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Verification of Deformable Dose Calculation with Phantom Measurements

J Adams

J Adams1*, H Zhong2, (1) Wayne State Univ School of Medicine, Detroit, MI, (2) Henry Ford Hospital System, Detroit, MI

TU-C-108-1 Tuesday 10:30AM - 12:30PM Room: 108

Purpose: Dose accumulation involves many computational procedures which may introduce error. The purpose of this study is to utilize a novel deformable dosimetry phantom to experimentally verify the total dose accumulated by 4D dose calculation algorithms.

Methods: Lung phantom consisting of heterogeneous sponge material and tissue-equivalent tumor. The sponge is enclosed in a latex cylinder, compressed by a piston to simulate the diaphragm motion. TLD-100 thermoluminescent dosimeters were placed in the sponge and inside the tumor for 4D-dosimetry verification. Three TLDs were used at each point for better statistics. A four phase 4DCT was taken for planning purposes. A 5-beam 6MV 3D conformal plan was developed with 95% of the ITV receiving the prescribed dose of 200 cGy, which may help to maintain a linear TLD response. The plan was delivered under the same deformation pattern with the BrainLab Novalis system, and exported to Eclipse and Pinnacle for comparison. CT phase images were registered to end-exhale (EE) phase, when sponge is maximally compressed, using a B-Spline-based algorithm. The resultant displacement vector field was output for dose mapping. Dose was warped to EE phase using an in-house developed, subvoxel projection method and accumulated for comparison with the TLD measurements.

Results: The tumor dose agreed very closely to the accumulated dose and was measured to be 0.5% lower with the TLDs. Another point of measurement was ~2 cm from the tumor, near the edge of the field, and was measured 5.4% higher with the dosimeter. A point of high field gradient ~5 mm from the tumor was measured 12.5% higher than calculated. Larger uncertainties in the TPS are expected in this gradient region. Dose warping improved the tumor dose calculation by 29.4%.

Conclusion: The deformable phantom could be a valuable tool to verify the total dose calculated by adaptive treatment plans.

Funding Support, Disclosures, and Conflict of Interest: The research was supported by NIH/NCI R01CA140341.

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