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A Novel Phantom for Dosimetric Validation of SBRT for Spinal Lesions

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K Papanikolaou

KN Papanikolaou1*, LT Watts2 , E Pappas3 , T Maris4 , C Ha1 , N Kirby1 , K Rasmussen1 , AN Gutierrez1 , S Stathakis1 , (1) University of Texas HSC SA, San Antonio, TX, (2) RII-UTHSCSA, San Antonio, TX, (3) Technological Educational Institute Of Athens, Athens, Attiki, (4) University Of Crete, Heraklion, Crete


SU-F-T-585 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: SBRT is proving to be a very efficacious treatment modality for an increasing number of indications, including spine lesions. We have developed a novel phantom to serve as an end-to-end QA tool for either patient specific QA or commissioning QA of SBRT for spine lesions.

Methods: In this feasibility study, we have selected a patient with a single metastatic lesion in the L5 vertebral body. The patient’s CT simulation scan was used to develop a VMAT treatment plan delivering 18Gy to at least 90% of the target volume, following the guidelines of RTOG 0631. The treatment plan was developed with the Pinnacle planning system using the adaptive convolution superposition calculation mode. The approved plan was re-calculated using the Monaco planning system. We performed a pseudo-in-vivo study whereby we manufactured two copies of a phantom to the exact shape and anatomy of the patient. The phantom was made from the CT images of the patient using a 3D printer with sub-millimeter accuracy. One phantom was filled with a gel dosimeter and the other was made with two ion chamber inserts to allow us to obtain point dose measurements in the target’s center and the spinal cord.

Results: The prescribed dose of 18Gy was planned for the target while keeping the maximum spinal cord dose to less than 14Gy in 0.03cc of the cord. The VMAT plan was delivered to both the gel dosimeter filed phantom and the phantom with the ion chambers. The 3D gel dosimetry revealed a very good agreement between the monte carlo and measured point and volumetric dose.

Conclusion: A patient like phantom was developed and validated for use as an end-to-end tool of dose verification for SBRT of spine lesions. We found that gel dosimetry is ideally suited to assess positional and dosimetric accuracy in 3D.

Funding Support, Disclosures, and Conflict of Interest: RTsafe provided the phantoms and the gel dosimeter used for this study

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