Evaluation of a single-scan protocol for radiochromic film dosimetry.

The purpose of this study was to evaluate a single-scan protocol using Gafchromic EBT3 film (EBT3) by comparing it with the commonly used 24-hr measurement protocol for radiochromic film dosimetry. Radiochromic film is generally scanned 24 hr after film exposure (24-hr protocol). The single-scan protocol enables measurement results within a short time using only the verification film, one calibration film, and unirradiated film. The single-scan protocol was scanned 30 min after film irradiation. The EBT3 calibration curves were obtained with the multichannel film dosimetry method. The dose verifications for each protocol were performed with the step pattern, pyramid pattern, and clinical treatment plans for intensity-modulated radiation therapy (IMRT). The absolute dose distributions for each protocol were compared with those calculated by the treatment planning system (TPS) using gamma evaluation at 3% and 3 mm. The dose distribution for the single-scan protocol was within 2% of the 24-hr protocol dose distribution. For the step pattern, the absolute dose discrepancies between the TPS for the single-scan and 24-hr protocols were 2.0 ± 1.8 cGy and 1.4 ± 1.2 cGy at the dose plateau, respectively. The pass rates were 96.0% for the single-scan protocol and 95.9% for the 24-hr protocol. Similarly, the dose discrepancies for the pyramid pattern were 3.6 ± 3.5cGy and 2.9 ± 3.3 cGy, respectively, while the pass rates for the pyramid pattern were 95.3% and 96.4%, respectively. The average pass rates for the four IMRT plans were 96.7% ± 1.8% for the single-scan protocol and 97.3% ± 1.4% for the 24-hr protocol. Thus, the single-scan protocol measurement is useful for dose verification of IMRT, based on its accuracy and efficiency.

Validation of fluence-based 3D IMRT dose reconstruction on a heterogeneous anthropomorphic phantom using Monte Carlo simulation.

In this study, we evaluated the performance of a three-dimensional (3D) dose verification system, COMPASS version 3, which has a dedicated beam models and dose calculation engine. It was possible to reconstruct the 3D dose distributions in patient anatomy based on the measured fluence using the MatriXX 2D array. The COMPASS system was compared with Monte Carlo simulation (MC), glass rod dosimeter (GRD), and 3DVH, using an anthropomorphic phantom for intensity-modulated radiation therapy (IMRT) dose verification in clinical neck cases. The GRD measurements agreed with the MC within 5% at most measurement points. In addition, most points for COMPASS and 3DVH also agreed with the MC within 5%. The COMPASS system showed better results than 3DVH for dose profiles due to individual adjustments, such as beam modeling for each linac. Regarding the dose-volume histograms, there were no large differences between MC, analytical anisotropic algorithm (AAA) in Eclipse treatment planning system (TPS), 3DVH, and the COMPASS system. However, AAA underestimated the dose to the clinical target volume and Rt-Parotid slightly. This is because AAA has some problems with dose calculation accuracy. Our results indicated that the COMPASS system offers highly accurate 3D dose calculation for clinical IMRT quality assurance. Also, the COMPASS system will be useful as a commissioning tool in routine clinical practice for TPS.

Validation of a quick three-dimensional dose verification system for pre-treatment IMRT QA.

