On the effect of dose delivery temporal domain on the biological effectiveness of central nervous system CyberKnife radiosurgery applications: theoretical assessment using the concept of biologically effective dose.

OBJECTIVE: The diversity in technical configuration between clinically available radiosurgery systems, results in accordingly diverse treatment times for the same physical dose prescription, spanning from several min to more than 1 h. This, combined with evidence supporting the impact of dose delivery temporal pattern on the bio-effectiveness of low-LET radiation treatments, challenges the 'acute exposure' assumption adopted clinically to estimate the biological outcome of a given treatment scheme under the concept of biologically effective dose (BED).Approach: In this work, the treatment plans of 30 patients underwent CyberKnife radiosurgery for vestibular schwannoma (VS), prescribing a marginal dose of 13 Gy to the tumor, were retrospectively reviewed and the corresponding dose distributions were resolved in the temporal domain. For this purpose, the dose delivery timeline for each treatment was calculated based on relevant treatment plan data and technical specifications of the CyberKnife system, while dosimetry data were independently acquired on a CT-based digital model of each patient using an in-house developed dose calculation algorithm.Main results: Results showed that CyberKnife delivers highly inhomogeneous dose rate distributions in the temporo-spatial domain. This influences the delivered BED levels due to alterations in the sublethal damage repair (SLR) occurring within the treatment session. Using a BED framework involving SLR effects, it was shown that each physical dose iso-surface is associated with a BEDslrrange. For the patient cohort studied, a typical range of 2%, with respect to the mean BEDslrvalue was found at 1σ.Significance: The marginal BEDslrdelivered to the tumor by the prescription dose iso-surface deteriorates with treatment time, involving both beam-on time and beam-off gaps. For treatment time,T, between 21 and 50 min, this can be expressed byBEDslr(Gy2.47)=-0.35±2.8%∙Tmin+(76.74±0.4%).Compared to the acute exposure approach, a BED 'loss' of 21% is associated with the delivery of 13 Gy to the VS-tumor in 35 min.

Assessing the dose rate delivery of helical TomoTherapy prostate and head & neck treatments.

The dose rate distributions delivered to 55 prostate and head & neck (H&N) cancer patients treated with a helical TomoTherapy (HT) system were resolved and assessed with regard to pitch and field width defined during treatment planning. Statistical analysis of the studied cases showed that the median treatment delivery time was 4.4 min and 6.3 min for the prostate and H&N cases, respectively. Dose rate volume histogram data for the studied cases showed that the 25% and 12% of the volume of the planning target volumes of the prostate and H&N cases are irradiated with a dose rate of greater or equal to 1 Gy min-1. Quartile dose rate (QDR) data confirmed that in HT, where the target is irradiated in slices, most of the dose is delivered to each voxel of the target when it travels within the beam. Analysis of the planning data from all cases showed that this lasts for 68 s (median value). QDRs results showed that using the 2.5 cm field width, 75% of the prescribed dose is delivered to target voxels with a median dose rate of at least 3.2 Gy min-1and 4.5 Gy min-1, for the prostate and H&N cases, respectively. Systematically higher dose rates were observed for the H&N cases due to the shallower depths of the lesions in this anatomical site. Delivered dose rates were also found to increase with field width and pitch setting, due to the higher output of the system which, in general, results in accordingly decreased total treatment time. The biological effect of the dose rate findings of this work needs to be further investigated using in-vitro studies and clinical treatment data.

On the total system error of a robotic radiosurgery system: phantom measurements, clinical evaluation and long-term analysis.

The total system error (TSE) of a CyberKnife® system was measured using two phantom-based methods and one patient-based method. The standard radiochromic film (RCF) end-to-end (E2E) test using an anthropomorphic head and neck phantom and isocentric treatment delivery was used with the 6Dskull, Fiducial and Xsight® spine (XST) tracking methods. More than 200 RCF-based E2E results covering the period from installation in 2006 until 2017 were analyzed with respect to tracking method, system hardware and software versions, secondary collimation system, and years since installation. An independent polymer gel E2E method was also applied, involving a 3D printed head phantom and multiple spherical target volumes widely distributed within the brain. Finally, the TSE was assessed by comparing the delineated target in the planning computed tomography images of a patient treated for a thalamic functional target with the radiation-induced lesion defined on the six-month follow-up magnetic resonance (MR) images. Statistical analysis of the RCF-based TSE results showed mean ± standard deviation values of 0.40 ± 0.18 mm, 0.40 ± 0.19 mm, and 0.55 ± 0.20 mm for the 6Dskull, Fiducial, and XST tracking methods, respectively. Polymer gel TSE values smaller than 0.66 mm were found for seven targets distributed within the brain, showing that the targeting accuracy of the system is sustained even for targets situated up to 80 mm away from the center of the skull. An average clinical TSE value of 0.87 ± 0.25 mm was also measured using the FSE T2 and FLAIR post-treatment MR image data. Analysis of the long-term RCF-based E2E tests showed no changes of TSE over time. This study is the first to report long-term (>10 years) analysis of TSE, TSE measurement for targets positioned at large distances from the virtual machine isocenter, or a clinical assessment of TSE for the CyberKnife system. All of these measurements demonstrate TSE consistently < 1 mm.

