P233] Investigation of the response of cylindrical ionization chamber in non-reference conditions when using high-energy photon beams

Abstract

Purpose To investigate the response of CC13 cylindrical ionization chamber in non-reference conditions with focus on the penumbra region and the depth of maximum dose when using high-energy photon beams. This chamber is used for beam data measurements and also for pre-treatment quality assurance procedures for dynamic treatments. Methods Measurements and Monte Carlo calculations are performed, employing a Varian iX Linac (6 MV photon beam), at the depths of 15 mm and 100 mm for the field sizes of 20 × 20 mm 2 ; 20×100 mm 2 and 100×100 mm 2 . The Monte Carlo model is developed using the egs-chamber in the EGSnrc package. Conversion factors are calculated relating the absorbed dose to air in the air cavity of the chamber to the absorbed dose to water through using: (a) water volume equal to the air cavity of the chamber, (b) 1×1×1 mm 3 water voxel representing point dose. Corresponding correction factors, C w/chamber and C w,point/chamber , are determined to quantify the variations in the conversion factors. Results Agreement within 3.5%/0.5 mm is obtained between the measurements and calculations. The correction factors for a chamber position at the central axis of the beam and at the center of the field penumbra are between 0.98 and 1.02 for all field sizes and depths examined. The largest corrections are seen for off-axis positions 3 mm beyond the penumbra center where C w,point/chamber varies between 0.54 and 0.71 for the depth of 100 mm and 0.45 and 0.58 for the depth of 15 mm indicating an overestimation of the absorbed dose to water when using the CC13 chamber. C w,point/chamber factors up to 1.27 are obtained in regions close to the penumbra inside the 20×20 mm 2 and 20×100 mm 2 fields indicating an underestimation of the absorbed dose to water. The maximum variation of the C w/chamber factors is between 0.70 and 1.06 for the 20×20 mm 2 field at the depth of 15 mm. Conclusions Uncorrected chamber response of the CC13 chamber, or similar, results in under or overestimation of the absorbed dose to water at positions inside or outside of the beam close to the penumbra for small as well as large field sizes.