Comparison of beam data using diode and ion chamber in small field of CyberKnife

Abstract

Ion chamber, diode detector and film are used for measuring small field of SRS. The diode detector is recommended to measure the beam from CyberKnife recently. In this study, various detectors had been used to measure the beam data and compared that for each CyberKnife of domestic and foreign departments.

All measurements of the beam data (TPR and OCR and output factor) for all 12 collimators were performed by several detectors: diode detector, small sensitive volume ion chamber, Gafchromic EBT film. These measured data are compared to a set of nationwide average data, which provided by Accuray.

For the collimators over than 20 mm, the beam data (TPR and OCR) from the different detectors showed a good agreement within ±3%. However, it is easily observed that the ion chamber was broadens the measured penumbra width in the OCR data. The broadening effect is more enhanced with decreasing beam diameter. For the collimators less than 20 mm, the beam data have discrepancy of 10% or more for each detector. Comparing the measured output factors in this study and the average of foreign departments provided by Accuary, the agreement was less than ±0.5% except for 5 mm collimators for diode detector and ±1.5% for Gafchromic EBT film. The discrepancy of output factor for 5 mm collimators measured with both Gafchromic EBT film and diode detector were 3.5%.

For the collimators over than 20 mm, the beam data from the multi-sites showed a good agreement within ±1%. For the collimators less than 20 mm, it was found that the effective volume and the material of the detector affect the beam data.