An efficient method of measuring the 4 mm helmet output factor for the Gamma Knife

It is essential to have accurate measurements of the 4 mm helmet output fac tor in the treatment of trigeminal neuralgia patients using the Gamma Knife . Because of the small collimator size and the sharp dose gradient at the b eam focus, this measurement is generally tedious and difficult. We have dev eloped an efficient method of measuring the 4 mm helmet output factor using regular radiographic films. The helmet output factor was measured by expos ing a single Kodak XV film in the standard Leksell spherical phantom using the 18 mm helmet with 30-40 of its plug collimators replaced by the 4 mm pl ug collimators. The 4 mm helmet output factor was measured to be 0.876 +/- 0.009. This is in excellent agreement with our EGS4 Monte Carlo simulated v alue of 0.876 +/- 0.005. This helmet output factor value also agrees with m ore tedious TLD, diode and radiochromic film measurements that were each ob tained using two separate measurements with the 18 mm helmet and the 4 mm h elmet respectively. The 4 mm helmet output factor measured by the diode was 0.884 +/- 0.016, and the TLD measure ment was 0.890 +/- 0.020. The radioch romic film measured value was 0.870 +/- 0.018. Because a single-exposure me asurement was performed instead of a double-exposure measurement, most of t he systematic errors that appeared in the double-exposure measurements due to experimental setup variations were cancelled our. Consequently, the 4 mm helmet output factor is more precisely determined by the single-exposure a pproach. Therefore, routine measurement and quality assurance of the 4 mm h elmet output factor of the Gamma Knife could be efficiently carried out usi ng the proposed single-exposure technique.