Monte Carlo and experimental determination of correction factors for gamma knife perfexion small field dosimetry measurements.

Detector-, field size- and machine-specific correction factors are required for precise dosimetry measurements in small and non-standard photon fields. In this work, Monte Carlo (MC) simulation techniques were used to calculate the [Formula: see text] and [Formula: see text] correction factors for a series of ionization chambers, a synthetic microDiamond and diode dosimeters, used for reference and/or output factor (OF) measurements in the Gamma Knife Perfexion photon fields. Calculations were performed for the solid water (SW) and ABS plastic phantoms, as well as for a water phantom of the same geometry. MC calculations for the [Formula: see text] correction factors in SW were compared against corresponding experimental results for a subset of ionization chambers and diode detectors. Reference experimental OF data were obtained through the weighted average of corresponding measurements using TLDs, EBT-2 films and alanine pellets. [Formula: see text] values close to unity (within 1%) were calculated for most of ionization chambers in water. Greater corrections of up to 6.0% were observed for chambers with relatively large air-cavity dimensions and steel central electrode. A phantom correction of 1.006 and 1.024 (breaking down to 1.014 from the ABS sphere and 1.010 from the accompanying ABS phantom adapter) were calculated for the SW and ABS phantoms, respectively, adding up to [Formula: see text] corrections in water. Both measurements and MC calculations for the diode and microDiamond detectors resulted in lower than unit [Formula: see text] correction factors, due to their denser sensitive volume and encapsulation materials. In comparison, higher than unit [Formula: see text] results for the ionization chambers suggested field size depended dose underestimations (being significant for the 4 mm field), with magnitude depending on the combination of contradicting phenomena associated with volume averaging and electron fluence perturbations. Finally, the presence of 0.5 mm air-gap between the diodes' frontal surface and their phantom-inserts may considerably influence OF measurements, reaching 4.6% for the Razor diode.

Characterization of system-related geometric distortions in MR images employed in Gamma Knife radiosurgery applications.

This work provides characterization of system-related geometric distortions present in MRIs used in Gamma Knife (GK) stereotactic radiosurgery (SRS) treatment planning. A custom-made phantom, compatible with the Leksell stereotactic frame model G and encompassing 947 control points (CPs), was utilized. MR images were obtained with and without the frame, thus allowing discrimination of frame-induced distortions. In the absence of the frame and following compensation for field inhomogeneities, measured average CP disposition owing to gradient nonlinearities was 0.53 mm. In presence of the frame, contrarily, detected distortion was greatly increased (up to about 5 mm) in the vicinity of the frame base due to eddy currents induced in the closed loop of its aluminum material. Frame-related distortion was obliterated at approximately 90 mm from the frame base. Although the region with the maximum observed distortion may not lie within the GK treatable volume, the presence of the frame results in distortion of the order of 1.5 mm at a 7 cm distance from the center of the Leksell space. Additionally, severe distortions observed outside the treatable volume could possibly impinge on the delivery accuracy mainly by adversely affecting the registration process (e.g. the position of the lower part of the N-shaped fiducials used to define the stereotactic space may be miss-registered). Images acquired with a modified version of the frame developed by replacing its front side with an acrylic bar, thus interrupting the closed aluminum loop and reducing the induced eddy currents, were shown to benefit from relatively reduced distortion. System-related distortion was also identified in patient MR images. Using corresponding CT angiography images as a reference, an offset of 1.1 mm was detected for two vessels lying in close proximity to the frame base, while excellent spatial agreement was observed for a vessel far apart from the frame base.

On the development of a comprehensive MC simulation model for the Gamma Knife Perfexion radiosurgery unit.

