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PShield: An Algorithm for Optimization of PET/CT Shielding


A Pasciak

A Pasciak1*, A Jones2, (1) University of Tennessee Medical Center, Knoxville, TN, (2) UT MD Anderson Cancer Center, Houston, TX

MO-F-213CD-10 Monday 4:30:00 PM - 6:00:00 PM Room: 213CD

Purpose:
Calculation of radiation shielding requirements for high-workload PET installations using the methods proposed by AAPM TG-108 can be difficult. The principle challenge that makes PET shielding design more complex than other diagnostic imaging modalities, aside from the higher photon energy, is that it is a multisource problem for which no unique solution exists. The PShield algorithm incorporates three-dimensional numerical methods to optimize PET shielding and deliver a cost-optimized solution while making no approximations.

Methods:
PShield uses a sequential quadratic programming routine to optimize PET shielding by minimizing a cost function in 3-dimensions using extrapolations of the TG-108 formulas. PShield makes no approximations and accounts for the contribution of every radiation source to the dose rate at every location in the problem using a discrete mesh. We used two simple examples of shielding problems to compare PShield with the TG-108 methods.

Results:
The benefit of applying an optimization routine to an indeterminate problem is the identification of the only solution to the problem that minimizes the desired cost function. Choosing a poorly optimized solution can result in a shielding design that requires as much as 50% more shielding than an optimized design to reach the same dose rate at a given control point. The increased accuracy afforded by PShield ensures that dose rates at every point in a control area never exceed the design dose, whereas a reasonable design based on the TG-108 methods may have hot spots where the dose rate exceeds the design dose by a factor of 2 or more.

Conclusions:
PShield is an exact three-dimensional numerical solution for optimal PET shielding which identifies a singular solution which is cost-optimized. This is especially important for modern PET/CT suites, where increases in scanner capabilities have resulted in more complex shielding problems and the potential for high occupational doses.

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