Comprehensive evaluation and new recommendations in the use of Gafchromic EBT3 film

BACKGROUND

Gafchromic film's unique properties of tissue-equivalence, dose-rate independence, and high spatial resolution make it an attractive choice for many dosimetric applications. However, complicated calibration processes and film handling limits its routine use.

PURPOSE

We evaluated the performance of Gafchromic EBT3 film after irradiation under a variety of measurement conditions to identify aspects of film handling and analysis for simplified but robust film dosimetry.

METHODS

The short- (from 5 min to 100 h) and long-term (months) film response was evaluated for clinically relevant doses of up to 50 Gy for accuracy in dose determination and relative dose distributions. The dependence of film response on film-read delay, film batch, scanner type, and beam energy was determined.

RESULTS

Scanning the film within a 4-h window and using a standard 24-h calibration curve introduced a maximum error of 2% over a dose range of 1-40 Gy, with lower doses showing higher uncertainty in dose determination. Relative dose measurements demonstrated <1 mm difference in electron beam parameters such as depth of 50% of the maximum dose value (R50 ), independent of when the film was scanned after irradiation or the type of calibration curve used (batch-specific or time-specific calibration curve) if the same default scanner was used. Analysis of films exposed over a 5-year period showed that using the red channel led to the lowest variation in the measured net optical density values for different film batches, with doses >10 Gy having the lowest coefficient of variation (<1.7%). Using scanners of similar design produced netOD values within 3% after exposure to doses of 1-40 Gy.

CONCLUSIONS

This is the first comprehensive evaluation of the temporal and batch dependence of Gafchromic EBT3 film evaluated on consolidated data over 8 years. The relative dosimetric measurements were insensitive to the type of calibration applied (batch- or time-specific) and in-depth time-dependent dosimetric signal behaviors can be established for film scanned outside of the recommended 16-24 h post-irradiation window. We generated guidelines based on our findings to simplify film handling and analysis and provide tabulated dose- and time-dependent correction factors to achieve this without reducing the accuracy of dose determination.

Performance evaluation of buildup bolus during external radiotherapy of mastectomy patients: treatment planning and film dosimetry

A buildup bolus is used during the post-mastectomy radiotherapy (PMRT) to overcome under-dosage issues in the chest wall. The current study is aimed at evaluating the performance of a bolus in dose enhancement through both film dosimetry and treatment planning approaches. Twenty patients were enrolled in current research. The received dose by the skin at the lateral and medial regions of the chest wall in the presence and absence bolus was evaluated. Film dosimetry results showed that the presence of the bolus can averagely increase the skin dose by about 80% (P value < 0.001) and 92% (P value < 0.001) in lateral and medial regions, respectively. No significant difference was observed between the measured and treatment planning system (TPS)-calculated dose values in the presence of bolus. The presence of the bolus can considerably increase the absorbed dose by superficial chest wall regions. The TPS shows a favorable performance in superficial dose calculations in the presence of the buildup bolus. Hosseini et al.: demonstration of implemented research in the current study.

Evaluation of OrthoChromic OC-1 films for photon radiotherapy application

A new film dosimetry system consists of the new OrthoChromic™ OC-1 film, and a novel calibration procedure was evaluated. Two films, C1 and C2, were exposed simultaneously using the 6FFF beam with a step-wedge pattern of five steps ranging from 590 to 3000 cGy. C1 was used for calibration, and C2 was used for calibration curve validation. The second scan of C2 was done by rotating the film by 90-deg. To evaluate the effectiveness of the non-uniform scanner response correction with the new system, a film was exposed to a 20 × 20 cm2 field. The beam profile measured with the film was compared to the IBA cc04 measurements in water. Films were irradiated to characterize the energy response, dynamic range and temporal growth effect. Open (MLC-defined) and clinical fields were radiated to evaluate the overall performance of the new system. The new calibration procedure was validated with an average dose difference of 1.6% and a gamma (2%,2 mm) passing rate of 100%. With C2 scanned 90-deg rotated, the average dose difference was 1.3%. The average difference between cc04 and film was 0.4%. The St between films and diode/cc04 were within -0.3% difference for 1 × 1 to 14 × 14 cm2 and -2.8% for 0.5 × 0.5 cm2. For clinical fields, the average gamma (3%,2 mm) was 98.8%. These results were consistent with EBT3 film and MapCheck measurements with a dose > 400 cGy. The results have shown that the OC-1 film system can achieve accurate results for QA measurements, but more considerable uncertainty was observed within the low dose range.