This work presents a comprehensive Monte Carlo (MC) simulation model for the Gamma Knife Perfexion (PFX) radiosurgery unit. Model-based dosimetry calculations were benchmarked in terms of relative dose profiles (RDPs) and output factors (OFs), against corresponding EBT2 measurements. To reduce the rather prolonged computational time associated with the comprehensive PFX model MC simulations, two approximations were explored and evaluated on the grounds of dosimetric accuracy. The first consists in directional biasing of the (60)Co photon emission while the second refers to the implementation of simplified source geometric models. The effect of the dose scoring volume dimensions in OF calculations accuracy was also explored. RDP calculations for the comprehensive PFX model were found to be in agreement with corresponding EBT2 measurements. Output factors of 0.819 ± 0.004 and 0.8941 ± 0.0013 were calculated for the 4 mm and 8 mm collimator, respectively, which agree, within uncertainties, with corresponding EBT2 measurements and published experimental data. Volume averaging was found to affect OF results by more than 0.3% for scoring volume radii greater than 0.5 mm and 1.4 mm for the 4 mm and 8 mm collimators, respectively. Directional biasing of photon emission resulted in a time efficiency gain factor of up to 210 with respect to the isotropic photon emission. Although no considerable effect on relative dose profiles was detected, directional biasing led to OF overestimations which were more pronounced for the 4 mm collimator and increased with decreasing emission cone half-angle, reaching up to 6% for a 5° angle. Implementation of simplified source models revealed that omitting the sources' stainless steel capsule significantly affects both OF results and relative dose profiles, while the aluminum-based bushing did not exhibit considerable dosimetric effect. In conclusion, the results of this work suggest that any PFX simulation model should be benchmarked in terms of both RDP and OF results.

Assessment and characterization of the total geometric uncertainty in Gamma Knife radiosurgery using polymer gels.

PURPOSE: This work proposes and implements an experimental methodology, based on polymer gels, for assessing the total geometric uncertainty and characterizing its contributors in Gamma Knife (GK) radiosurgery. METHODS: A treatment plan consisting of 26, 4-mm GK single shot dose distributions, covering an extended region of the Leksell stereotactic space, was prepared and delivered to a polymer gel filled polymethyl methacrylate (PMMA) head phantom (16 cm diameter) used to accurately reproduce every link in the GK treatment chain. The center of each shot served as a "control point" in the assessment of the GK total geometric uncertainty, which depends on (a) the spatial dose delivery uncertainty of the PERFEXION GK unit used in this work, (b) the spatial distortions inherent in MR images commonly used for target delineation, and (c) the geometric uncertainty contributor associated with the image registration procedure performed by the Leksell GammaPlan (LGP) treatment planning system (TPS), in the case that registration is directly based on the apparent fiducial locations depicted in each MR image by the N-shaped rods on the Leksell localization box. The irradiated phantom was MR imaged at 1.5 T employing a T2-weighted pulse sequence. Four image series were acquired by alternating the frequency encoding axis and reversing the read gradient polarity, thus allowing the characterization of the MR-related spatial distortions. RESULTS: MR spatial distortions stemming from main field (B0) inhomogeneity as well as from susceptibility and chemical shift phenomena (also known as sequence dependent distortions) were found to be of the order of 0.5 mm, while those owing to gradient nonlinearities (also known as sequence independent distortions) were found to increase with distance from the MR scanner isocenter extending up to 0.47 mm at an Euclidean distance of 69.6 mm. Regarding the LGP image registration procedure, the corresponding average contribution to the total geometric uncertainty ranged from 0.34 to 0.80 mm. The average total geometric uncertainty, which also includes the GK spatial dose delivery uncertainty, was found equal to (0.88 ± 0.16), (0.88 ± 0.26), (1.02 ± 0.09), and (1.15 ± 0.24) mm for the MR image series acquired with the read gradient polarity (direction) set toward right, left, posterior, and anterior, respectively. CONCLUSIONS: The implemented methodology seems capable of assessing the total geometric uncertainty, as well as of characterizing its contributors, ascribed to the entire GK treatment delivery (i.e., from MR imaging to GK dose delivery) for an extended region of the Leksell stereotactic space. Results obtained indicate that the selection of both the frequency encoding axis and the read gradient polarity during MRI acquisition may affect the magnitude as well as the spatial components of the total geometric uncertainty.

Dosimetric accuracy of a deterministic radiation transport based (192)Ir brachytherapy treatment planning system. Part III. Comparison to Monte Carlo simulation in voxelized anatomical computational models.

