Calibration of 125I seeds at Mexico City altitude

摘要

Introduction.Several publications had indicated the fail of conventional correction for air density kTP to x rays [1,2],for low energy [3,4] and 60Co photons [5] in dosimetry measurements with ionization chamber;therefore,it is necessary employ an additional correction factor for air density corrections k'PT.Objectives.(1).-To calibrate 125I seeds in terms Sk - for interstitial brachytherapy to treatment of prostate cancer- with and without this additional factor k'PT.(2).- To show that Sk values of a sample seeds are statistically equal within the relative expanded uncertainty ±7％ (k=2) specify by the manufacturer.Method.(1) Nine 125I seeds,Amersham model 6711 are calibrated on Sk with an ionization well chamber traceable to Wisconsin University,following AAPM recommendations [6].(2) The Tukey's ANOVA mean and ANOM test [7] are used to demonstrate the hypothesis H0: That Sk mean values –without the additional correction– are statistically equal.Also is used the capability sixpack test (between/within) to verify the uncertainty claimed for the manufacturer,[8,9].Results.The kPT=1.3287±0.0047 and k'PT= 0.8966±0.0022,both for air density condition of 23.9 oC and 76.76 kPa;therefore (kPT)total=1.1913±0.0052 which is less 10.8％ than the conventional kPT.For α=0.05 is rejected the H0,the mean test shows that the manufacturer's Sk value is inaccuracy 4％ above the Sk great mean value with the kPT conventional correction-,nevertheless that uncertainty is consistent with the uc=3.5％ established.Conclusions.The application of conventional kPT overestimates the Sk values for the environment conditions of the Mexican upland.The ANOVA mean test is a quality tool to accomplish the recommendations of the report 59 [10] to verify the manufacturer's Sk value.