SU-E-T-734: The Feasibility of Using Polymer Gel Dosimetry to Measure the Dose Around Thin High Z Materials Irradiated with High Energy Photon Beams

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Abstract

Purpose:

The purpose of this study is to investigate the feasibility of 3D dose measurements near thin high z materials using high energy photon beams.

Methods:

A MAGAT-type polymer gel was manufactured in-house. Lead foils of dimensions 1cm × 1cm × 0.5mm were encapsulated in 0.06mm thick tape and placed in two foil supports with 4cm depth and 0.7cm separation. The foils and supports were placed in a 500ml polystyrene container and the gel was poured into the phantom. The phantom was irradiated with a 18MV photon beam of 5cm × 5cm field size perpendicular to the foil at 100cm SSD. The gel was imaged with a Siemens 3T MRI scanner using the CPMG pulse sequence. DOSXYZnrc was used to calculate the expected dose and the Monte Carlo dose was compared with the gel. Enough histories were simulated so that the average error was less than 1%. The percentage difference of the dose between the heterogeneous gel and the homogeneous water was computed along and perpendicular to the beam axis.

Results:

Depth dose measurements made on the central axis showed increased dose upstream and between the foils, but had a decreased dose downstream. The range of increased dose was 1.8cm above and 0.7cm below the foils. The minimum dose point between the foils had an enhancement of 7.4%. The dose reduction downstream averaged around 5%. There was very good agreement between the Monte Carlo measurements and the gel doses. The gel and Monte Carlo results were within 2% at the minimum point.

Conclusion:

It was demonstrated that the MAGAT polymer-gel can be used to measure the fine structures around small metallic objects. The results suggest that the dose of a 18MV photon beam can be increased in between thin high Z foils while reducing the dose downstream.

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