PURPOSE: To compare TG43-based and Acuros deterministic radiation transport-based calculations of the BrachyVision treatment planning system (TPS) with corresponding Monte Carlo (MC) simulation results in heterogeneous patient geometries, in order to validate Acuros and quantify the accuracy improvement it marks relative to TG43. METHODS: Dosimetric comparisons in the form of isodose lines, percentage dose difference maps, and dose volume histogram results were performed for two voxelized mathematical models resembling an esophageal and a breast brachytherapy patient, as well as an actual breast brachytherapy patient model. The mathematical models were converted to digital imaging and communications in medicine (DICOM) image series for input to the TPS. The MCNP5 v.1.40 general-purpose simulation code input files for each model were prepared using information derived from the corresponding DICOM RT exports from the TPS. RESULTS: Comparisons of MC and TG43 results in all models showed significant differences, as reported previously in the literature and expected from the inability of the TG43 based algorithm to account for heterogeneities and model specific scatter conditions. A close agreement was observed between MC and Acuros results in all models except for a limited number of points that lay in the penumbra of perfectly shaped structures in the esophageal model, or at distances very close to the catheters in all models. CONCLUSIONS: Acuros marks a significant dosimetry improvement relative to TG43. The assessment of the clinical significance of this accuracy improvement requires further work. Mathematical patient equivalent models and models prepared from actual patient CT series are useful complementary tools in the methodology outlined in this series of works for the benchmarking of any advanced dose calculation algorithm beyond TG43.

On the output factor measurements of the CyberKnife iris collimator small fields: Experimental determination of the k(Q(clin),Q(msr) ) (f(clin),f(msr) ) correction factors for microchamber and diode detectors.

PURPOSE: To measure the output factors (OFs) of the small fields formed by the variable aperture collimator system (iris) of a CyberKnife (CK) robotic radiosurgery system, and determine the k(Q(clin),Q(msr) ) (f(clin),f(msr) ) correction factors for a microchamber and four diode detectors. METHODS: OF measurements were performed using a PTW PinPoint 31014 microchamber, four diode detectors (PTW-60017, -60012, -60008, and the SunNuclear EDGE detector), TLD-100 microcubes, alanine dosimeters, EBT films, and polymer gels for the 5 mm, 7.5 mm, 10 mm, 12.5 mm, and 15 mm iris collimators at 650 mm, 800 mm, and 1000 mm source to detector distance (SDD). The alanine OF measurements were corrected for volume averaging effects using the 3D dose distributions registered in polymer gel dosimeters. k(Q(clin),Q(msr) ) (f(clin),f(msr) ) correction factors for the PinPoint microchamber and the diode dosimeters were calculated through comparison against corresponding polymer gel, EBT, alanine, and TLD results. RESULTS: Experimental OF results are presented for the array of dosimetric systems used. The PinPoint microchamber was found to underestimate small field OFs, and a k(Q(clin),Q(msr) ) (f(clin),f(msr) ) correction factor ranging from 1.127 ± 0.022 (for the 5 mm iris collimator) to 1.004 ± 0.010 (for the 15 mm iris collimator) was determined at the reference SDD of 800 mm. The PinPoint k(Q(clin),Q(msr) ) (f(clin),f(msr) ) correction factor was also found to increase with decreasing SDD; k(Q(clin),Q(msr) ) (f(clin),f(msr) ) values equal to 1.220 ± 0.028 and 1.077 ± 0.016 were obtained for the 5 mm iris collimator at 650 mm and 1000 mm SDD, respectively. On the contrary, diode detectors were found to overestimate small field OFs and a correction factor equal to 0.973 ± 0.006, 0.954 ± 0.006, 0.937 ± 0.007, and 0.964 ± 0.006 was measured for the PTW-60017, -60012, -60008 and the EDGE diode detectors, respectively, for the 5 mm iris collimator at 800 mm SDD. The corresponding correction factors for the 15 mm iris collimator were found equal to 0.997 ± 0.010, 0.994 ± 0.009, 0.988 ± 0.010, and 0.986 ± 0.010, respectively. No correlation of the diode k(Q(clin),Q(msr) ) (f(clin),f(msr) ) correction factors with SDD was observed. CONCLUSIONS: This work demonstrates an experimental procedure for the determination of the k(Q(clin),Q(msr) ) (f(clin),f(msr) ) correction factors required to obtain small field OF results of increased accuracy.

