TY - JOUR
T1 - SU‐GG‐T‐516
T2 - Investigation of Large Discrepancies in Dose‐Rates of Gamma‐Knife Units at Various Institutions
AU - Shiu, A.
AU - Drzymala, R.
AU - Alvarez, P.
AU - Followill, D.
AU - Tailor, R.
PY - 2010/6
Y1 - 2010/6
N2 - Purpose: Investigate the culprits responsible for large discrepancies in dose rates of Gamma‐knife units as measured by the RPC‐TLD. Materials and Method: Measurements, conducted at the Advanced‐Medicine Center, Washington University (WU) and MD Anderson (MDA) Cancer Center, involved combinations of different (i) ion‐chamber types, (ii) ion‐chamber planes, and (iii) calibration phantoms/calibration protocols (TG21, TG51). Three ion chambers: Capintec PR‐05 (WU‐Ch‐C), Exradin A‐16 (WU‐Ch‐E), and PTW TN13010 (MDA‐Ch‐P) were employed. Four phantoms were: WU's indigenous water phantom, Elekta AB‐plastic spheres belonging to WU (WU‐Ph‐E) and MDA (MDA‐Ph‐E), and Elekta SW (SW‐Ph‐E) sphere. In addition, RPC‐TLD measurements were performed in the SW‐Ph‐E sphere to provide an independent check of dose rate. Results: All measurements were made with 16‐mm collimator. Dose‐rate comparisons are normalized to the TG21 calibration with the MDA‐Ch‐P in the MDA‐Ph‐E sphere. The TG51 dose‐rate measured with the MDA‐Ch‐P in the SW‐Ph‐E sphere agrees very well with the RPC‐TLD result. Moreover, the TG21 dose rate with the same chamber in the MDA‐Ph‐E sphere provides TG51/TG21 to be 1.009 which is also in good agreement with the established ratio. Surprisingly, the TG21 dose‐rate measured with the WU‐Ch‐C in the WU‐Ph‐E is found to be 2.4% higher than that measured with the MDA‐Ch‐P chamber in the similar phantom MDA‐Ph‐E. This is consistent with the discrepancy seen by the RPC‐TLD in the past. Impact of variation in composition among the two AB‐plastic spheres (WU‐Ph‐E and MDA‐Ph‐E) on dose‐rate measurement is seen to be negligible (<0.5%). Conclusion: The measured 2.4% difference in TG21 dose‐rate appears to be due to combination of difference in ion‐chamber type and its orientation. Compounding this with ∼1% due to the protocol difference (TG51 versus TG‐21) explains the past RPC‐TLD results. Our measurements rule out any significant contribution due to variation in phantom composition.
AB - Purpose: Investigate the culprits responsible for large discrepancies in dose rates of Gamma‐knife units as measured by the RPC‐TLD. Materials and Method: Measurements, conducted at the Advanced‐Medicine Center, Washington University (WU) and MD Anderson (MDA) Cancer Center, involved combinations of different (i) ion‐chamber types, (ii) ion‐chamber planes, and (iii) calibration phantoms/calibration protocols (TG21, TG51). Three ion chambers: Capintec PR‐05 (WU‐Ch‐C), Exradin A‐16 (WU‐Ch‐E), and PTW TN13010 (MDA‐Ch‐P) were employed. Four phantoms were: WU's indigenous water phantom, Elekta AB‐plastic spheres belonging to WU (WU‐Ph‐E) and MDA (MDA‐Ph‐E), and Elekta SW (SW‐Ph‐E) sphere. In addition, RPC‐TLD measurements were performed in the SW‐Ph‐E sphere to provide an independent check of dose rate. Results: All measurements were made with 16‐mm collimator. Dose‐rate comparisons are normalized to the TG21 calibration with the MDA‐Ch‐P in the MDA‐Ph‐E sphere. The TG51 dose‐rate measured with the MDA‐Ch‐P in the SW‐Ph‐E sphere agrees very well with the RPC‐TLD result. Moreover, the TG21 dose rate with the same chamber in the MDA‐Ph‐E sphere provides TG51/TG21 to be 1.009 which is also in good agreement with the established ratio. Surprisingly, the TG21 dose‐rate measured with the WU‐Ch‐C in the WU‐Ph‐E is found to be 2.4% higher than that measured with the MDA‐Ch‐P chamber in the similar phantom MDA‐Ph‐E. This is consistent with the discrepancy seen by the RPC‐TLD in the past. Impact of variation in composition among the two AB‐plastic spheres (WU‐Ph‐E and MDA‐Ph‐E) on dose‐rate measurement is seen to be negligible (<0.5%). Conclusion: The measured 2.4% difference in TG21 dose‐rate appears to be due to combination of difference in ion‐chamber type and its orientation. Compounding this with ∼1% due to the protocol difference (TG51 versus TG‐21) explains the past RPC‐TLD results. Our measurements rule out any significant contribution due to variation in phantom composition.
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U2 - 10.1118/1.3468912
DO - 10.1118/1.3468912
M3 - Article
AN - SCOPUS:84946135454
SN - 0094-2405
VL - 37
SP - 3305
JO - Medical physics
JF - Medical physics
IS - 6
ER -