Purpose:The Leksell Gamma Knife (GK) B & C series contains 201 Cobalt‐60 sources with a helmet. The new model, Perfexion uses 192 Cobalt‐60 sources without a helmet; using IRIS system for collimation and stereotactic guidance to deliver SRS to brain tumors. Relative dose to extracranial organs at risk (OARs) is measured in phantom in this study for Perfexion and C‐series GK.Materials & Methods:Measurements were performed in a Rando anthropomorphic phantom on both systems using a large ion chamber (Keithley‐175) for each collimator. The Keithley‐175 cc ion chamber was sandwiched between phantom slices at various locations in the phantom to correspond to different extracranial OARs (thyroid, heart, kidney, ovary and testis, etc.) The dose measurement was repeated with OSL detectors for each position and collimator.Results:A large variation is observed in the normalized dose between these two systems. The dose beyond the housing falls off exponentially for Perfexion. Dose beyond the C‐series GK housing falls off exponentially from 0–20cm then remains relatively constant from 20–40cm and again falls off with distance but less rapidly. The variation of extracranial dose with distance for each collimator is found to be parallel to each other for both systems.Conclusion:Whole body dose is found to vary significantly between these systems. It is important to measure the extracranial dose, especially for young patients. It is estimated that dose falls off exponentially from the GK housing and is about 1% for large collimators at 75 cm. The dose is two‐orders of magnitude smaller for the 4mm collimator. However, this small dose for patient may be significant radiologically.