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Use of High-Z Backscatter Material For Dose Enhancement in HDR Surface Applicators

M Bhagwat*, R Cormack , I Buzurovic , D O'Farrell , T Harris , P Devlin , Brigham and Women's Hospital, Harvard Medical School, Boston, MA


(Wednesday, 8/1/2018) 10:00 AM - 10:30 AM

Room: Exhibit Hall | Forum 4

Purpose: AAPM's Task group-43 calculation algorithm includes full scatter conditions. During HDR therapy with surface applicators these conditions are not met, and the delivered dose is lower than the planned dose. The purpose of this work is to investigate if thin slabs of high-Z material placed atop the applicator can provide the required dose enhancement.

Methods: Radiation dose deposited by a coplanar 3cmx3cm applicator at a distance of 1.0cm perpendicular to this plane at the centre of the square was calculated using MCNP6. A Freiberg applicator has an inherent backscatter material of 0.5cm which was used as reference. The calculation was repeated by placing additional backscatter slabs of thickness 0.5cm, 1cm, 2cm. The materials used were water, Al, Ti, Fe, Cu, Pb and W. Additional calculation with 5cm of water provided dose under full backscatter conditions.10^8 histories were simulated on a dual Xeon 12-core processors running 48 threads to achieve a statistical uncertainty of less than 1%.

Results: A plot of dose enhancement with increasing Z is shown in figure 1. The dose is enhanced with increasing Z until, reaching a peak at Z=26(Fe) and then begins to drop. The dose enhancement also reaches saturation for Pb and W. Figure 2 shows that 5cm of water is sufficient to provide full scatter conditions, and there is 1.6% dose increase compared to zero backscatter in this simple model. Thus, an equivalent dose enhancement of 1.6% can be achieved, as seen from figure 1, by placing a smaller amount higher Z material. About 0.2cm of Fe or Cu can provides 1.6% of dose enhancement.

Conclusion: A practical method to compensate for the decrease in dose due to lack of water as backscatter material in surface applicators is to use a thin slab of Fe or Cu.


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