Influence of Chamber Wall Material On Ionization Chamber Absorbed Dose Energy Response: A Numerical and Experimental Study
G Aldosary*, J Seuntjens, A Sarfehnia, McGill University, Montreal, QCSU-E-T-72 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Purpose: To study the energy response of ionization chambers with different wall materials.
Methods:The dose inside the cavity of an accurately modeled Exradin A12 ionization chamber was scored with Monte Carlo user code egs++/egs_chamber. The C552 plastic chamber wall material was changed in the simulations with materials of higher atomic number Z (aluminum, copper, molybdenum and tungsten) and simulations were carried out with five beam energies ranging from 120 kVp to 18 MV. The dose scored inside the cavity for each wall material was normalized to that from a C552 plastic wall. The mean secondary electron energy for each beam Ee- was also calculated at the level of chamber cavity using MC user code FLURZnrc.
The simulations were experimentally verified by replacing the Exradin A12 chamber C552 wall with Al or Cu walls of identical dimensions. AAPM TG51 and TG61 setups were followed for high and low energy beams, respectively. Large attenuation of kilovoltage photons by high Z wall materials was accounted for by correcting the readings with a CAVRZnrc-calculated chamber wall attenuation and scatter correction Awall.
Results:The relative readings obtained showed that with the use of higher Z wall materials, the chamber signal increased by up to a factor of 2.96 for MV photons, and 54.71 at 120 kVp. Higher Z walls result in larger contribution of photoelectrons, and as such changing the wall material significantly affects the absorbed dose energy-dependence of the chambers. Experimental results agree with simulations to within 9.8 %. The discrepancy is largest at kV beams and can be mitigated if the impurities found in each wall material were considered in the MC simulations.
Conclusion:The change in ionization chamber absorbed-dose energy dependence is studied (numerically and experimentally), by replacing the original wall chamber with walls of different Z material, while keeping the wall dimensions identical.