AbstractID: 9647 Title: Study of the Effect of Ambient Pressure on Well Ionization
Chamber Response
Well-type re-entrant ionization chambers are used clinically to measure the air kerma strength of brachytherapy seeds for effective
treatment planning before patient implantation. Those chambers designed for atmospheric communication must be corrected to STP.
The relationship between the response of well chambers and the ambient pressure has not been studied, particularly at the reduced
pressures at high elevations. It has been seen that the standard correction for temperature and pressure (CTP) does not fully account for
high altitude pressure effects seen with air-communicating chambers at low photon energies in the range of 20 – 100 keV. When the
chamber response is corrected using the standard correction, the response of the well chambers is over-corrected, giving readings that
are too high and resulting in errors up to 16% at a pressure corresponding to an elevation of 8000 feet for photon energies of 20 keV.
For high-energy photons (Cesium-137, Iridium-192) the CTP factor corrects appropriately. It has also been seen that the ambient
pressure does not significantly affect the response of pressurized well chambers to low energy photons. When used with beta emitters,
air-communicating well chambers follow the CTP correction more closely than they do when used with low energy photon sources.
Using both Monte Carlo simulations and experimental data, the response and any further corrections will be determined for both aircommunicating well chambers and pressurized well chambers with photon and beta sources at elevations above sea level.