Radiation Physics

PV QA 3 - Poster Viewing Q&A 3

TU_3_3142 - 3D gel dosimetry for commissioning and routing QA in mono and poly-isocentric SRS

Tuesday, October 23
1:00 PM - 2:30 PM
Location: Innovation Hub, Exhibit Hall 3

3D gel dosimetry for commissioning and routing QA in mono and poly-isocentric SRS
K. Papanikolaou1, D. Saenz2, K. Rasmussen2, N. Kirby2, S. Stathakis2, E. Pappas3, and Y. Li4; 1University of Texas Health San Antonio, San Antonio, TX, 2University of Texas Health San Antonio Cancer Center, San Antonio, TX, 3Department of Radiology and Radiotherapy, Technological Educational Institute of Athens, Athens, Greece, 4The University of Texas Health Science Center San Antonio, San Antonio, TX

Purpose/Objective(s): Stereotactic radiosurgery of multiple brain metastases represents a clinical challenge requiring extended treatment times in the case of multiple multi-focal and poly-isocenteric plans. Using a single isocenter is a more efficient dose delivery, but is subject to planning complications and sensitive to errors in localization and rotational registration. Pre-treatment patient-specific quality assurance is essential to ensure proper dose delivery to multiple targets. The use of a gel dosimeter may facilitate this process and allow for the verification of dose delivery in a customized patient-emulating phantom.

Materials/Methods: An inversely-optimized volumetrically modulated arc therapy plan was devised in a Monte Carlo-based treatment planning system to treat six targets throughout the brain. Targets varied in volume from 0.058 cc to 5.074 cc. In addition, a larger 5.39 cc target was treated to a homogeneous dose in a region where phantoms facilitating the insertion of film and an ion chamber could be used. The 3D dose distribution was measured throughout the brain with a gel dosimeter. The phantom was localized with cone-beam CT in conjunction with a six-degree-of-freedom treatment table.

Results: The 3D dose distribution obtained from the MRI scan of the phantom post-irradiation was compared with the 3D DICOM exported dose from the treatment planning system. 3D gamma analysis in the larger QA target revealed a 94.3% passing rate with 3%/2mm criteria. Dose volume histograms agreed well with planning dose volume histograms. Mean dose in all targets matched calculations within 3%. Absolute dose verification measurement with an ionization chamber showed agreement within 1.3% while 2D gamma analysis for the film insert at 3%/2mm showed a passing rate of 95.7%.

Conclusion: 3D gel dosimetry can be an effective quality assurance tool for the dosimetric verification and localization accuracy of stereotactic radiosurgery on a patient-specific basis. Comparisons between measured and calculated dose distributions showed good agreement with the advantage of measurements being made in a phantom geometry which closely mimics that of the patient’s anatomy.

Author Disclosure: K. Papanikolaou: None. D. Saenz: None. K. Rasmussen: None. E. Pappas: None. Y. Li: Independent Contractor; Univ. of Texas Health Science Center at SA.

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