Air Kerma Strength Measurement Using a Seven-Distance Ionization Technique and Comparison With the Gamma Spectroscopy Method

The “seven-distance measurement” technique is the accepted method of measuring the Air Kerma Strength (Sk) of a high dose rate brachytherapy source. The basic process consists of making seven ionization chamber measurements at seven different source-to-chamber distances. These seven sets of data are then used to solve a series of seven equations to obtain a value for Sk. This indirect measurement method introduces uncertainty in each measurement and in the fit used to obtain Sk. Additionally, this technique utilizes correction factors for ionization chamber readings and source-to-chamber distance, thus introducing additional error into the final air kerma strength calculation. A viable alternative for obtaining Sk, obtained by making a single gamma spectroscopic measurement, is presented here and compared against the “seven-distance measurement” technique in terms of Sk precision and accuracy. To obtain Sk through the gamma spectroscopy method, a single spectroscopic measurement is obtained and the resulting Sk calculated for each measured photo-peak. A cylindrical, 17 mCi Cs-137 source initially was used with a HPGe detector to test the feasibility of the gamma spectroscopy technique. Cs-137 was chosen because it has only one energy peak at 661.7 keV, thus simplifying the gamma spectroscopy calculations. Seven-distance measurements were then taken using an Exradin A12 ion chamber suspended about 30 cm above the work surface, aligned with the suspended source. The support for the ion chamber was mounted to a movable track, allowing for the source-to-chamber distance to be easily and accurately changed. At each of the seven distances, an electrometer recorded the charge collected by the ion chamber over a time interval of exactly 60 seconds. Additionally, each measurement was corrected for the temperature and pressure of the room and for background. The precision and accuracy of the gamma spectroscopy method was estimated by performing a comparison of Sk values obtained using gamma spectroscopy and Monte Carlo simulations (MCNP5) of six Yb-169 seeds. The photo-peaks were again obtained using a HPGe detector. The air kerma strength per unit activity of the Cs-137 source was calculated to be 3.16 +/- 0.13 (cGy cmˆ2 hrˆ-1 mCiˆ-1) using the “seven-distance” method and 3.29 +/- 0.13 (cGy cmˆ2 hrˆ-1 mCiˆ-1) using the gamma spectroscopy method. The percent difference between these two results is approximately 4%, and the total relative uncertainties in the “seven-distance” and gamma specroscopy measurements were found to be approximately 4%. Monte Carlo simulations of the Yb-169 seeds yielded an average Sk value of 1.45 +/- 0.03 (cGy cmˆ2 hrˆ-1 mCiˆ-1). Gamma spectroscopy analysis of the same seeds gave an average Sk value of 1.42 +/- 0.07 (cGy cmˆ2 hrˆ-1 mCiˆ-1). The percent difference between these two results is approximately 2% and the total uncertainty in measuring Sk was found to be approximately 4.5%. The overlap in these results suggests that gamma spectroscopy is a viable alternative to the “seven-distance measurement” technique. A comparison of the uncertainties in measurement suggests that the gamma spectroscopy method offers a precision comparable to that of the “seven-distance” method. The small percent difference between the gamma spectroscopy method and Monte Carlo simulation suggest that the gamma spectroscopy method also offers high accuracy in obtaining Sk values. Similar Sk measurements of an Ir-192 HDR source are also being taken in order to explore the feasibility of the gamma spectroscopy method for use with HDR seeds. These data are to be obtained by January 2015.