Dosimetric characterization of a mobile accelerator dedicated for intraoperative radiation therapy: Monte Carlo simulations and experimental validation

PURPOSE

This Technical Note validates previously published data about the dosimetry of the electron beams produced by a mobile accelerator dedicated for intraoperative radiation therapy (IORT). The evaluation of the directional response of a PTW microDiamond detector is presented together with a detailed analysis of the output factors (OFs) for bevelled applicators.

METHODS

The OFs of the 6, 8, 10 and 12 MeV electron beams produced by a light intraoperative accelerator (LIAC, SIT, Italy) were measured in a commercial water phantom using the microDiamond. A set of flat and bevelled applicators with sizes ranging from 4 to 10 cm was characterized. For bevelled applicators, a correction for the angular dependence of the microDiamond was calculated using a home-made spherical phantom. Correction factors were obtained through measurements performed rotating the accelerator treatment head at 0°, 15°, 30° and 45°.

RESULTS

For flat applicators, the average deviation between measured and simulated OFs was (-1.1 ± 0.7)%. The microDiamond showed a higher angular dependence for the 6 MeV beam (∼8% for angles up to 45°, range 92 % ÷ 100 %), while the variations for 8, 10 and 12 MeV beams were ∼ 4 % (range 97 % ÷ 101 %). Correcting for this dependence, the average deviation of the OFs for bevelled applicators was (-0.9 ± 1.6)%.

CONCLUSIONS

The presented results were in very good agreement with those reported in literature. Very similar deviations were found between flat and bevelled applicators confirming the suitability of our method to determine the angular dependence correction factors of the microDiamond detector.

Reusable radiochromic hackmanite with gamma exposure memory

Radiochromic films are used as position-sensitive dose meters in e.g. medical physics and radiation processing. The currently available films like those based on lithium-10,12-pentacosdiynoate or leucomalachite green are either toxic or non-reusable, or both. There is thus a great need for a sustainable solution for radiochromic detection. In the present work, we present a suitable candidate: hackmanite with the general formula Na₈Al₆Si₆O₂₄(Cl,S)₂. This material is known as a natural intelligent material capable of changing color when exposed to ultraviolet radiation or X-rays. Here, we show for the first time that hackmanites are also radiochromic when exposed to alpha particles, beta particles (positrons) or gamma radiation. Combining experimental and computational data we elucidate the mechanism of gamma-induced radiochromism in hackmanites. We show that hackmanites can be used for gamma dose mapping in high dose applications as well as a memory material that has the one-of-a-kind ability to remember earlier gamma exposure. In addition to satisfying the requirements of sustainability, hackmanites are non-toxic and the films made of hackmanite are reusable thus showing great potential to replace the currently available radiochromic films.

Characterization of GafChromic EBT2 film dose measurements using a tissue-equivalent water phantom for a Theratron® Equinox Cobalt-60 teletherapy machine

Purpose

In vivo dosimetry is a quality assurance tool that provides post-treatment measurement of the absorbed dose as delivered to the patient. This dosimetry compares the prescribed and measured dose delivered to the target volume. In this study, a tissue-equivalent water phantom provided the simulation of the human environment. The skin and entrance doses were measured using GafChromic EBT2 film for a Theratron® Equinox Cobalt-60 teletherapy machine.

Methods

We examined the behaviors of unencapsulated films and custom-made film encapsulation. Films were cut to 1 cm × 1 cm, calibrated, and used to assess skin dose depositions and entrance dose. We examined the response of the film for variations in field size, source to skin distance (SSD), gantry angle and wedge angle.

Results

The estimated uncertainty in EBT2 film for absorbed dose measurement in phantom was ±1.72%. Comparison of the measurements of the two film configurations for the various irradiation parameters were field size (p = 0.0193, α = 0.05, n = 11), gantry angle (p = 0.0018, α = 0.05, n = 24), SSD (p = 0.1802, α = 0.05, n = 11) and wedge angle (p = 0.6834, α = 0.05, n = 4). For a prescribed dose of 200 cGy and at reference conditions (open field 10 cm x 10 cm, SSD = 100 cm, and gantry angle = 0º), the measured skin dose using the encapsulation material was 70% while that measured with the unencapsulated film was 24%. At reference irradiation conditions, the measured skin dose using the unencapsulated film was higher for open field configurations (24%) than wedged field configurations (19%). Estimation of the entrance dose using the unencapsulated film was within 3% of the prescribed dose.

