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Extraction of depth‐dependent perturbation factors for parallel‐plate chambers in electron beams using a plastic scintillation detector
Author(s) -
Lacroix Frédéric,
Guillot Mathieu,
McEwen Malcolm,
Cojocaru Claudiu,
Gingras Luc,
Beddar A. Sam,
Beaulieu Luc
Publication year - 2010
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 - 10.1118/1.3463383
Subject(s) - scintillation , dosimetry , ionization , electron , detector , monte carlo method , ionization chamber , optics , physics , materials science , atomic physics , computational physics , nuclear physics , nuclear medicine , medicine , ion , statistics , mathematics , quantum mechanics
Purpose: This work presents the experimental extraction of the overall perturbation factorP Qin megavoltage electron beams for NACP‐02 and Roos parallel‐plate ionization chambers using a plastic scintillation detector (PSD). Methods: The authors used a single scanning PSD mounted on a high‐precision scanning tank to measure depth‐dose curves in 6, 12, and 18 MeV clinical electron beams. The authors also measured depth‐dose curves using the NACP‐02 and PTW Roos chambers. Results: The authors found that the perturbation factors for the NACP‐02 and Roos chambers increased substantially with depth, especially for low‐energy electron beams. The experimental results were in good agreement with the results of Monte Carlo simulations reported by other investigators. The authors also found that using an effective point of measurement (EPOM) placed inside the air cavity reduced the variation of perturbation factors with depth and that the optimal EPOM appears to be energy dependent. Conclusions: A PSD can be used to experimentally extract perturbation factors for ionization chambers. The dosimetry protocol recommendations indicating that the point of measurement be placed on the inside face of the front window appear to be incorrect for parallel‐plate chambers and result in errors in theR 50of approximately 0.4 mm at 6 MeV, 1.0 mm at 12 MeV, and 1.2 mm at 18 MeV.