Dosimetric accuracy of a deterministic radiation transport based 192Ir brachytherapy treatment planning system. Part II: Monte Carlo and experimental verification of a multiple source dwell position plan employing a shielded applicator.

PURPOSE: The aim of this work is the dosimetric validation of a deterministic radiation transport based treatment planning system (BRACHYVISION v. 8.8, referred to as TPS in the following) for multiple 192Ir source dwell position brachytherapy applications employing a shielded applicator in homogeneous water geometries. METHODS: TPS calculations for an irradiation plan employing seven VS2000 192Ir high dose rate (HDR) source dwell positions and a partially shielded applicator (GM11004380) were compared to corresponding Monte Carlo (MC) simulation results, as well as experimental results obtained using the VIP polymer gel-magnetic resonance imaging three-dimensional dosimetry method with a custom made phantom. RESULTS: TPS and MC dose distributions were found in agreement which is mainly within +/- 2%. Considerable differences between TPS and MC results (greater than 2%) were observed at points in the penumbra of the shields (i.e., close to the edges of the "shielded" segment of the geometries). These differences were experimentally verified and therefore attributed to the TPS. Apart from these regions, experimental and TPS dose distributions were found in agreement within 2 mm distance to agreement and 5% dose difference criteria. As shown in this work, these results mark a significant improvement relative to dosimetry algorithms that disregard the presence of the shielded applicator since the use of the latter leads to dosimetry errors on the order of 20%-30% at the edge of the "unshielded" segment of the geometry and even 2%-6% at points corresponding to the potential location of the target volume in clinical applications using the applicator (points in the unshielded segment at short distances from the applicator). CONCLUSIONS: Results of this work attest the capability of the TPS to accurately account for the scatter conditions and the increased attenuation involved in HDR brachytherapy applications employing multiple source dwell positions and partially shielded applicators.

On the implementation of a recently proposed dosimetric formalism to a robotic radiosurgery system.

PURPOSE: The aim of this work is to implement a recently proposed dosimetric formalism for nonstandard fields to the calibration and small field output factor measurement of a robotic stereotactic radiosurgery system. METHODS: Reference dosimetry measurements were performed in the nonstandard, 60 mm diameter machine specific reference (msr) field using a Farmer ion chamber, five other cylindrical chambers with cavity lengths ranging from 16.25 down to 2.7 mm, and alanine dosimeters. Output factor measurements were performed for the 5, 7.5, 10, and 15 mm field sizes using microchambers, diode detectors, alanine dosimeters, TLD microcubes, and EBT Gafchromic films. Measurement correction factors as described in the proposed formalism were calculated for the ion chamber and diode detector output factor measurements based on published Monte Carlo data. Corresponding volume averaging correction factors were calculated for the alanine output factor measurements using 3D dose distributions, measured with polymer gel dosimeters. RESULTS: Farmer chamber and alanine reference dosimetry results were found in close agreement, yielding a correction factor of k(Q(msr),Q)(f(msr),f(ref)) = 0.999 +/- 0.016 for the chamber readings. These results were also found to be in agreement within experimental uncertainties with corresponding results obtained using the shorter cavity length ionization chambers. The mean measured dose values of the latter, however, were found to be consistently greater than that of the Farmer chamber. This finding, combined with an observed inverse relationship between the mean measured dose and chamber cavity length that follows the trend predicted by theoretical volume averaging calculations in the msr field, implies that the Farmer k(Q(msr),Q)(f(msr),f(ref)) correction is greater than unity. Regarding the output factor results, deviations as large as 33% were observed between the different dosimeters used. These deviations were substantially decreased when appropriate correction factors were applied to the measured microchamber, diode, and alanine values. After correction, all diode and microchamber measured output factors agreed within 1.6% with the corresponding alanine measurements, and within 3.1% with the TLD measurements. The weighted mean output factors were 0.681 +/- 0.001, 0.824 +/- 0.001, 0.875 +/- 0.001, and 0.954 +/- 0.001 for the 5, 7.5, 10, and 15 mm beams, respectively. CONCLUSIONS: The comparison of Farmer chamber measurements versus alanine reference dosimetry validates the use of the former for dosimetry in the msr field of this treatment delivery system. The corresponding results of this work obtained using chambers with different cavity lengths, combined with previous literature findings, suggest that a k(Q(msr),Q)(f(msr),f(ref)) Farmer chamber dose response correction factor of 1.01 may improve calibration measurement accuracy when using the proposed dosimetric formalism. The k(Q(msr),Q)(f(msr),f(ref)) correction factor is within 0.5% from unity for ion chambers with cavity lengths less than 10 mm. Substantial improvements in small field output factor measurement accuracy can be obtained when using microchambers and diodes by applying appropriately calculated correction factors to the detector measurements according to the proposed dosimetric formalism, and their routine use is therefore recommended.

