Room: AAPM ePoster Library
Purpose: determine the correction factors necessary to correct for altitude effects on ambient air pressure when determining the air kerma strength of Cs-131 low energy photon brachytherapy seeds.
Methods: purpose-built pressure chamber was constructed in the past which could achieve pressures ranging from 560 mmHg to 800 mmHg. Three Cs-131 sources were tested over this pressure range in increments of 20 mmHg in three HDR1000+ and three IVB1000 air communicating well-type ionization chambers, both from Standard Imaging. Three measurements of each source/chamber combination were completed. The standard temperature and pressure correction was applied to the average result at each pressure then normalized to the result at 760 mmHg. A Monte Carlo model was also built in MCNP6 and both chambers were simulated to confirm the experimentally derived factors.
Results: the HDR and IVB results followed the expected form of a power fit with residuals below 0.8 % for all points and R² values of 0.9973 and 0.9941, respectively. The correction factors obtained from the power fits were 0.0580 for k1 and 0.429 for k2 in the HDR1000+ chamber and 0.0825 for k1 and 0.376 for k2 in the IVB1000 chamber. The Monte Carlo model accurately predicted the chamber response with the maximum difference between calculated and measured response being under 2.5 % for both the HDR1000+ and IVB1000 chambers.
Conclusion: expected, the correction factors for Cs-131 are different from other low energy brachytherapy sources. However, unlike other sources, the factors are different for HDR1000+ and IVB1000 chambers, caused by the comparatively higher average energy of Cs-131. When testing the air kerma strength of their Cs-131 sources, clinics must take care to use the appropriate factors for their chamber.
Funding Support, Disclosures, and Conflict of Interest: Larry DeWerd has a proprietary interest in Standard Imaging, which provided the well chambers used in this study.