In this study, we evaluated the dosimetric performance of the three-dimensional (3D) dose verification system, COMPASS version 3 (IBA Dosimetry, GmbH, Germany). The COMPASS has the function of a dedicated beam modeling and dose calculation. It is able to reconstruct 3D dose distributions on patient CT images, using the incident fluence from a linear accelerator measured with the MatriXX 2D array (IBA Dosimetry). The dose profiles measured with various multi-leaf collimator (MLC) test patterns for the COMPASS were checked by comparison with those of EDR2 (Eastman Kodak, Rochester, NY) films and Monte Carlo (MC) simulations. The COMPASS was also used for dose verification in clinical intensity-modulated radiation therapy (IMRT) plans for head and neck cases. The dose distributions were compared with those measured by 3DVH (Sun Nuclear, Melbourne, FL) and MC. In addition, the quality assurance (QA) times among the COMPASS, 3DVH, and EDR2 were compared. For MLC test patterns, the COMPASS dose profiles agreed within 3 % with those of EDR2 films and MC simulations. The physical resolution of the COMPASS detectors was lower than that of film, but the dose resolution for MLC patterns was comparable to that of film. In clinical plans, the dose-volume-histograms were equal for all systems. The average QA times of the COMPASS, 3DVH, and EDR2 film were 40.1, 59.4, and 121.4 min, respectively. The COMPASS system provides fast and reliable 3D dose verification for clinical IMRT QA. The COMPASS QA process does not require phantom plans. Therefore, it allows a simple QA workflow.

Optimization of acquisition parameters and accuracy of target motion trajectory for four-dimensional cone-beam computed tomography with a dynamic thorax phantom.

Our purpose in this study was to evaluate the performance of four-dimensional computed tomography (4D-CBCT) and to optimize the acquisition parameters. We evaluated the relationship between the acquisition parameters of 4D-CBCT and the accuracy of the target motion trajectory using a dynamic thorax phantom. The target motion was created three dimensionally using target sizes of 2 and 3 cm, respiratory cycles of 4 and 8 s, and amplitudes of 1 and 2 cm. The 4D-CBCT data were acquired under two detector configurations: "small mode" and "medium mode". The projection data acquired with scan times ranging from 1 to 4 min were sorted into 2, 5, 10, and 15 phase bins. The accuracy of the measured target motion trajectories was evaluated by means of the root mean square error (RMSE) from the setup values. For the respiratory cycle of 4 s, the measured trajectories were within 2 mm of the setup values for all acquisition times and target sizes. Similarly, the errors for the respiratory cycle of 8 s were <4 mm. When we used 10 or more phase bins, the measured trajectory errors were within 2 mm of the setup values. The trajectory errors for the two detector configurations showed similar trends. The acquisition times for achieving an RMSE of 1 mm for target sizes of 2 and 3 cm were 2 and 1 min, respectively, for respiratory cycles of 4 s. The results obtained in this study enable optimization of the acquisition parameters for target size, respiratory cycle, and desired measurement accuracy.

[Development of a quality assurance phantom for brachytherapy: the feasibility of daily check with the phantom].

PURPOSE: We developed a quality assurance (QA) phantom to enable easy confirmation of radiation source output measurements of a high dose rate (192)Ir intracavitary brachytherapy unit in gynecology. The purpose of this study was to evaluate the feasibility of daily checks using the QA phantom. METHODS AND MATERIALS: The QA phantom was designed with tough water phantoms to hold a Farmer-type ionization chamber, with semiconductor detectors used as in vivo dosimeters to measure rectal dose, and three transfer tubes for gynecology. To test the reliability of our QA phantom for the detection of abnormalities in source output or semiconductor detectors, we applied different doses. RESULTS: Variations due to different settings of the QA phantom were within 2%. The temporal variations were less than 2% and 5% in the Farmer-type ionization chamber and semiconductor detectors, respectively. Interobserver variations were below 3%. CONCLUSIONS: With tolerance levels of 2% and 5% for a Farmer-type ionization chamber and semiconductor detectors, respectively, a QA phantom is potentially useful for easily detecting abnormalities by applying daily checks of the brachytherapy unit.

[Comparison of various image guided radiation therapy systems; image-guided localization accuracy and patient throughput].

In this study, we evaluated various image guided radiation therapy (IGRT) systems regarding accuracy and patient throughput for conventional radiation therapy. We compared between 2D-2D match (the collation by 2 X-rays directions), cone beam computed tomography (CBCT), and ExacTrac X-Ray system using phantom for CLINAC iX and Synergy. All systems were able to correct within almost 1 mm. ExacTrac X-Ray system showed in particular a high accuracy. As for patient throughput, ExacTrac X-Ray system was the fastest system and 2D-2D match for Synergy was the slowest. All systems have enough ability with regard to accuracy and patient throughput on clinical use. ExacTrac X-Ray system showed superiority with accuracy and throughput, but it is important to note that we have to choose the IGRT technique depending on the treatment site, the purpose, and the patient's state.

