Objectives: Describe results of AAPM Task Group 178 on Gamma Knife Dosimetry and Quality Assurance Round Robin dosimetry intercomparison

Methods: Nine Gamma Knife Centers in six states participated in a round robin intercomparison w four phantoms, two ionization chambers and one electrometer comparing and contrasting three different calibration protocols

Results: AAPM Task Group 51 protocol for external beam dosimetry, suitably modified following the recommendations of Alfonso et al (2008) gave the most consistent calibration results

Conclusions: AAPM Task Group 178 final report is about to be reviewed by the Working Group on Recommendations for Radiotherapy External Beam QA before publication