Medical Physics Seminar – Monday, May 6, 2013
Is Dose Accumulation Required? Implication on Dose Guided Adaptive Radiotherapy
Jialu Yu (student of Dr. Wolfgang Tome)
Research Assistant, Department of Medical Physics, UW-School of Medicine & Public Health, Madison, WI - USA –
Planning image is only a snapshot of the patient anatomy at that point and does not stand for the anatomy in the whole treatment process, thus the planned dose is rarely the same with actually delivered dose to patients. The true dose distribution in normal tissues by accumulating dose over the whole treatment process is, in general, poorly quantified. Knowledge of the true dose delivered to organs opens up the opportunity of dose guidance adaptive radiotherapy. The goal is to achieve better tumor control while keeping the normal tissue toxicity well tolerated.
We first studied if deformable image registration (DIR) helped to estimate total delivered dose for prostate patients treated with or without balloons, by comparing fractional and total delivered dose DVHs to planned dose DVHs. The results showed DIR error was acceptable for patients treated with balloons, but not for those treated without balloons. We further studied how the setup parameters affect the dose delivery for patients treated with balloons.
We then did dosimetric predictor study using a cohort of prostate cancer patients with long term follow up. We fitted an NTCP model using the planned dose and included latency effect. We calculated total delivered dose for all patients we can, and statistically compared the dosimetric predictors of total delivered and planned dose. The interesting thing was 91% of the patients had lower total delivered dose than planned.
The next step we did was developing iso-NTCP dose guidance adaptive planning strategies for qualified patients. Since any dose escalation to target must be done in a conservative way, we proposed a pretreatment test that uses historical images for iso-NTCP adaptive plan delivery and produce 95% confidence interval of predictive total delivered dose for the patients. We then delivered the adaptive plan on future images to yield the real total delivered dose. We found both the real and predictive total delivered dose were in acceptable level for this iso-NTCP adaptive planning strategy. The maximum tumor control probability gain we saw was 4%.