Conclusions

GafChromic EBT2 film measurements were significantly affected at larger field sizes and gantry angles. Furthermore, we determined a high accuracy in entrance dose estimations using the film.

Marker-Free, Molecule Sensitive Mapping of Disturbed Falling Fluid Films Using Raman Imaging

Technical liquid flow films are the basic arrangement for gas fluid transitions of all kinds and are the basis of many chemical processes, such as columns, evaporators, dryers, and different other kinds of fluid/fluid separation units. This publication presents a new method for molecule sensitive, non-contact, and marker-free localized concentration mapping in vertical falling films. Using Raman spectroscopy, no label or marker is needed for the detection of the local composition in liquid mixtures. In the presented cases, the film mapping of sodium sulfate in water on a plain surface as well as an added artificial streaming disruptor with the shape of a small pyramid is scanned in three dimensions. The results show, as a prove of concept, a clear detectable spectroscopic difference between air, back plate, and sodium sulfate for every local point in all three dimensions. In conclusion, contactless Raman scanning on falling films for liquid mapping is realizable without any mechanical film interaction caused by the measuring probe. Surface gloss or optical reflections from a metallic back plate are suppressed by using only inelastic light scattering and the mathematical removal of background noise.

THREE-DIMENSIONAL DOSIMETRY OF MAMMOGRAPHY USING A BLOCK CUBE BREAST PHANTOM AND RADIOCHROMIC FILM

This study developed a phantom with a shape similar to that of the breast and use GAFCHROMIC films that can be placed inside the phantom to measure the detailed breast dose distribution in mammography. GAFCHROMIC EBT3 was placed on the block cube breast phantom and irradiated with a mammography device to measure the absorbed dose distribution inside the phantom in the horizontal and depth directions. The dose distribution in the horizontal plane was the highest in the centre on the chest wall side, and it decreased in a fan shape. Along the depth of the phantom, the doses absorbed across the entire cross-section were 16.15 mGy at the surface and 7.51, 3.25 and 1.68 mGy at depths of 10, 20 and 30 mm, respectively. Compared with the mean glandular dose, the proposed method can measure breast dose distributions in greater detail and is applicable to various breast shapes.

Evaluation of dosimetric functions for a new 169 Yb HDR Brachytherapy Source

169 Yb has been recently used as an HDR brachytherapy source for cancer treatment. In this paper, dosimetric parameters of a new design of 169 Yb HDR brachytherapy source were determined by Monte Carlo (MC) method and film dosimetry. In this new source, the radioactive core has been encapsulated twice for safety purposes. The calculations of dosimetric parameters carried out using MC simulation in water and air phantom. In order to exclude photon contamination's cutoff energy, δ was set at 10 keV. TG-43U1 data dosimetric, including Sk , Λ, g(r), F(r, θ) was computed using outputs from the simulation and their statistical uncertainties were calculated. Dose distribution around the new prototype source in PMMA phantom in the framework of AAPM TG-43 and TG-55 recommendations was measured by Radiochromic film (RCF) Gafchromic EBT3. Obtained air kerma strength, Sk , and the dose rate constant, Λ, from simulation has a value of 1.03U ± 0.03 and 1.21 cGyh-1 U-1  ± 0.03, respectively. The radial dose function was calculated at radial distances between 0.5 and 10 cm with a maximum value of 1.15 ± 0.03 at 5-6 cm distances. The anisotropy functions for radial distances of 0.5-7 cm and angle distances 0° to180° were calculated. The dosimetric data of the new HDR 169 Yb source were compared with another reference source of 169 Yb-HDR and were found that has acceptable compatibility. In addition, the anisotropy function of the MC simulation and film dosimetry method at a distance of 1 cm from this source was obtained and a good agreement was found between the anisotropy results.

USE OF ULTRAVIOLET RAY PRE-IRRADIATION TO IMPROVE THE ACCURACY OF LOW-DOSE MEASUREMENTS OF THE CT USING A GAFCHROMIC RTQA2 FILM

The purpose of this study is to develop a method for use at extremely low-dose ranges and to decrease the uncertainty outside the recommended range of Gafchromic RTQA2 (RTQA2). By this method, the CT dose including the scattered radiation region can be grasped. The base density was increased by ultraviolet (UV)-ray preirradiation. RTQA2 was irradiated with UV-A rays for 26 and 40 h. Subsequently, RTQA2 was exposed to 2, 4, 6, 8, 10, 25, 50, 75, 100, 150, 200 and 250 mGy X-rays using a segmentation method. Calibration curves with and without UV-A irradiation were compared. The calibration curve with 40-h UV-A ray irradiation was the most linear, and a steeper slope area was not observed. The uncertainty in the calibration curve was reduced (p < 0.05). UV-A ray irradiation is an effective method for treating RTQA2; the accuracy in the extremely low-dose range of RTQA2 was improved.

