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A New Device for Quality Assurance of a Single Isocenter Technique for the Simultaneous Treatment of Multiple Brain Metastases

J Maurer

J Maurer1*, B Sintay2 , V Varchena3 , (1) Cone Health Cancer Center, Greensboro, NC, (2) Cone Health, Greensboro, NC, (3) Cirs, Inc., Norfolk, Virginia


SU-E-T-52 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Comprehensive quality assurance (QA) of a single isocenter technique for the simultaneous treatment of multiple brain metastases is presently impractical due to the time consuming nature of measuring each lesion’s dose on film or with a micro-chamber. Three dimensional diode array and full field film measurements are sometimes used to evaluate these plans, but gamma analysis may not reveal local errors that have significant effects on one or a few of several targets. This work aimed to design, build and test a phantom to simplify comprehensive measurement and evaluation.

Methods: A phantom was designed with 28 stackable slabs. The top and bottom slabs are 1.5 centimeters (cm) in thickness, and central 26 slabs are 0.5 cm thick. When assembled with radiochromic film in all 27 gaps, the phantom measures 16.5 x 15 x 19 cm. Etchings were designed to aide in identification of specific film planes on computed tomography (CT) images and correlation of individual PTVs with closest bisecting planes. Patient verification plans with a total of 16 PTVs were calculated on the phantom CT, and test deliveries both with and without couch kicks were performed to test the ability to identify correct film placements and subsequent PTV specific dose distributions on the films.

Results: Bisecting planes corresponding to PTV locations were easily identified, and PTV specific dose distributions were clear for all 16 targets. For deliveries with couch kicks, the phantom PTV dose distributions closely approximated those calculated on the patient’s CT. For deliveries without couch kicks, PTV specific dosimetry was also possible, although the distributions had ‘ghosts’ equaling the number of couch kicks, with distance between ghosts increasing with distance from the isocenter.

Conclusion: A new phantom facilitates fast comprehensive commissioning validation and PTV specific dosimetry for a single isocenter technique for treating multiple brain metastases.

Funding Support, Disclosures, and Conflict of Interest: This work was partially funded by CIRS, Inc.

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