The effect of air cavity size in cylindrical ionization chambers on the measurements in high-energy radiotherapy photon beams--an experimental study

Cylindrical ionization chamber response in static and dynamic 6 and 15 MV photon beams

Purpose.To investigate the response of the CC13 ionization chamber under non-reference photon beam conditions, focusing on penumbra and build-up regions of static fields and on dynamic intensity-modulated beams.Methods. Measurements were performed in 6 MV 100 × 100, 20 × 100, and 20 × 20 mm²static fields. Monte Carlo calculations were performed for the static fields and for 6 and 15 MV dynamic beam sequences using a Varian multi-leaf collimator. The chamber was modelled using EGSnrc egs_chamber software. Conversion factors were calculated by relating the absorbed dose to air in the chamber air cavity to the absorbed dose to water. Correction and point-dose correction factors were calculated to quantify the conversion factor variations.Results. The correction factors for positions on the beam central axis and at the penumbra centre were 0.98-1.02 for all static fields and depths investigated. The largest corrections were obtained for chamber positions beyond penumbra centre in the off-axis direction. Point-dose correction factors were 0.54-0.71 at 100 mm depth and their magnitude increased with decreasing field size and measurement depth. Factors of 0.99-1.03 were obtained inside and near the integrated penumbra of the dynamic field at 100 mm depth, and of 0.92-0.94 beyond the integrated penumbra centre. The variations in the ionization chamber response across the integrated dynamic penumbra qualitatively followed the behaviour across penumbra of static fields.Conclusions. Without corrections, the CC13 chamber was of limited usefulness for profile measurements in 20-mm-wide fields. However, measurements in dynamic small irregular beam openings resembling the conditions of pre-treatment patient quality assurance were feasible. Uncorrected ionization chamber response could be applied for dose verification at 100 mm depth inside and close to large gradients of dynamically accumulating high- and low-dose regions assuming 3% tolerance between measured and calculated doses.

Reference dose determination in 60Co and high-energy radiotherapy photon beams by using Farmer-type cylindrical ionization chambers - an experimental investigation

Ionization chamber dosimetry is predominantly used for determination of the absorbed dose to water in ⁶⁰Co and high-energy radiotherapy photon beams. The most widespread ionization chambers employed for absolute or reference dose determinations in reference conditions are the Farmer-type cylindrical ionization chambers. The Farmer-type ionization chambers have a variety of constructions and materials and their responses vary in the radiation beam. Clinical accelerators, in addition to conventional photon beams with flattening-filter, can also deliver flattening-filter-free (FFF) photon beams. The responses of five different Farmer-type cylindrical ionization chambers were experimentally examined with reference to absorbed dose determination in reference conditions when using the International Atomic Energy Agency (IAEA) - American Association of Physicists in Medicine (AAPM) Technical Reports Series no. 483 (TRS-483) and the IAEA TRS-398 dosimetry protocol in the present investigation. The irradiations were performed using ⁶⁰Co and megavoltage photon beams with 6 MV, 15 MV, 6 MV FFF and 10 MV FFF nominal photon energies. The chamber calibrations were performed at different Secondary Standard Dosimetry Laboratories and are traceable to primary standards at different Primary Standard Dosimetry Laboratories. The chambers were also cross-calibrated at our laboratory using ⁶⁰Co γ-beam. The variation found in the data regarding the reference dose determination using the various Farmer-type chambers in the photon beams employed was about 1% at maximum. Thus, the selection of the ionization chamber in reference dose determinations may affect the outcomes. The differences in the absorbed dose values were similar in the conventional as well as in the FFF photon beams. For the FFF photon beams the absorbed dose computations were performed using the IAEA-AAPM TRS-483 dosimetry protocol. Two of the ionization chambers used had identical construction but different central electrodes, i.e. graphite versus aluminium. The results obtained using these two chambers show that, in the photon beams examined, the employed correction for the central electrode (p _cel ) regarding these two chambers is associated with an inaccuracy which is larger than the calculated uncertainty for this correction. The outcomes found in the present experimental investigation using the various ionization chambers also indicate possible inaccuracy in the employed beam quality correction factors (k _Q ) and imply the need for a revision of these factors.

Calculated beam quality correction factors for ionization chambers in MV photon beams

The beam quality correction factor, [Formula: see text], which corrects for the difference in the ionization chamber response between the reference and clinical beam quality, is an integral part of radiation therapy dosimetry. The uncertainty of [Formula: see text] is one of the most significant sources of uncertainty in the dose determination. To improve the accuracy of available [Formula: see text] data, four partners calculated [Formula: see text] factors for 10 ionization chamber models in linear accelerator beams with accelerator voltages ranging from 6 MV to 25 MV, including flattening-filter-free (FFF) beams. The software used in the calculations were EGSnrc and PENELOPE, and the ICRU report 90 cross section data for water and graphite were included in the simulations. Volume averaging correction factors were calculated to correct for the dose averaging in the chamber cavities. A comparison calculation between partners showed a good agreement, as did comparison with literature. The [Formula: see text] values from TRS-398 were higher than our values for each chamber where data was available. The [Formula: see text] values for the FFF beams did not follow the same [Formula: see text], [Formula: see text] relation as beams with flattening filter (values for 10 MV FFF beams were below fits made to other data on average by 0.3%), although our FFF sources were only for Varian linacs.

Addendum to the AAPM's TG-51 protocol for clinical reference dosimetry of high-energy photon beams

An addendum to the AAPM's TG-51 protocol for the determination of absorbed dose to water in megavoltage photon beams is presented. This addendum continues the procedure laid out in TG-51 but new kQ data for photon beams, based on Monte Carlo simulations, are presented and recommendations are given to improve the accuracy and consistency of the protocol's implementation. The components of the uncertainty budget in determining absorbed dose to water at the reference point are introduced and the magnitude of each component discussed. Finally, the consistency of experimental determination of ND,w coefficients is discussed. It is expected that the implementation of this addendum will be straightforward, assuming that the user is already familiar with TG-51. The changes introduced by this report are generally minor, although new recommendations could result in procedural changes for individual users. It is expected that the effort on the medical physicist's part to implement this addendum will not be significant and could be done as part of the annual linac calibration.