THE CONCEPT OF X-RAY CT DOSE EVALUATION METHOD USING RADIOCHROMIC FILM AND FILM-FOLDING PHANTOM

In this paper, we propose a novel radiochromic film (RCF)-based computed tomography (CT) dosimetry method, which is different from the method based on CT dose index. RCF dosimetry using Gafchromic QA2 films was performed using two lengths of film-folding phantoms. The phantom was exposed to X-ray CT through a single scan, while the RCF was sandwiched between the phantoms. We analysed the dose profile curve in two directions to investigate the dose distribution. We observed a difference in the dose distribution as the phantom size changed. Our results contradict with the results of previous studies such as Monte Carlo simulation or direct measurement. The ability to visually evaluate 2D dose distributions is an advantage of RCF dosimetry over other methods. This research investigated the ability of 2D X-ray CT dose evaluation using RCF and film-folding phantom.

Temperature dependence and temporal stability of stacked radiochromic sheets for three-dimensional dose verification

PURPOSE

Recently a novel radiochromic sheet dosimeter, termed as PRESAGE sheets, consisting of leuco crystal violet dye and radical initiator had been developed and characterized. This study examines the dosimeter's temporal stability and storage temperature dependence postirradiation, and its applicability for dose verification in three dimensions (3D) as a stack dosimeter.

METHODS

PRESAGE sheets were irradiated using 6 MV photons at a dose range of 0-20 Gy with the change in optical density measured using a flatbed scanner. Following their irradiation, PRESAGE sheets were stored in different temperature environments (-18 °C, 4 °C, and 22 °C) and scanned at different time points, ranging from 1 to 168 h postirradiation, to track changes in measured signal and linearity of dose response. Multiple PRESAGE sheets were bound together to create a 12 × 13 × 8.7 cm³ film stack, with EBT3 film inserted between the sheets in the central region of the stack, that was treated using a clinical VMAT plan. Based on the results from the time and storage temperature study, two-dimensional (2D) relative dose distribution measurements in PRESAGE were acquired promptly following irradiation at selected planes in the coronal, sagittal, and axial orientation of the film stack and compared to the treatment planning system calculations in their respective axes. Dose distribution measurements on the coronal axis of the stack dosimeter were also independently verified using EBT3 film.

RESULTS

The dose response was observed to be linear (R² > 0.995) with sheets stored in colder temperatures retaining their signal and dose response sensitivity for extended periods postirradiation. Sheets stored in 22 °C environment should be measured within an hour postirradiation. Sheets stored in a 4 °C and -18 °C environment can be scanned up to 20- and 72 h postirradiation, respectively, while preserving the integrity of their dose response sensitivity and linearity of dose response within a mean absolute percent error of 2.0%. For instance, at 20 h postirradiation the dose response sensitivity for sheets stored in a -18 °C, 4 °C, and 22 °C temperature environment was measured to be 97%, 91%, and 77% of their original values measured within an hour postirradiation, respectively. The 2D gamma pass rate for central slices exceed 95% for PRESAGE film stack compared with treatment planning system on selected planes in the axial, coronal, and sagittal orientation and EBT3 film in the coronal orientation using a 2D gamma index of 2%/2mm. The gamma pass rate in comparing the calculated dose distribution with the measured dose distribution from PRESAGE-LCV was observed to decrease in sheets scanned at later elapsed times postirradiation. In one example, the gamma pass rate for 2%/2mm criteria in the coronal plane was observed to decrease from 97.7% pass rate when scanned within an hour postirradiation to 92.1% pass rate when scanned at 20 h postirradiation under room temperature conditions.

CONCLUSIONS

This is the first study to demonstrate that the temporal stability of PRESAGE sheets can be enhanced through its storage in colder temperature environments postirradiation and that sheets as a film stack dosimeter hold promise for precise relative dose distribution measurements in 3D where advanced optical CT is unavailable.

