Purpose: Since the Gafchromic film EBT has been recently replaced by the newer model EBT2, its characterization, especially energy dependence, has become critically important. The energy dependence of the dose response of Gafchromic EBT2 film is evaluated for a broad range of energies from different radiation sources used in radiation therapy. Methods: The beams used for this study comprised of kilovoltage x rays (75, 125, and 250 kVp), C 137 s gamma (662 KeV), C 60 o gamma (1.17–1.33 MeV), megavoltage x rays (6 and 18 MV), electron beams (6 and 20 MeV), and protonbeams (100 and 250 MeV). The film’s response to each of the above energies was measured over the dose range of 0.4–10 Gy, which corresponds to optical densities ranging from 0.05 to 0.74 for the film reader used. Results: The energy dependence of EBT2 was found to be relatively small within measurement uncertainties ( 1 σ = ± 4.5 % ) for all energies and modalities. Conclusion: For relative and absolute dosimetry of radiation therapybeams, the weak energy dependence of the EBT2 makes it most suitable for clinical use compared to other films.

Energy dependence and dose response of Gafchromic EBT2 film over a wide range of photon, electron, and proton beam energies

In this paper we present the results of a dosimetric evaluation of a 2D ionization chamber array with the objective of its implementation for quality assurance in clinical routine. The pixel ionization chamber MatriXX (Scanditronix Wellhofer, Germany) consists of 32x32 chambers with a distance of 7.6 mm between chamber centres. The effective depth of measurement under the surface of the detector was determined. The dose and energy dependence, the behaviour of the device during its initial phase and its time stability as well as the lateral response of a single chamber of the detector in cross-plane and diagonal directions were analysed. It could be shown, that the detector's response is linear with dose and energy independent. Taking the lateral response into account, two different dose profiles, for a pyramidal and an IMRT dose distribution, were applied to compare the data generated by a treatment planning system with measurements. From these investigations it can be concluded that the detector is a suitable device for quality assurance and 2D dose verifications.

https://iopscience.iop.org/article/10.1088/0031-9155/52/4/023/pdf

Dosimetric evaluation of a 2D pixel ionization chamber for implementation in clinical routine

Challenges in calculation of the gamma index in radiotherapy - Towards good practice.

Evaluation of the ArcCHECK QA system for IMRT and VMAT verification

Two commercially available detector arrays were compared for their use in the quality assurance of patient-specific IMRT treatment plans: one a diode-based array (MapCHECK) and the other an ion chamber-based array (MatriXX). The dependence of the response of detectors on the field size, dose rate, and radiation energy were measured and compared with reference measurements using a Farmer-type ionization chamber. The linearity of the detector response, short-term and long-term reproducibility, statistical uncertainty as a function of delivered dose, and the validity of the array calibration were also examined to understand the stability and uncertainty of the systems. No field size or SSD dependence were observed within the range of the field sizes and SSDs used in the study at both 6 MV and 18 MV photon energies. Both detector arrays showed negligible errors (< 1%) when measuring doses of more than ~8 cGy, but exhibited errors of ~3% when measuring doses on the order of 1 cGy. While the MapCHECK showed a stable short-term reproducibility to within the measurement errors, the MatriXX showed a slow but continuously increase in reading during the one-hour period (about 0.8%). The MapCHECK also showed a slightly better array sensitivity correction with all the detectors having less than 1% discrepancy and more than 90% of the detectors within 0.5% variation, whereas about 60% of the MatriXX detectors showed a less than 0.5% variation and approximately 8% exhibited a larger than 1% discrepancy. MatriXX detectors also displayed a volume-averaging effect consistent with its detector size of approximately 4.5 mm in diameter. Excellent passing rates were obtained for both detector arrays when compared with the planar dose distributions from the treatment planning system for three 6 MV IMRT fields and three 18 MV IMRT fields after the volume-averaging effect of the MatriXX was taken into account.

https://aapm.onlinelibrary.wiley.com/doi/pdf/10.1120/jacmp.v10i2.2942

Comparison of two commercial detector arrays for IMRT quality assurance.

Dynamic intensity-modulated radiotherapy (D-IMRT) using the sliding-window technique is currently applied for selected treatments of head and neck cancer at Institute for Cancer Research and Treatment of Candiolo (Turin, Italy). In the present work, a PiXel-segmented ionization Chamber (PXC) has been used for the verification of 19 fields used for four different head and neck cancers. The device consists of a 32 × 32 matrix of 1024 parallel-plate ionization chambers arranged in a square of 24 × 24 cm2 area. Each chamber has 0.4 cm diameter and 0.55 cm height; a distance of 0.75 cm separates the centre of adjacent chambers. The sensitive volume of each single ionization chamber is 0.07 cm3. Each of the 1024 independent ionization chambers is read out with a custom microelectronics chip.The output factors in water obtained with the PXC at a depth of 10 cm were compared to other detectors and the maximum difference was 1.9% for field sizes down to 3 × 3 cm2. Beam profiles for different field dimensions were measured with the PXC and two other types of ionization chambers; the maximum distance to agreement (DTA) in the 20–80% penumbra region of a 3 × 3 cm2 field was 0.09 cm. The leaf speed of the multileaf collimator was varied between 0.07 and 2 cm s−1 and the detector response was constant to better than 0.6%. The behaviour of the PXC was measured while varying the dose rate between 0.21 and 1.21 Gy min−1; the mean difference was 0.50% and the maximum difference was 0.96%. Using fields obtained with an enhanced dynamic wedge and a staircase-like (step) IMRT field, the PXC has been tested for simple 1D modulated beams; comparison with film gave a maximum DTA of 0.12 cm. The PXC was then used to check four different IMRT plans for head and neck cancer treatment: cervical chordoma, parotid, ethmoid and skull base. In the comparison of the PXC versus film and PXC versus treatment planning system, the number of pixels with γ parameter ≤1 was 97.7% and 97.6%, respectively.

/pdf/d-imrt-verification-with-a-2d-pixel-ionization-chamber-4tn6dskg4q.pdf

A. Sardo

Papers

D-IMRT verification with a 2D pixel ionization chamber: dosimetric and clinical results in head and neck cancer.