[A comparison of the helmet output factors for five Gamma-Knife units.]

A radiophotoluminescent (RPL) glass rod dosimeter (GRD) and a small active volume p-type silicon diode detector are used for the measurement of the helmet output factors from five Gamma-Knife units, which include four Model B units and a Model C unit. The output factors for the five units measured with the GRD from 14, 8 and 4 mm helmets relative to the 18 mm helmet are 0.984 +/- 0.003, 0.951 +/- 0.003 and 0.884 +/- 0.006, respectively. Similarly, the corresponding output factors measured with the p-type silicon diode detector are 0.983 +/- 0.002, 0.952 +/- 0.003 and 0.867 +/- 0.015, respectively. The output factors are corrected with the end effect for each helmet of the five units. The end effect time for the four Model B units ranges from 4 sec for the 18 mm helmet to 2 sec for the 4 mm helmet. The results for the Model C unit are within 1 sec for all the helmets. The output factors for the five units obtained from both detectors are in good agreement with the values in a recent publication and the values recommended by Elekta, the device manufacturer, except for that of the 4 mm helmet measured with the GRD. The average GRD output factor for the 4 mm helmet is 1.6% higher than Elekta's value, 0.870, but is in good agreement with the published value which was measured using small active volume detectors. The helmet output factors for the five Gamma-Knife units measured with the GRD agree within measurement deviation.