Estimation of the Effective Dose of Radiation Workers: Optimization Based on the Weight Percentile

Radiation workers might be exposed to polyenergetic photon radiation beams at different directions in their working environments. In this regard, their effective dose (E) should be accurately estimated using a two-dosimeter algorithm (TDA), based on the measurements of two thermoluminescent dosimeters (TLDs) or film badges that are mounted on the front and back of the body. However, considering different human anatomies, radiation workers may have a variety of weight percentiles. This work sought to find whether TDA obtained for the reference weight percentile (50) can be used for higher weight percentiles (including; 65, 75, 85, and 95). MCNPX was used to simulate different weight percentiles on the revised ORNL phantom by adding extra layers of muscle and adipose on the torso. Then front and back TLD responses were calculated for external beam photon energies of 40 keV to 10 MeV in different irradiation geometries. The results revealed that E value declines with increasing the weight percentile. In this study, three TDA were investigated consisting of Eest = 0.73 Rf + 0.53 Rb (73/53), Eest = 0.55 Rf + 0.50 Rb (55/50), and Eest = 0.70 Rf + 0.30 Rb (70/30). The ratio of Eest/E was calculated for each TDA in different energy bins and weight percentiles. Results obtained using the 55/50 and 70/30 showed higher underestimation for most of the energy bins, especially for PA and AP geometries. Compared to these two TDA, the 73/53 algorithm resulted in higher overestimation for RLAT and LLAT geometries for the same energy bins. Variation of the algorithms showed a similar trend for the studied weight percentiles. To conclude, results obtained by TDA for the 50% weight percentile are applicable for weight percentiles >50%.

Comparison of three film analysis softwares using EBT2 and EBT3 films in radiotherapy

Introduction The purpose of the study was to compare the results of gamma value based film analysis according to the used type of self-developer film and software product. Material and methods The films were irradiated with different treatment techniques such as 3D conformal and intensity modulated radiotherapy with static and rotational delivery. Stereotactic plans with conformal and intensity modulated arc techniques, using coplanar and non-coplanar beam setup were also evaluated. The data of irradiated film were compared with the planned planar dose distribution exported from the treatment planning system. Three film analysis software programs were evaluated: PTW Mephysto (PTW), FilmQA Pro (FQP) and radiohromic.com(RC). Both EBT2 and EBT3 types of films were examined. The comparisons of dose distributions were performed with gamma analysis using 10% cut-off level. Results The results of the gamma analysis for larger fields were between 78.3% and 98.3%, 75.7% and 100%, 80.2% and 98.8% with PTW, FQP and RC, respectively. The results of evaluation in case of stereotactic measurements were 76.8%-99.2% for PTW, 95.7%-100% for FQP and 91.2%-99.9% for RC. Conclusions All the three software programs are suitable for calibrating and evaluating films, performing gamma analysis, and can be used for patient specific quality assurance measurements. There is no direct connection between gamma passing rate and absolute accuracy or software quality, it is just a feature of the software. The interpretation of own results has to be defined on an institutional level according to given workflow and preliminary results.

PATIENT-SPECIFIC DOSIMETRY USING IN-HOUSE DEVELOPED OSL DISC DOSEMETERS

Circular discs of diameter 5 mm were made from three indigenously developed optically stimulated luminescent (OSL) phosphors for medical dosimetry. Dosimetric characteristics of these discs were evaluated for their use in machine and patient-specific dosimetry in radiotherapy. Uncertainty in dosimetric measurements using these discs was also estimated, and combined standard uncertainty in measurement of absorbed dose was found to be 3.34%. Characterisation studies indicate that OSL discs are suitable for dosimetric application in radiotherapy. These discs were also used for patient-specific dosimetry in conventional and conformal radiotherapy treatments (five different cases) vis-à-vis ionisation chamber and Gafchromic EBT3 film. Doses measured by OSL discs were found comparable to ionisation chamber and Gafchromic EBT3 film measured values and radiotherapy treatment planning system (TPS) calculated dose values in all the cases. The variation between TPS calculated dose values and OSL discs measured dose values was found within the measurement uncertainty.

SKIN DOSE MAPPING IN INTERVENTIONAL CARDIOLOGY: A PRACTICAL SOLUTION

Numerous cases of radiation-induced tissue reactions following interventional cardiology (IC) procedures have been reported, resulting in the need for an optimized and personalized dosimetry. At present, there are many fluoroscopy units without Digital Imaging and Communications in Medicine (DICOM) Radiation Dose Structured Report globally installed. Many of these have not been updated yet, and may never be, therefore, the main objectives of this paper are to develop an offline skin dose mapping application, which uses DICOM headers for the peak skin dose (PSD) assessment and to compare the PSD assessment results to XR-RV3 Gafchromic film for common IC procedures. The mean deviation between the measured and the calculated PSD was 8.7 ± 26.3%. Simulated skin dose map showed good matching with XR-RV3 Gafchromic film. The skin dose mapping application presented in this paper is an elegant solution and a suitable alternative to XR-RV3 Gafchromic film.

