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Investigation of the Potential for Dosimetric Accuracy Improvements When Utilizing the Gammex 1467 Advanced Electron Density Phantom for CT Number Conversions of High-Density Biologic Material

A Plypoo1*, E Lobb2, K Ruchala3, (1) Ascension St. Elizabeth Hospital, Appleton, WI, (2) Ascension St. Elizabeth Hospital, Appleton, WI, (3) GAMMEX Inc., Madison, WI


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: investigate the dosimetric effect of using tissue-mimicking bone-like materials with the Gammex 1467 Advanced Electron Density Phantom versus proxy Teflon materials with the Catphan 504.

Methods: number to relative electron density (CT-RED) curves were generated for the Gammex 1467 and Catphan 504 phantoms and entered into the Varian Eclipse 15.5 treatment planning system. Treatment fields through a heterogeneous phantom consisting of plastic water and 2cm of bone-equivalent material were computed using each CT-RED curve and dose values to an embedded Farmer-type chamber were extracted. Dose measurements for this geometry were compared to calculated values for 6MV, 10MV, 10MVFFF, and 15MV photon energies and 5x5, 10x10, and 20x20 cm² field sizes. Ten retrospective patient plans treating in or adjacent to bony anatomy were optimized using the Catphan CT-RED curve and then recalculated using identical field parameters using the Gammex CT-RED curve. Dosimetric changes for target volumes and organs-at-risk (OARs) were evaluated for each plan.

Results: of up to 595 HU are seen in the bone region when comparing the CT-RED curves. Mean agreement between the measured and calculated dose in the bone phantom was 3.6% and 1.1% when using the Catphan and Gammex curves, respectively. For clinical patients, the mean Dmax dose difference in the target, bone, and OARs were 1.2%±0.6% (2.0% max), 1.1%±0.9% (2.3% max), and 1.1%±0.7% (2.6% max) when calculated with the Catphan curve compared to the Gammex curve, while average Dmean differed by 1.3%±0.6% (2.3% max), 0.6%±0.3% (1.2% max), and 0.5%±0.3% (1.3% max).

Conclusion: Catphan 504 and the Gammex 1467 phantoms produced CT-RED curves that differ at higher densities such as bony material. The Gammex 1467 CT-RED curve resulted in better agreement between calculated and measured dose and clinical case investigation showed that proxy materials for bone tissue may lead to underestimation of delivered dose.

Funding Support, Disclosures, and Conflict of Interest: Gammex, INC provided the Gammex 1467 Advanced Electron Density Phantom for evaluation.


Phantoms, Dosimetry, Treatment Planning


TH- External Beam- Photons: treatment planning/virtual clinical studies

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