Investigation Of The Culprits Responsible For The Significant Variation Of The Calibrated Dose-rate Among Gamma-knife Institutions

Almon Shiu1, Robert E Drzymala2, Paola Alvarez1, Ramesh C Tailor1, David S Followill1

1The University of Texas M.D. Anderson Cancer Center 2 USA Washington University School of Medicine

Keywords: physics, Dose, dosimetry, technique, gamma knife

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     RPC performed TLD-measurements on 16 GK-institutions for audits of the TG-51 non-compliant beams. The average RPC/INST ratio is 0.962 with SD of ±0.022. In 06/09, MDACC acquired GK-Perfexion. RPC-TLD was used as an independent check of dose-rate calibration. The TLD-result indicated the 16-mm-collimator dose-rate was 1% higher than that of TG-21 ion-chamber measurement in a plastic sphere-phantom (PSP), but it agreed with that of TG-51ion-chamber measurement in an Elekta solid-water phantom (ESWP).  
     The purpose of this study is to investigate what are the culprits responsible for this significant difference between the doses were measured with RPC-TLD and calculated based on the institution dose-rate calibration.
     This study was carried out at CAM, Washington University. First, TG-21 calibration was performed using the Capintec PR-05P ion-chamber (C-PR-05P) placed in x-y plane within the WU PSP, then with MDACC PSP. The same measurements were repeated using the PTW TN3010 ion-chamber (PTW-TN3010). Secondly, TG-51 calibrations were also carried with ESWP with a PTW-TN3010 and the WU SRS water-phantom (WUWP) with an Extradin-A16 ion-chamber (E-A16).
     The difference of the TG-21 calibration measured in WU and MDACC PSP is within 0.5% (The comparison results are all normalized to the calibration based on TG-21 using PTW-TN3010 in MDACC PSP.). The result is suggested that the variation of the composition of PSP is very small. It is to our surprise the dose-rate was calibrated based TG-21 with C-PR-05P in a PSP is 2.4% higher that was calibrated with PTW-TN3010 in a PSP. The ratio of the calibrated dose-rate based on TG-51 over that on TG-21 is 1.009. The dose-rate based on TG-51 using WUWP with the E-A16 is 1% higher than that of the dose rate determined by the PTW-TN3010 in the ESWP.
     This is a retrospective study.
     We could safely rule out the uncertainties of the PSP is the culprit for the significant dose-rate variation.
     The difference of about 2.5% is due to the type and the sensitive volume of the ion-chambers used for the calibration and the other 1% is due to the differences between the TG-21 in polystyrene and TG-51 in water dose calculations.


Project Roles:

A. Shiu (), R. Drzymala (), P. Alvarez (), R. Tailor (), D. Followill ()