Accepting uncertainty.

Top Line: Have you ever considered dosimetric trade-offs with avoiding entrance dose through hip prostheses during pelvic radiation?

The Study: The reason we do it is to be more confident in our dosimetry. This is all well and good but an potential unanticipated side effect could be excess skin dose–especially when there are bilateral prostheses. This Canadian group did a very practical dosimetric analysis of prostate VMAT plans by incrementally reducing prosthesis avoidance to achieve maximum skin doses between 30-50 Gy. The riddle becomes: you have a set amount of water to fill the three competing interests of target coverage, prosthesis avoidance, and curtailing max skin dose. To further fill one cup, it must come from another. Traditionally we’ve (understandably) focused all our energies on target coverage and thus prosthesis avoidance so we can be uber sure about said target coverage. These authors, however, channeled Heisenberg to land on the approach of accepting more uncertainty in order to limit max skin dose to <40 Gy. This specifically meant lowering the permissible estimated drop in target coverage from <5% to <20%. Yes, this sounds ghastly, but the key word is “estimated” based on a technique that intentionally grossly overestimates the density of the typical prostheses by 4-5 fold. Other pearls of wisdom include switching out your carbon fiber couch top to styrofoam.

TBL: Don’t let perfect be the enemy of good when it comes to allowing dose entrance through hip prostheses during prostate radiation in order to avoid unanticipated skin toxicity. | Mak, Pract Radiat Oncol 2022

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