CyberKnife® M6™: Peripheral dose evaluation for brain treatments.

PURPOSE: This study evaluates the peripheral dose (PD) delivered to healthy tissues for brain stereotactic radiotherapy treatments (SRT) performed with a CyberKnife M6™ Robotic Radiosurgery System and proposes a model to estimate PD before treatment. METHOD: PD was measured with thermoluminescent dosimeters. Measurements were performed to evaluate the influence of distance, collimator type (fixed or Iris™) and aperture size on PD for typical brain treatment plans simulated on an anthropomorphic phantom. A model to estimate PD was defined by fitting functions to these measurements. In vivo measurements were subsequently performed on 30 patients and compared to the model-predicted PD. RESULTS: PD (in cGy) was about 0.06% of MU at 15cm for a 20mm fixed collimator and 0.04% of MU for the same aperture with Iris™ collimator. In vivo measurements showed an average thyroid dose of 55mGy (σ=18.8mGy). Computed dose for thyroid, breast, umbilicus and gonads showed on average a relative difference of 3.4% with the in vivo dose (σ=12.4%). CONCLUSION: PD at the thyroid with Iris™ was about a third lower than with a fixed collimator in case of brain SRT. Despite uncertainties (use of anthropomorphic PD to estimate patient specific PD, surface PD to estimate OAR PD) the model allows PD to be estimated without in vivo measurements. This method could be used to optimise PD with different planning strategies.