Jun 6, 2012
A closer look at ion chamber dosimetry
Dosimetry of radiotherapy treatment fields is currently performed using ionization chambers to determine the absorbed dose in water. To account for effects due to the displacement of water by the ion chamber itself, the measured depth-dose curves can be corrected using an effective point of measurement (EPOM), which shifts the position of the ion chamber so that the resulting depth-dose profile agrees with the true profile.
In a new study from DKFZ, the German Cancer Research Center in Heidelberg, researchers examined the accuracy of two dosimetry protocols that employ this EPOM concept: the International Atomic Energy Agency's IAEA TRS-398 and the German protocol DIN 6800-2. In particular, they investigated the dependence of the displacement effect on the radius of the ion chamber cavity (Phys. Med. Biol. 57 3463).
"Current dosimetry protocols for high-energy photons report a relative standard uncertainty in the order of 1% when using ionization chambers, and a significant contribution to this uncertainty originates in the displacement effect," explained authors Carine Legrand, Christian Karger and Günther Hartmann. "Only a few experimental studies exist, and experiments and Monte-Carlo simulations don't always fully agree on the size of the displacement effect."
Deriving the dose
Legrand and colleagues examined six Farmer-type chambers with inner radii ranging from 1.0 to 6.0 mm. A plane-parallel Roos chamber was used to generate a reference depth dose curve. The chambers were cross-calibrated under reference conditions (i.e., at specified values of temperature, pressure, field size, detector depth and so on), and depth-dose measurements were performed in a water phantom using a vertical Co-60 beam. The position of the cavity centre of the ionization chamber was taken as the point of measurement.
To calculate absorbed dose, the collected charge is multiplied by the chamber's calibration factor for Co-60 radiation, and by correction factors defined for each applied protocol. For IAEA TRS-398 this includes, for example, corrections for temperature and air pressure, while for DIN 6800-2, the measured depth-dose profile is shifted by 0.5r (where r is the ion chamber inner radius) towards the beam source and multiplied by a displacement correction factor (these cancel out under reference conditions).
For IAEA TRS-398, this procedure provides the absorbed dose under reference conditions. Under non-reference conditions, percentage depth-dose (PDD) values are measured using an EPOM shift of 0.6r towards the beam source. For DIN 6800-2, the above procedure yields the absorbed dose at all depths.
Looking at the measured dose under reference conditions following IAEA TRS-398, the relative standard deviation between the chambers was (as expected) less than 0.1%. The absorbed dose obtained with the DIN 6800-2 protocol also agreed within 0.1%.
For IAEA TRS-398 under non-reference conditions, the variation in PDD was within 0.1% at 5 cm depth. At depths of between 1 and 10 cm, the curves agreed with that of the Roos chamber to within 0.3%, while deviations of more than 1% were observed in the build-up and dose maximum regions.
The depth-dose profile determined according to DIN 6800-2 showed doses at 5 cm depth to agree within 0.1%. At depths of between 1 and 10 cm, dose values agreed with the reference chamber to within 0.5%. In the maximum and build-up regions, differences of more than 1% were seen.
Although both methods produce coherent results behind the maximum, they fail to describe the maximum and build-up depths consistently. According to IAEA TRS-398, accurate measurements in the build-up region should be performed with an extrapolation chamber or a plane-parallel chamber. DIN 6800-2, on the other hand, allows measurements with cylindrical chambers in the build-up region as soon as the dose exceeds 80% of the maximum dose, which can lead to significant deviations.
The researchers postulate that the discrepancy arises because the displacement effect is generally assumed to be almost constant beyond the maximum, and a fixed EPOM is thus used throughout the depth profile. However, this work shows that, in fact, a depth-dependent EPOM is required.
To demonstrate this shift in EPOM with depth, the DKFZ team determined the EPOM from measured depth-dose curves assuming a fixed EPOM throughout the depth profile. They then calculated the shift of the EPOM from the reference point with chamber radius. To obtain good agreement between the different PDDs, a depth-dependent EPOM is required.
"The displacement effect originates from several effects, including lack of attenuation of the beam within the cavity, the shape of the entrance surface, and perturbation of the secondary electron fluence by the air cavity and the central electrode," explained the authors. "Most likely, the change of the electron spectrum from the build-up region towards greater depths is responsible for the depth dependent EPOM-shift."
The researchers conclude that the corrections recommended in the two protocols are not fully appropriate when using cylindrical ionization chambers for dosimetry in a Co-60 beam. "For practical measurements, we suggest an alternative procedure, which consists of a constant EPOM shift together with a displacement correction factor," said Legrand. "This produces more accurate results in the build-up region – with maximum deviations of up to 0.6% – as well as beyond the maximum, with particular accuracy at the reference depth."
• Related articles in PMB
Experimental determination of the effective point of measurement for cylindrical ionization chambers in 60Co gamma radiation C Legrand et al Phys. Med. Biol. 57 3463
Experimental investigation of the effect of air cavity size in cylindrical ionization chambers on the measurements in 60Co radiotherapy beams John Swanpalmer and Karl-Axel Johansson Phys. Med. Biol. 56 7093
Experimental determination of the effective point of measurement for various detectors used in photon and electron beam dosimetry Hui Khee Looe et al Phys. Med. Biol. 56 4267
About the author
Tami Freeman is editor of medicalphysicsweb.