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Validation of a New Tool for Testing Spatial Accuracy of Off-Axis Beam Apertures Used in Single-Isocenter Stereotactic Treatment of Multiple-Metastases of the Brain

D Pinkham1*, A Murray2, H Kudrolli2, S Hancock3 (1) Yale School of Medicine, New Haven, CT, (2) Sun Nuclear Corporation, Melbourne, FL, (3) Southeast Missouri Hospital, Cape Girardeau, MO


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

A prerequisite for simultaneous SRS treatment of multiple targets is validation of the spatial accuracy of off-axis beam apertures. The purpose of this project is to validate a method for testing the spatial accuracy of off-axis SRS beam apertures using a new commercial phantom and software with Elekta LINACs.

A commercial Acrylic phantom containing 6 tungsten ball bearings (BBs) covering a 10 cm span was used to develop a stereotactic off-axis targeting test. A set of ten beams, each containing multiple 2 cm square apertures was designed for the Elekta Agility MLC to irradiate the six targets from eight unique gantry, collimator, and couch angle combinations. Beam apertures were defined by leaf tips and edges without jaws. Targeting accuracy was evaluated for each BB’s projection on the EPID using stereotactic analysis software.

The new tool was used to test multiple Elekta LINACs with Agility collimator. The test analysis was found to be dependent upon accurate pixel density specification, which depends upon gantry angle due to gravitational effects on the EPID panel. The angle-dependent pixel density can be determined by observing the distance between the two most distant BBs. For one of the linacs, the test detected a lateral misalignment of the MLC leaf bank and misalignment of the couch axis from the gantry isocenter. After adjustment of both, the following results were obtained. The mean deviation for all projections was 0.65 mm ±0.3 mm. The greatest deviations, up to 1.33 mm, were observed for BBs farthest from the isocenter and with 90 degree couch rotation, suggesting that improvements might be obtained with further adjustment of couch axis and recalibration of angle readouts.

The tool provides for effective and efficient testing for commissioning and QA of a stereotactic program that includes simultaneous treatment of multiple metastasis.


Stereotactic Radiosurgery, Commissioning, Linear Accelerator


TH- External Beam- Photons: Quality Assurance - Linear accelerator

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