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Quality Assurance for Multi-Focal Single Isocenter Stereotactic Radiosurgery Through 3D Gel Dosimetry

D Saenz1*, K Papanikolaou1, K Rasmussen1 , N Kirby1 , S Stathakis1 , E Pappas2 , Y Li1 , (1) Ut Health San Antonio, San Antonio, Texas, (2) RTsafe P.C., Athens, Greece


(Wednesday, 8/1/2018) 10:00 AM - 10:30 AM

Room: Exhibit Hall | Forum 7

Purpose: Multiple brain metastases SRS is a challenging radiation therapy procedure, often characterized by lengthy treatment times. A more efficient dose delivery is possible with the use of one isocenter provided the localization accuracy is sufficient over the increased range of the brain. Patient-specific quality assurance is essential particularly in these cases to ensure proper dose delivery. Gel dosimetry can serve this purpose for dose verification and localization end-to-end testing in a pseudo patient phantom.

Methods: A Monaco VMAT treatment plan was created which targets six metastases throughout the brain in a patient-derived CT. Targets ranged in diameter from 6-25 mm. A quality assurance target was also treated in the same plan in a region which allows for the insertion of a film holder and an ionization chamber to verify the absolute dose. A customized 3D-printed gel phantom from RTsafe with the patient’s specific bony anatomy was acquired and irradiated after 6D correction with the Hexapod treatment table (CBCT image guidance). A 2D Half Fourier Single Shot Turbo Spin Echo PD-T2-weighted MRI scan of the phantom was done to obtain the relative dose distribution.

Results: The measured dose was compared with the planned 3D dose distribution. In the QA target, 3D gamma analysis (3%/2mm) showed 94.3% of points had gamma index < 1. Brain metastasis mean dose agreed with the plan by better than 3%. Agreement of the absolute dose was <1.3%.

Conclusion: Patient specific dosimetric quality assurance and localization verification can be performed for single isocenter multiple metastasis SRS using 3D gel dosimetry. Good agreement was obtained with calculations with the added benefit of comparing with a patient dose distribution rather than with a phantom recalculation.

Funding Support, Disclosures, and Conflict of Interest: This work was partially supported by a research grant from Elekta.


Stereotactic Radiosurgery, Quality Assurance


TH- External beam- photons: intracranial stereotactic/SBRT

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