Jun 26, 2012
Correction factors aid CyberKnife dosimetry
During commissioning of a radiotherapy system, the user must accurately measure radiation dose output at the centre of a reference beam and the output variation as a function of collimator size, often called the output calibration and output factor measurements, respectively. However, the increasing use of small fields in radiotherapy, such as those produced by Accuray's CyberKnife system creates some awkward problems. For example, standard dosimetry protocols specify a reference beam size of 10 x 10 cm, which cannot be achieved by CyberKnife with its maximum field diameter of 6 cm.
In response, a new formalism for reference dosimetry using small and non-standard fields was introduced in 2008 advising the use of two additional correction factors: one related to measurements in a non-standard reference field size, and one to small-field output factor measurements (Med. Phys. 35 5179). Although publications have detailed some of these correction factors for the CyberKnife system and a range of detectors, researchers based in Italy and at Accuray (Sunnyvale, CA) have published a new set of figures that they say improve on the previously available data (Phys. Med. Biol. 57 3741).
"Updated correction factors are needed because new models of CyberKnife with different designs and collimation systems are now available," Accuray's Warren Kilby told medicalphysicsweb. "Some of the detectors studied previously are now obsolete, and detectors not previously studied are now commonly used for small-field dosimetry. Also, the Monte Carlo codes used to calculate the correction factors have been improved, allowing more accurate calculations with lower statistical uncertainty to be performed."
Simulating all the options
The CyberKnife creates circular treatment fields of diameters ranging from 5 mm to a maximum of 60 mm using either a fixed collimator or an iris variable aperture collimator. Several versions of CyberKnife are in existence, with older systems having a dose rate of 400, 600 or 800 MU/min, and the latest CyberKnife VSI model having 1000 MU/min.
"Our publication provides a complete set of correction factor data needed to apply the 2008 formalism to all currently available CyberKnife versions and collimation options, for a range of commonly used detectors," explained Paolo Francescon from the Department of Medical Physics at ULSS 6 Vicenza in Italy. "Data presented in our older publications should now be replaced with the data in our new paper."
Small field effects
Francescon, Kilby and colleagues simulated the treatment heads of the 600 and 800 MU/min CyberKnife versions, as well as the fixed collimator for the 600 MU/min system and both collimator options for the 800 MU/min design. They also considered a range of detector types including Farmer chambers, liquid-filled ionization chambers, air-filled micro-chambers and diode detectors. A full list of the resulting correction factors can be found in their research paper.
One of the reasons why a correction factor needs to be applied to the detector response is that, as the size of the treatment beam approaches the detector dimensions, this creates large dose gradients over the sensitive volume. The team's analysis showed that the correction magnitude is larger at smaller field sizes for diodes and micro-chambers, with diodes over-responding by up to 6% and chambers under-responding by up to 11%. For field sizes greater than 15 mm, the corrections were all less than 1%.
"This was the first time that a liquid-filled micro-chamber was analysed for CyberKnife and it was pleasing to see that the corrections needed are smaller than those required for diodes or air-filled chambers," commented Kilby. Francescon added: "Our model evaluates the increased fraction of scatter from the material around the silicon sensitive volume (mainly the epoxy resin that has a density significantly different from 1). This effect becomes important at small fields."
Although the 1000 MU/min CyberKnife was not explicitly analysed in this study, the group reports that the equivalence of the 600 MU/min and the 800 MU/min correction factor data, and similarity between these designs and the 1000 MU/min system, strongly suggests that the same figures are applicable.
• Related articles in PMB
Monte Carlo simulated correction factors for machine specific reference field dose calibration and output factor measurement using fixed and iris collimators on the CyberKnife system
P Francescon et al Phys. Med. Biol. 57 3741
Assessment of small volume ionization chambers as reference dosimeters in high-energy photon beams
M Le Roy et al Phys. Med. Biol. 56 5637
A Monte Carlo method to evaluate the impact of positioning errors on detector response and quality correction factors in nonstandard beams
Hugo Bouchard et al Phys. Med. Biol. 56 2617
Investigation of systematic uncertainties in Monte Carlo-calculated beam quality correction factors
J Wulff et al Phys. Med. Biol. 55 4481
About the author
Jacqueline Hewett is a freelance science and technology journalist based in Bristol, UK.