2520 - Evaluation of Three Commercial Stereotactic Radiosurgery Phantoms for End-to-End Testing in a Self-Shielded Gyroscopic Radiosurgery System
Presenter(s)
Y. Lee1,2, A. Zens3, R. P. Tolakanahalli1,2, D. J. Wieczorek2,4, R. Kotecha1,2, M. D. Hall1,2, M. P. Mehta1,2, M. W. McDermott5,6, A. Gutierrez7, and U. Baharom3; 1Department of Radiation Oncology, Miami Cancer Institute, Baptist Health South Florida, Miami, FL, 2Department of Oncological Sciences, Herbert Wertheim College of Medicine, Florida International University, Miami, FL, 3Standard Imaging, Middleton, WI, 4Miami Cancer Institute, B, Miami, FL, 5Department of Neurosurgery, Miami Neuroscience Institute, Baptist Health South Florida, Miami, FL, 6Department of Neuroscience, Herbert Wertheim College of Medicine, Florida International University, Miami, FL, 7Miami Cancer Institute, Miami, FL
Purpose/Objective(s): End-to-end (E2E) testing performed during commissioning, annually and after major repairs is highly recommended as a best practice for stereotactic radiosurgery (SRS). This study evaluates three different commercial SRS phantoms for E2E testing in the Zap-X Gyroscopic Radiosurgery Platform.
Materials/Methods: Three Lucy phantoms (series 3D, EA and MR) from Standard Imaging, Inc (Middleton, WI) were used for this study. For each phantom with a calibrated ionization chamber (Standard Imaging Exradin A16) inserted, our Zap-X clinical workflow was followed: 1) Magnetic resonance (MR) images were acquired without the chamber due to MRI incompatibility; 2) Computed tomography (CT) simulation including immobilization (mask and headrest creation) and CT scan was performed; 3) MR-CT image registration was performed; 4) A target (planning target volume (PTV) [=gross tumor volume + 1-mm setup margin]) and ionization chamber inside the PTV were contoured; 5) A treatment plan (22 Gy in one fraction to PTV) was created in the Zap-X treatment planning system (TPS) following our SRS planning guidelines; 6) For each phantom plan, secondary dose calculations were executed using a Python-based in-house tool and patient-specific quality assurance (PSQA) was run using Sun Nuclear SRS MapCHECK and compared to TPS calculations with gamma criteria of 2%/1 mm and 10% threshold; 7) The Winston-Lutz test was performed; 8) After target localization by automatic alignments between kilovoltage planar images and digitally reconstructed radiographs, treatment delivery with ionization chamber measurements was completed.
Results: Secondary dose calculations agreed with TPS dose calculations within ±0.05% for all beams of each phantom plan. Gamma passing rates of PSQA were 99.6%, 97.4% and 98.8% for 3D, EA and MR phantom plans, respectively. Corresponding ionization chamber measured doses were -2.76%, -0.41% and -1.89% of TPS calculated mean doses.
Conclusion: E2E testing was feasible using all three Lucy phantoms in the Zap-X system and measured absolute dose was within the tolerance level (=±3% of TPS calculated dose) recommended by AAPM MPPG 8.b for E2E testing. However, some limitations were observed (Table 1): Although the Lucy 3D phantom does not require immobilization (mask and headrest) and is the most affordable option, phantom setup requires independent indexing or a secure tool on the Zap-X table/QA adapter due to its design primarily for a C-arm linear accelerator table with an index bar; The Lucy EA phantom is not visible on MR images preventing MR-CT fusion evaluation.
Table 1: Comparison and characteristic overview of three Lucy SRS phantoms investigated in this study| Lucy 3D | Lucy EA | Lucy MR | |
| MRI compatibility | Yes (with CT/MR insert) | No | Yes (entire phantom) |
| Immobilization requirement | No | Yes | Yes |
| Ion chamber measurement | Yes | Yes | Yes |
| Film measurement | Yes (=3 lesions depending on inserts purchased) | Yes (4 lesions) | Yes (2 lesions) |