Room: Karl Dean Ballroom A1
Purpose: Targeted marrow irradiation (TMI) as part of an investigational conditioning regimen prior to bone marrow transplantation is being studied as an alternative to total body irradiation (TBI). Due to the considerable difference in dose distributions and uniformity encountered between these modalities, we use biological modelling in quantifying the dosimetric improvement offered by TMI.
Methods: Ten patients treated at our institution with a multi-isocentric VMAT TMI technique on a standard linac were re-planned in Phillips Pinnacle ver9.10 for simulated extended-field TBI treatments. The CTV was defined as the entire bony anatomy plus the spleen. The generalized equivalent uniform dose (gEUD) of Niemierko was used to estimate the effective dose to the CTV and various organs at risk (OARs), varying the empirical volume parameter a in a robustness analysis.
Results: For volume parameters obtained from the published literature, the percent decreases in gEUD for the TMI versus the TBI plans for the lungs, brain, kidneys, liver, and heart were 47.8%, 16.1%, 54.8%, 51.0%, and 43.7%, respectively (p<0.001). Between the TMI and TBI plans, no significant difference in CTV gEUD was found for the volume parameter a varied from -20 to -5, despite the 31.7% lesser homogeneity index of the TMI plans (p=0.002). TMI provided significant (p<0.05) improvements to most of the studied OARs for a varied over a large range: from 0 to >25 for the lungs, kidneys, liver, and heart, and from 0 to 14.7 for the brain.
Conclusion: Despite the relative non-uniformity of dose in the TMI plans as compared to the TBI plans and the uncertainty in a, biological modelling demonstrates that TMI plans are significantly better in OAR avoidance, and not significantly different in CTV coverage. The robustness of this result to a wide range of assumed values of a provides improved confidence in TBIâ€™s dosimetric superiority.