(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: measure the setup accuracy of a patient positioning system for linac based single-isocenter, multiple target, and non-coplanar Stereotactic Radiosurgery (SRS) treatment deliveries.
Methods: were made using an anthropomorphic head phantom, into which eight radio-opaque markers were inserted simulating treatment targets within the head. An actual patient treatment procedure that consisted of fixation mask construction, acquisition of CT images, treatment planning and delivery was followed. A treatment plan was generated in Eclipse which contained 14 non-coplanar, multi-port, static fields defined by a high definition multi-leaf collimator (MLC) with sizes ranging from 10.0 x 10.0 mm2 to 15.0 x 15.0 mm2. The treatment plan was designed such that the target markers were exposed in each MLC port. The treatment plan was delivered on a TrueBeam STx linac equipped with a six degree of freedom couch, an On-board Imaging System and Mega-Voltage imager. A cone beam computed tomography (CBCT) acquisition was used to position the phantom to isocenter, and MV images of the treatment fields were acquired. A computer program was developed to segment the images and to locate the positions of the target marker centers with respect to the field edges for each MLC port. Expected field and target center positions were calculated from the treatment plan and the positional accuracy of the system was evaluated by comparing the expected and measured values.
Results: target markers setup deviations from expected positions ranged between 0.2 mm to 1.0 mm for all the treatment ports that were measured, with a mean deviation of 0.5 ± 0.2 mm.
Conclusion: automated quality assurance (QA) test has been developed to efficiently quantify the setup accuracy of a cranial SRS delivery system for central and off-axis targets. The test can be added to a QA schedule as a routine end to end monitoring of SRS setup accuracy.
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