Purpose. The GammaPlan™ treatment planning system does not fully account for intershot transit and shutter doses when multiple shots are required to deliver a patient’s treatment plan with the Gamma Knife® model 4C and Perfexion™, respectively. Gamma Knife radiosurgery is a highly precise modality for the treatment of intracranial diseases; however, there is potential for unaccounted exposures during treatment. These extracranial and intracranial exposures are measured for both the Gamma Knife® model 4C and Perfexion™; measured doses from this study are used to determine the lifetime potential for fatal secondary cancer and risks of aggregated detriment to exposed organs.
Materials and method. A stereotactic head frame was attached to a Leksell® 16 cm diameter spherical phantom, with a calibrated ionization chamber at its center. Using a fiducial-box, CT images of the phantom were acquired and registered in the GammaPlan™ treatment planning system (TPS) to plan test treatments to the center of the phantom. Measurements give the relationship of measured dose to the number of repositions with the automatic positioning system (APS, model 4C) and patient positioning system (PPS, Perfexion™) and to the collimator size. The target site was identical throughout entire study with no movement of the treatment coordinates between various shots; this allowed for measurement of intershot transit and shutter doses at the target site and its periphery. The study with the Perfexion™ was performed using two dosimeters: an ionization chamber and Gafchromic EBT film. The film measurements give greater spatial resolution of the shutter effect and measurements with the ionization chamber give more accurate measurements at lower dose ranges. For film, 3 Gy to the 50% isodose line was prescribed to the target site; for ionization chamber measurements, an absorbed dose of 10 Gy to the 50% isodose line was prescribed to the target site for all measurements. For extracranial exposures, organ doses were measured for a typical treatment for both models using film dosimetry (Gafchromic EBT) and a Rando phantom. The Linear-No Threshold (LNT) model is used to determine lifetime risk of radiation induced second cancer formation to various anatomical structures.
Results. Measured dose increases with frequency of repositioning and with collimator size for each model. As the radiation sectors transition between the beam on and beam off states for the Perfexion™, the target receives more shutter dose than peripheral regions. The shutter dose profile is nearly symmetric along the x-axis; however, the shutter dose profile along the z-axis is asymmetric. Shutter doses of 3.53 ± 0.04 and 1.59 ± 0.04 cGy/reposition (as measured by the ionization chamber) to the target site are observed for the 16 and 8 mm collimators, respectively. The target periphery receives additional dose that varies depending on its position relative to the target. Film data shows consistent results for the 8 and 16 mm collimators. The shutter effect for the 4 mm collimator could not be accurately measured at the target with the ionization chamber, but it was determined using film dosimetry. The shutter doses are comparable to intershot transit doses measured from the model 4C. The LNT-model predicts a lifetime fatal cancer risk as high as 6.2 incidences per 100,000 people for the model 4C, and 0.05 incidences per 100,000 for the Perfexion, assuming a homogeneous exposure to the brain. Considering areas around the target-site in the brain as separate organs, this model predicts increased fatal cancer risk of 50 and 132 incidences per 100,000 for the Perfexion™ and model 4C, respectively. The lifetime risk of fatal lung cancer can be 18 and 31 incidences per 100,000 for the Perfexion™ and model 4C, respectively. Fatal thyroid cancer risk is 1 and 2 in 100,000 incidences for the Perfexion™ and model 4C, respectively. Organ dose is lower for the Perfexion than the model-4C, reducing overall risk.
Conclusions. The radiation sector motions for the Leksell Gamma Knife® Perfexion™ result in an additional dose due to the shutter effect. The magnitude of exposure is comparable with that measured for the intershot transit dose of the model 4C. This additional dose to the patient may be clinically relevant, especially around critical structures within the brain. Further characterization of radiation dose from the sector motions accompanying coordinate repositioning with the PPS and development of a suitable correction to account for these doses could improve the accuracy of the delivered plan. Appropriate accounting for and minimization of intershot transit and shutter effects may reduce secondary cancer formation, especially when treating pediatric patients, patients with benign disease or those with greater life expectancy.