Abstract
The GammaPlan treatment planning system does not account for the leakage and scatter dose during APS repositioning. In this study, the dose delivered to the target site and its periphery from the defocus stage and intershot couch transit (couch motion from the focus to defocus position and back) associated with APS repositioning are measured for the Gamma Knife model 4C. A stereotactic head‐frame was attached to a Leksell 16 cm diameter spherical phantom with a calibrated ion chamber at its center. Using a fiducial box, CT images of the phantom were acquired and registered in the GammaPlan treatment planning system to determine the coordinates of the target (center of the phantom). An absorbed dose of 10 Gy to the 50% isodose line was prescribed to the target site for all measurements. Plans were generated for the 8, 14 and 18 mm collimator helmets to determine the relationship of measured dose to the number of repositions of the APS system and to the helmet size. The target coordinate was identical throughout entire study and there was no movement of the APS between various shots. This allowed for measurement of intershot transit dose at the target site and its periphery. The couch was paused in the defocus position, allowing defocus dose measurements at the intracranial target and periphery. Measured dose increases with frequency of repositioning and with helmet collimator size. During couch transit, the target receives more dose than peripheral regions; however, in the defocus position, the greatest dose is superior to the target site. The automatic positioning system for the Leksell Gamma Knife model 4C results in an additional dose of up to 3.87±0.07%,4.97±0.04%, and 5.71±0.07% to the target site; its periphery receives additional dose that varies depending on its position relative to the target. There is also dose contribution to the patient in the defocus position, where the APS repositions the patient from one treatment coordinate to another. This may be important for treatment areas around critical structures within the brain. Further characterization of the defocus and transit exposures and development of a dose calculation algorithm to account for these doses would improve the accuracy of the delivered plan.PACS numbers: 87.53.‐j, 87.53.Bn, 87.53.Dq, 87.53.Ly
Highlights
The Gamma Knife model 4C is a highly precise stereotactic radiosurgery tool for the treatment of intracranial lesions.[1,2,3,4] Arranged in five parallel-plane rings forming a portion of a hemisphere, 201 individual 60Co sources emit gamma radiation focused at a single isocenter
The Gamma Knife model 4C has an automatic positioning system (APS) used to reposition the patient’s head to different coordinates in a treatment run.[2,3,5] During patient repositioning, the couch moves out of the docking position to the defocus position (28 cm away) where the APS moves to the coordinates of the shot.[5]. Once the patient’s head is properly positioned, the couch moves back into the machine and into treatment position to allow delivery of dose to the target site
We look at the dose to the target site and its periphery, and map the distribution of additional intracranial dose from APS repositioning in the defocus and transit positions
Summary
The Gamma Knife model 4C is a highly precise stereotactic radiosurgery tool for the treatment of intracranial lesions.[1,2,3,4] Arranged in five parallel-plane rings forming a portion of a hemisphere, 201 individual 60Co sources emit gamma radiation focused at a single isocenter. The process is repeated until all shots within a run are delivered. There are three sources of undocumented doses during a typical treatment: the transportation dose, the intershot transit dose (or transit dose), and the defocus dose. The transportation dose results from the exposure the patient receives while moving between the setup and treatment positions at the beginning and end of a run; the shielding doors are open and the patient is exposed to the scatter radiation from the sources.[6,7,8] The transit dose results from exposure when the couch moves between the focus and defocus positions. The shielding doors are open as the couch moves away from the focal point of the 201 beamlets and into a defocus position, where the APS can transition between shot coordinates. While in the defocus position, exposure results from leakage and scatter from the unshielded sources
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