Target and peripheral dose from radiation sector motions accompanying couch repositioning of patient coordinates with the Gamma Knife(®) Perfexion(™)

BACKGROUND: The GammaPlan(™) treatment planning system (TPS) does not fully account for shutter dose when multiple shots are required to deliver a patient’s treatment. The unaccounted exposures to the target site and its periphery are measured in this study. The collected data are compared to a similar effect from the Gamma Knife(®) model 4C. MATERIALS AND METHODS. A stereotactic head frame was attached to a Leksell(®) 16 cm diameter spherical phantom; using a fiducial-box, CT images of the phantom were acquired and registered in the TPS. Measurements give the relationship of measured dose to the number of repositions with the patient positioning system (PPS) and to the collimator size. An absorbed dose of 10 Gy to the 50% isodose line was prescribed to the target site and all measurements were acquired with an ionization chamber. RESULTS: Measured dose increases with frequency of repositioning and with collimator size. As the radiation sectors transition between the beam on and beam off states, the target receives more shutter dose than the periphery. Shutter doses of 3.53±0.04 and 1.59±0.04 cGy/reposition 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. CONCLUSIONS: The radiation sector motions for the Gamma Knife(®) Perfexion(™) result in an additional dose due to the shutter effect. The magnitude of this exposure is comparable to that measured for the model 4C.