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Determination of the Water Equivalent Thickness of Strata XRT Gel and Evaluation of the Dosimetric Effect of Its Application On the Skin of Proton Therapy Patients

A Gautam*, S McGovern , X Zhu , N Sahoo , UT MD Anderson Cancer Center, Houston, TX


(Sunday, 7/14/2019)  

Room: ePoster Forums

Purpose: To determine the water equivalent thickness (WET) or proton relative stopping power (RSP) of Strata XRT Gel and to evaluate the dosimetric effect of the application of this gel on the skin of patients receiving proton therapy.

Methods: The radiological properties of the Strata XRT gel was studied by irradiating the gel with passive scattering proton beams. Measurements of depth dose curves were carried out by using Zebra (IBA, Schwarzenbruck, Germany) device. A container made of Plexi was used to put 5.2 mm of Gel in the path of the proton beam. The depth dose curves obtained with and without gel in the container were used to determine the WET and RSP of the Gel. A CT scan of Strata XRT was performed to evaluate the radiological path length calculated by the Varian Eclipse treatment planning system.

Results: Zebra data show that WET of 5.2 mm of Gel as 4.6 mm giving the RSP of the Gel as 0.893. When the beam enters through 1 mm of Gel, the skin dose will increase by no more than 0.5% of the skin dose without the Gel. CT scan data show a higher attenuation for the Gel compared to water.

Conclusion: The Zebra data show a lower RSP of Gel for proton beam. The CT scans with the Gel may not reflect the correct proton stopping power for the Gel. To avoid any inaccuracy in range calculation, it may be better not to scan the patients with Gel applied. It is suggested that when the Gel is applied during the treatment with a thickness of no more than 1 mm, the distal margin around the CTV should be increased by 1 mm in treatment planning to account for the presence of the Gel.




IM- Particle (e.g., proton) CT: General (Most aspects)

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