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Thermoplastic Shell Immobilization with Attached Repositionable Lung Blocks and CT Planning for Lateral Decubitus TBI

A Olch

A Olch*, K Wong , University of Southern California and Children's Hospital Los Angeles, Los Angeles, CA


WE-RAM1-GePD-T-6 (Wednesday, August 2, 2017) 9:30 AM - 10:00 AM Room: Therapy ePoster Lounge

Purpose: Lung shielding for total body irradiation is commonly performed to reduce lung toxicity. Because lung blocks are usually attached to an external frame, misalignment of the blocks relative to the lungs due to patient movement during treatment can easily occur. This paper describes a technique where a thermoplastic shell is molded over the patient in the lateral decubitus position and lung blocks are attached to the shell. The patient is simulated in this position, lung blocks are fabricated and dose calculated based on the CT dataset and APPA plan.

Methods: The patient is CT scanned in a body vacuum bag in the lateral decubitus position. A solid 3 mm thick thermoplastic sheet is applied from above the shoulders to the hips. Acuros V13.6 is used to calculate the dose in the treatment geometry and to create partial transmission lung blocks using the “bolus” function (with cerrobend density). Blocks are attached with Velcro to allow for repositioning based on portal images. Transmission through the patient and lung blocks is measured in-vivo with exit OSLDs. OSLDs also are used to measure entrance and exit doses at the umbilicus and head.

Results: Nine patients have been treated. Typically one or two images per side per fraction were required to reproduce the lung block positions with shifts of 0.5-1 cm. Patient movement during each treatment was negligible as determined by post-treatment imaging or observation. Mean OSLD doses at each location were within 5% of Eclipse calculated doses with the range being within 10%.

Conclusion: Thermoplastic shell immobilization with attached lung blocks can more reproducibly shield lungs than when lung blocks are mounted away from the patient, largely eliminating the risk of misalignment during treatment. A treatment planning system can be used to design and accurately calculate the dose distribution for the lung blocks.

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