Directional Dependence Of The Radiation Output Of Gamma Knife Perfexion On Ion Chamber Orientation

Keywords: galactorrhea, dose delivery, Perfexion, dosimetry, technique

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     The Gamma Knife Perfexion consisted of 192 Co-60 radioactive sources distributed evenly on 8 sectors, with 24 sources on each sectors respectively. The arrangement of the Co-60 sources is different from that for Gamma Knife Model C or previous model.
     This study is aimed to investigate the effects of different ion chamber orientations on the measured output for both individual sectors and all sectors opened.
     The Capintec model PR-05P ion chamber with collecting volume 0.07 cc. was used to measure the output by inserting the ion chamber to the center of the 16 cm diameter Elekta polystyrene spherical phantom. The ionic chamber was placed in three orientations, namely the X-Z; Y-Z and X-Z planes. This corresponded to placing the ion chamber along the X, Y and Z axis. It should be noted that the ion chamber made an angle with the X-axis due to the phantom adapter. The radiation outputs from individual designated sector with other sectors blocked and finally all sectors opened were measured. Totally 9 output values for each ion chamber orientation was obtained.
     The measured output for individual sectors showed that for sectors 3 and 7, radiation outputs were lowered by 1.8%-4.5% for the three ion chamber orientations as discussed by other authors. The total radiation outputs obtained by summing up individual sector output for different ion chamber orientations showed that it was decreased in the order of Z axis, X axis (0.4%) and then Y axis (0.8%). For all the sectors opened situation, the measured outputs for the ion chamber along the X and Z axis were almost the same, while the measured output for the Y axis was 0.4% lower. 
     This is a retrospective study.
     The orientation of the ion chamber along different axes may affect the measured radiation output of the Gamma Knife Perfexion.
     When the ionic chamber was placed along the Z axis, it showed the largest sensitivity for measuring the output. Even the average measured output for all sectors opened was comparable for both X and Z axes, it is suggested that the ion chamber be placed along the Z axis (X-Z plane) for output measurement.


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