Shot in the dark.
Because you don’t need the lights on when you’re managing prostate motion with electromagnetic transponders. That’s the technique used in this trial of prostate SBRT to deliver 7.4 Gy x 5. The early grade 2 GU toxicity rate of 23% fell to 9% “long-term” (with a median follow-up of 36 months). Acute and “chronic” grade 2 GI toxicity rates were in the single-digits. Zero grade 3 bladder or bowel toxicities were recorded. Ultimately, baseline issues predicted respective toxicities. This should help your next risk-benefit discussion on prostate SBRT be more than a shot in the dark.
Still not 100% certain the transponders and their real-time tracking are needed. I have seen the random motions, somewhat (almost unbelievably) chaotic in some patients. Back when I first tracked the accuracy of the fiducial method we found great accuracy (~2-3 mm PTV margins feasible, http://www.redjournal.org/article/S0360-3016(06)00105-2/fulltext). Later when we assessed intrafraction motion by re-IGRTing at the end of the day's fraction minutes later, >95% of shifts were <2 mm FWIW. A 23% acute and 9% late G2 GU toxicity rate would be a little concerning, to me, as would greater than 1-2% G2 rectal toxicities (http://www.redjournal.org/article/S0360-3016(07)02668-5/fulltext, http://www.redjournal.org/article/S0360-3016(08)01599-X/fulltext). But perhaps that's the price of extreme hypofx and progress.
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