Oncol Biol Phys 2020


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