Room: Exhibit Hall
Purpose: To commission a simplified total body irradiation (TBI) scatter dose calculation.
Methods: Our TBI patients are positioned semi standing for APâ‚‹PA treatments and lying down in a crib for lateral and decubitus treatments at 340 cm source-to-axis distance (SAD). An extended SAD dose reference is utilized, which is measured with a stack of solid water at this SAD. Scatter, depth, and off-axis corrections are made to this reference. The scatter correction is made with dose calculations after density overrides of elliptical contours, with various widths and thicknesses, relative to a contour of the solid water stack. The depth corrections were made with solid water based tissue phantom ratio measurements. Our TBI treatment field is divided into fixed sectors for the use of dose compensators. Offâ‚‹axis measurements were performed at these sectors across the field. Endâ‚‹toâ‚‹end measurements were performed in solid water stacks and in an anthropometric phantom. Dosimeter measurements were performed on the phantom surface, to replicate the inâ‚‹vivo measurement setup for our patients, and additionally at the phantom midline. Our final dose equation included all correction factors previously mentioned and was computed in Excel from the measured patient width and thicknesses, which yielded the number of beam monitor units necessary for treatment and compensator thickness for each sector.
Results: Internal and external dose verification readings were within a 10 percent dose deviation as recommended by TGâ‚‹29. Our previous dose calculation, which excluded offâ‚‹axis and patient scatter correction factors, had an average dose measurement deviation of 6 percent. This same number is reduced to 2 percent with the new technique.
Conclusion: The lateral scatter correction factor improved dose accuracy. We have implemented a simplified procedure for commissioning of a total body irradiation program with scatter corrections.