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WE‐E‐AUD B‐03: Analysis of Dose Perturbation Factors for An NACP‐02 Parallel‐Plate Ionization Chamber in Clinical Electron Beams
Author(s) -
Chin E,
Seuntjens J,
Bailey M,
Shipley D,
Palmans H,
Dusautoy A,
Verhaegen F
Publication year - 2008
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.2962777
Subject(s) - ionization chamber , electron , ionization , scaling , atomic physics , cathode ray , monte carlo method , physics , ion , dosimetry , perturbation (astronomy) , materials science , nuclear physics , nuclear medicine , geometry , medicine , statistics , mathematics , quantum mechanics
Purpose: To investigate the dose perturbation factors p wall , p cav , p Q in water of an NACP‐02 parallel‐plate ionization chamber in clinical electron beams (4–18 MeV). These perturbations factors are assumed unity in current dosimetry protocols, but recent publications have cast doubts on this. Method and Materials: An EGSnrc Monte Carlo model was developed for the NACP‐02 parallel‐plate ionization chamber. Perturbation factors for the presence of the non‐water wall p wall , the presence of an air cavity p cav and the total perturbation factor p Q were calculated in water. The influence of the front and back walls were investigated. The influence of electron beam energy and depth in water were investigated. Results: p wall is significantly different from unity for low energy electron beams, and decreases with energy. p cav is close to unity for all beam energies. Both p wall and p cav increase significantly with depth in water, leading to an increasing p Q , which reaches a value of 1.23 at a depth of R 50 for 4 MeV electrons. This was, however, found to depend strongly on whether scaling of the non‐water equivalent front wall of the chamber was performed; p Q decreased from 1.23 to 1.02 for 4 MeV electrons at R 50 when scaling is done. At the reference depth z ref , little effect of the window scaling was noted. The back and front wall of the ion chamber were found to have opposite influences on p wall . The effect of in‐scattering and track obliquity on the depth dependence of p cav was investigated. Conclusion: The findings of an increased p Q with depth in water could have important consequences for clinical depth dose measurements. However, the issue of front window scaling needs further study, and recommendations need to be issued. The behaviour of p cav and p wall with depth and energy were explained. Studies like these may allow improved chamber design.