[Comparison of dose accuracy between 2D array detectors for pre-treatment IMRT QA].

The dosimetric properties between various 2D array detectors were compared and were evaluated with regard to the accuracy in absolute dose and dose distributions for clinical treatment fields. We used to check the dose accuracy: 2D array detectors; MapCHECK (Sun Nuclear), EPID (Varian Medical Systems), EPID-based dosimetry (EPIDose, Sun Nuclear), COMPASS (IBA) and conventional system; EDR2 film (Eastman Kodak), Exradin A-14SL ion chamber (0.016 cc, Standard Imaging). First, we compared the dose linearity, dose rate dependence, and output factor between the 2D array detectors. Next, the accuracy of the absolute dose and dose distributions were evaluated for clinical fields. All detector responses for the dose linear were in agreement within 1%, and the dose rate dependence and output factor agreed within a standard deviation of ±1.2%, except for EPID. This is because EPID is fluence distributions. In all the 2D array detectors, the point dose agreed within 5% with treatment planning system (TPS). Pass rates of each detector for TPS were more than 97% in the gamma analysis (3 mm/3%). EPIDose was in a good agreement with TPS. All 2D array detectors used in this study showed almost the same accuracy for clinical fields. EPIDose has better resolution than other 2D array detectors and thus this is expected for dose distributions with a small field.

[Dosimetric correction for a six degrees carbon fiber couch].

We investigated experimentally and clinically the influence of a six degree (6D) carbon fiber couch on conventional radiation therapy. We used 4, 6 and 10 MV X-rays and compared dose distributions based on correction methods, i.e. monitor unit (MU) addition, including computed tomography (CT) couch, and the couch modeling. Additionally, we evaluated the clinical value of dosimetric correction for the 6D couch in 30 patients treated with multi-field irradiation. In the phantom study, the maximum difference of isocenter doses attributable to the 6D couch was 5.1%; the difference was reduced with increasing X-ray energy. Although the isocenter dose based on each correction method was precise within ±1%, MU addition underestimated the surface dose. In the clinical study, the maximum difference of isocenter doses attributable to the 6D couch was 2.7%. The correction methods for the 6D couch provide for highly precise treatment planning. However, the clinical indication of complicated correction methods should be considered for each institution or each patient, because the influence of the 6D couch was reduced with multi-field irradiation.

Radiation exposure of operator performing interventional procedures using a flat panel angiography system: evaluation with photoluminescence glass dosimeters.

PURPOSE: Using glass rod dosimeters we investigated the radiation dose to the operator performing interventional procedures in 43 patients with the aid of a monoplane flat detector-based angiography system. MATERIALS AND METHODS: During the procedures we recorded the number of radiographic frames and the radiographic conditions. After treatment we recorded the fluoroscopy time and the fluoroscopic, radiographic, and total air kerma. To obtain the total operator exposure dose we took measurements at five sites: left orbital fossa, thyroid, left hand, left chest, and pubic symphysis. RESULTS: The mean operator exposure dose to the left hand was higher than at the other sites we measured; it was 387.0, 209.6, 174.3, and 237.1 microGy for the stentgraft, percutaneous transluminal arteriography, transarterial chemoembolization, and hepatic infusion port placement procedures. There was a positive correlation between the fluoroscopic and radiographic air kerma value and the operator exposure dose at the left orbital fossa, thyroid, and left hand. CONCLUSION: The operator exposure dose correlated with the radiographic and fluoroscopic air kerma. Exposure of the operator's left hand was higher than at the other sites studied.

[Combinations of scan parameters and image quality at C-arm CT for abdominal imaging].

