Medical Physics Seminar – Monday, September 30, 2013
Microchambers
Jessica Snow (student of Dr. Larry DeWerd)
Research Assistant, Department of Medical Physics, UW-School of Medicine & Public Health, Madison, WI - USA
An increase in the delivery of small and non-standard radiation fields has led to the development of smaller-volume ionization chambers (<0.02 cm3), called microchambers. Microchambers are used for dosimetry measurements in radiation therapy clinics around the world. The University of Wisconsin Accredited Dosimetry Calibration Laboratory has experienced a significant increase in requests for the calibration of microchambers for low- and medium-energy x-ray beams as well as the 60Co beam. This indicated that these chambers are being used in reference-dosimetry measurements for a wide range of therapy applications and beam energies.
Unfortunately, microchambers demonstrate a series of anomalous behaviors not seen in larger-volume chambers. Several publications have highlighted these undesirable behaviors, such as a strong energy dependence, large voltage-dependent polarity effects, nonsensical ion recombination correction factors, and an inverse proportionality between chamber response and applied voltage. However, a comprehensive characterization of microchamber behavior does not exist. Furthermore, no work has been done to determine the cause of these behaviors. Without an understanding of the processes that compete with the collection of the true radiation-induced gas ionization in microchambers, the reliability and applicability of these dosimeters in uncertain.
Here in lies the motivation for this work; while microchambers are used in reference-dosimetry measurements and relied upon for accurate dose determination, it is necessary that the behavior of these chambers is either optimized or accounted for correctly. In this work a thorough characterization of the current microchamber models for low- and high-energy photon beams was performed to determine their dosimetric issues and limitations. The source of the anomalous behaviors was determined and removed through an optimized microchamber design.
The dosimetric challenges of small and non-standard-radiation fields are great. The first step to achieving accurate reference-dosimetry measurements in these complex radiation fields is to ensure the reliability and accuracy of the employed dosimeter. The aim of this work was to identify and resolve the obstacles impeding the performance of microchambers for small and non-standard radiation therapy dosimetry.