SU-E-T-553: Dose-Mass Vs. Dose-Volume Optimization: A Phantom Study

PURPOSE

To demonstrate the merits of mass-based optimization in comparison with volume-based optimization using a simple test phantom.

METHODS

Dose-volume-histogram-based (DVH-based) quadratic objective functions are converted into dose-mass-histogram-based (DMH-based) objective functions by multiplying per-voxel volumes by per-voxel densities within the objective. A digital phantom with a 1.0 g/cm³ target is constructed for irradiation with two beams: one beam path contains a 0.2 g/cm³ volume-of-interest (VOI0.2) while the orthogonal beam-path contains an equal-volume 0.8 g/cm³ VOI (VOI0.8). Monitor-units are computed to achieve a 100 cGy average target dose for each individual beam, and for two-beam DVH-based and DMH-based optimizations.

RESULTS

For single-beam irradiation through VOI0.2, the average dose to VOI0.2 is 20.5 cGy. For single-beam irradiation through VOI0.8, the average dose to VOI0.8 is 25.2 cGy. Traversing the low density volume results in ∼23% lower dose. When DVH- and DMH-based optimizations are performed such that target dose-volume-histograms of the optimizations match, for the DVH optimization 60% vs. 40% of the dose is delivered through VOI0.2 vs. VOI0.8. For DMH-optimization, the split between dose delivered through VOI0.2 vs. VOI0.8 is 70% vs. 30%.

CONCLUSIONS

When density is constant, there is no difference between DVH- and DMH-based optimizations. However, in heterogeneous media, DMH and DVH solutions differ when low and high density materials have the same dose objectives. Delivering target dose through lower density VOIs facilitates target dose deposition due to a decrease in attenuating material, and the decreased attenuation lowers dose to the low density VOI. From mathematical and physical points of view dose-mass optimization is more general than dose-volume optimization.

SU-E-J-103: Characteristics of Bladder Wall Deformation as a Function of Bladder Filling

PURPOSE

ICRU report 83 recommends delineation of the wall for hollow organs such as the bladder. For image guided radiotherapy, there is no guidance as to how to delineate deformed walls as the filling of hollow organs change. This work investigates characteristics of bladder wall deformation as a function of bladder filling in a controlled process.

METHODS

CT images of a fresh pig bladder are obtained in-air with different bladder air fillings. A plastic hose inserted into and glued to the bladder neck forms an air-tight seal for filling with a syringe. At each air-filling level, a helical CT (90kV, 90mAs) of the bladder is obtained. The pixel size on all CT images is <0.5 mm. Images are imported into a commercial planning system for delineation and statistical evaluation. Bladder walls at each filling status are auto-contoured, then edited for quality assurance slice by slice. The auto-contouring threshold is selected to obtain consistent volumes of the resulting region-of-interest at each filling status.

RESULTS

When the bladder wall interior volume increases by 360 cm³ from injecting air, the wall thickness decreases from 2.9 mm to 1.2 mm. The decrease in wall thickness is accompanied by a decrease in the wall CT number.

CONCLUSIONS

For a pig bladder, the wall thickness decreases with increased air filling. The CT number (hence apparent density) of the volume decreases as the bladder expands. Further investigation is ongoing to determine if the apparent density decrease is real, due to CT reconstruction, or due to the bladder being in an air, rather than liquid environment. Characterization of hollow organ wall thickness variations may be important for image guided radiation therapy and organ wall dose evaluation. Support: P01CA116602.