Dosimetric accuracy of a deterministic radiation transport based 192Ir brachytherapy treatment planning system. Part I: single sources and bounded homogeneous geometries.

PURPOSE: The aim of this work is to validate a deterministic radiation transport based treatment planning system (TPS) for single 192Ir brachytherapy source dosimetry in homogeneous water geometries. METHODS: TPS results were obtained using the deterministic radiation transport option of a BRACHYVISION v. 8.8 system for three characteristic source designs (VS2000, GMPlus HDR, and GMPlus PDR) with each source either centered in a 15 cm radius spherical water phantom, or positioned at varying distance away from the phantom center. Corresponding MC simulations were performed using the MCNPX code v.2.5.0 and source geometry models prepared using information provided by the manufacturers. RESULTS: Comparison in terms of the AAPM TG-43 dosimetric formalism quantities, as well as dose rate distributions per unit air kerma strength with a spatial resolution of 0.1 cm, yielded close agreement between TPS and MC results for the sources centered in the phantom. Besides some regions close to the source longitudinal axes where discrepancies could be characterized as systematic, overall agreement for all three sources studied is comparable to the statistical (type A) uncertainty of MC simulations (1% at the majority of points in the geometry increasing to 2%-3% at points lying both away from the source center and close to the source longitudinal axis). A corresponding good agreement was also found between TPS and MC results for the sources positioned away from the phantom center. CONCLUSIONS: Results of this work attest the capability of the TPS to accurately account for the scatter conditions regardless of the size or shape of a given geometry of dosimetric interest, and the position of a source within it. This is important since, as shown in the literature and summarized also in this work, these factors could introduce a significant dosimetric effect that is currently ignored in clinical treatment planning. It is concluded that the implementation of the deterministic radiation transport option of the BRACHYVISION v. 8.8 system for 192Ir brachytherapy dosimetry in homogeneous water geometries yields results of comparable accuracy to the golden standard of Monte Carlo simulation, in clinically viable calculation times.

Gamma knife output factor measurements using VIP polymer gel dosimetry.

PURPOSE: Water equivalent polymer gel dosimeters and magnetic resonance imaging were employed to measure the output factors of the two smallest treatment fields available in a Gamma Knife model C radiosurgery unit, those formed employing the 4 and 8 mm final collimator helmets. METHODS: Three samples of the VIP normoxic gel formulation were prepared and irradiated so that a single shot of the field whose output factor is to be measured and a single shot of the reference 18 mm field were delivered in each one. Emphasis is given to the development and benchmarking of a refined data processing methodology of reduced uncertainty that fully exploits the 3D dose distributions registered in the dosimeters. RESULTS: Polymer gel results for the output factor of the 8 mm collimator helmet are found to be in close agreement with the corresponding value recommended by the vendor (0.955 +/- 0.007 versus 0.956, respectively). For the 4 mm collimator helmet, however, polymer gel results suggest an output factor 3% lower than the value recommended by the vendor (0.841 +/- 0.009 versus 0.870, respectively). CONCLUSIONS: A comparison with corresponding measurements published in the literature indicates that output factor results of this work are in agreement with those obtained using dosimetric systems which, besides fine spatial resolution and lack of angular and dose rate dependence of the dosimeter's response, share with polymer gels the favorable characteristic of minimal radiation field perturbation.

Correlation of image quality with exposure index and processing protocol in a computed radiography system.

The correlation of image quality with the exposure index (EI) and the processing protocol was investigated in a Kodak computed radiography (CR) system using clinical radiographs and a water phantom containing an aluminium and a copper step-wedge. The phantom was exposed to different dose levels and the acquired images were processed using four clinical protocols. The quality of these images was evaluated in terms of image brightness, contrast and noise. In clinical radiographs, there was no straightforward correlation of image quality with EI. In phantom images, higher EI values improved contrast and reduced noise but after a point this improvement does not justify the implied increase in patient dose. Image brightness, contrast and noise were also strongly dependent on the processing protocol. To obtain the images of satisfactory quality with the Kodak CR system, a dose slightly higher than those used in 400 relative speed screen-film systems and a processing protocol designated for the specific radiographic examination are required.