We measured the contrast-to-noise ratio (CNR) and evaluated low-contrast images and streak artifacts to optimize abdominal C-arm CT imaging, and we investigated the view number, acquisition matrix, and pixel depth. To measure CNR, we filled 0.125-1.0-inch cavities in an American Association of Physicists in Medicine (AAPM) CT performance phantom with a sodium chloride solution. Five radiological technologists visually evaluated the noise, signal conspicuity at low-and high-signal density, and the overall image quality using paired comparisons based on Thurstone's law. In a given acquisition matrix, the total view number had the greatest effect on the image noise, artifacts, and signal detectability on C-arm CT images. For a given incident dose per view on the flat-panel detector (FPD), fewer images with noise and streak artifacts resulted when a larger view number was selected.

Radiation dose reduction without degradation of low-contrast detectability at abdominal multisection CT with a low-tube voltage technique: phantom study.

PURPOSE: To reduce radiation dose from abdominal computed tomography (CT) without degradation of low-contrast detectability by using a technique with low tube voltage (90 kV). MATERIALS AND METHODS: The institutional review board approved the participation of the radiologists in the observer performance test, and informed consent was obtained from all participating radiologists. A phantom for measurement of the radiation dose and a phantom containing low-contrast objects were scanned with a 16-detector row CT scanner at 120 kV and 90 kV. For determination of the radiation dose at both 90 kV and 120 kV, the tube current-time product settings were 100-560 mAs, and the doses at the center and periphery of the phantom were measured. To assess low-contrast detectability, we used a 300-mAs setting at 120 kV and 250-560-mAs settings at 90 kV. Five observers participated in the receiver operating characteristic analysis. Area under the receiver operating characteristic curve (A(z)) values were calculated in each observer. A(z) values obtained with each of the scanning techniques were recorded, and differences were examined for significance by using the Dunnet method. RESULTS: The mean A(z) value was 0.951 at 120 kV and 300 mAs. A(z) values were 0.927-0.973 at 90 kV and 450-560 mAs, and the differences between those values and values obtained at 120 kV and 300 mAs were not significant (P = .937-.952). A value of 100% was assigned to the radiation dose delivered to the center of the phantom at 120 kV and 300 mAs. The relative dose delivered at 90 kV ranged from 65% at 450 mAs to 79% at 560 mAs. CONCLUSION: A reduction from 120 kV to 90 kV led to as much as a 35% reduction in the radiation dose, without sacrifice of low-contrast detectability, at CT.

Reduction of radiation dose at HRCT of the temporal bone in children.

PURPOSE: The purpose of this study was to reduce the radiation exposure of the eye lens in high resolution computed tomography (HRCT) of the temporal bone using an experimental phantom. MATERIALS AND METHODS: The HRCT image that was used for analysis was obtained by changing parameters including effective-mAs (E-mAs), distance coverage, and height of object in the Y-axis. Radiation exposure was measured to calculate equivalent doses by glass rod dosimeters that were fixed above the right orbit parallel to the body axis. Deterioration in image quality was evaluated by three radiologists and the following three-point rating method was employed: grade 1 (good image quality without diagnostic limitations), grade 2 (image was deteriorated, but there were no diagnostic limitations), and grade 3 (image was deteriorated with diagnostic limitations). RESULTS: Assuming that the equivalent dose was y (mSv), and E-mAs was x, a simple regression line, y=0.506x-0.494 (decision coefficient, R2=0.999), was obtained. A standard deviation (S.D.) less than 120 (E-mAs, 220-120) was judged as grade 1, an S.D. between 120 and 150 was judged as grade 2, and an S.D. higher than 150 was judged as grade 3, indicating that deterioration of the quality of images with reduced E-mAs affected the diagnosis by imaging at S.D. higher than 150. CONCLUSION: Radiation dose at the eye lens in HRCT could be reduced up to an equivalent dose corresponding to 70 mAs without compromising diagnostic quality in the phantom experiment.