Radiochromic film dosimetry in synchrotron radiation breast computed tomography: a phantom study

This study relates to the INFN project SYRMA-3D for in vivo phase-contrast breast computed tomography using the SYRMEP synchrotron radiation beamline at the ELETTRA facility in Trieste, Italy. This peculiar imaging technique uses a novel dosimetric approach with respect to the standard clinical procedure. In this study, optimization of the acquisition procedure was evaluated in terms of dose delivered to the breast. An offline dose monitoring method was also investigated using radiochromic film dosimetry. Various irradiation geometries have been investigated for scanning the prone patient's pendant breast, simulated by a 14 cm-diameter polymethylmethacrylate cylindrical phantom containing pieces of calibrated radiochromic film type XR-QA2. Films were inserted mid-plane in the phantom, as well as wrapped around its external surface, and irradiated at 38 keV, with an air kerma value that would produce an estimated mean glandular dose of 5 mGy for a 14 cm-diameter 50% glandular breast. Axial scans were performed over a full rotation or over 180°. The results point out that a scheme adopting a stepped rotation irradiation represents the best geometry to optimize the dose distribution to the breast. The feasibility of using a piece of calibrated radiochromic film wrapped around a suitable holder around the breast to monitor the scan dose offline is demonstrated.

[Improvement of Nonuniformity on Flatbed Scanner for Radiochromic Film Dosimetry Using Average Correction Factor with Multi-direction Scan Data]

In order to correct the lateral effect caused by the light source of the flatbed scanner in the Gafchromic film EBT3, the usefulness of the correction method using the average value of the correction coefficient considering the scan directions were evaluated. EBT3 was scanned from four directions to measure the optical density (OD) of the red, blue, and, red/blue components and the correction coefficient were calculated. For the correction coefficients, average values were calculated for the purpose of use, when the scan directions could not be aligned (average lateral effect correction). Correction accuracy was verified with the pass rate of gamma analysis (3 mm/3%, threshold 30%) of the dose distribution using the EBT3 film irradiated with the step pattern. OD of the red, blue, and, red/blue components in the scanning vertical direction tended to be higher in the center than in the peripheral portion. The pass rate of the step pattern was the red component's before correction, from 26.9 to 45.1% (before correction), from 84.1 to 96.7% (after correction), the red/blue component, from 37.6 to 48.4% (before correction) and from 84.4 to 96.7% (after correction). When using the correction coefficient using the average value, the pass rate was 89.8% for the red component and 94.7% for the red/blue component. The lateral effect correction improves the accuracy of the dose distribution verification, and the correction coefficient using the average value is useful when the scanning direction is different from that at the time of obtaining the dose concentration curve.

Development and validation of a comprehensive patient-specific quality assurance program for a novel stereotactic radiation delivery system for breast lesions

PURPOSE

The GammaPod is a dedicated prone breast stereotactic radiosurgery (SRS) machine composed of 25 cobalt-60 sources which rotate around the breast to create highly conformal dose distributions for boosts, partial-breast irradiation, or neo-adjuvant SRS. We describe the development and validation of a patient-specific quality assurance (PSQA) system for the GammaPod.

METHODS

We present two PSQA methods: measurement based and calculation based PSQA. The measurements are performed with a combination of absolute and relative dose measurements. Absolute dosimetry is performed in a single point using a 0.053-cc pinpoint ionization chamber in the center of a polymethylmethacrylate (PMMA) breast phantom and a water-filled breast cup. Relative dose distributions are verified with EBT3 film in the PMMA phantom. The calculation-based method verifies point doses with a novel semi-empirical independent-calculation software.

RESULTS

The average (± standard deviation) breast and target sizes were 1263 ± 335.3 cc and 66.9 ± 29.9 cc, respectively. All ion chamber measurements performed in water and the PMMA phantom agreed with the treatment planning system (TPS) within 2.7%, with average (max) difference of -1.3% (-1.9%) and -1.3% (-2.7%), respectively. Relative dose distributions measured by film showed an average gamma pass rate of 97.0 ± 3.2 when using a 3%/1 mm criteria. The lowest gamma analysis pass rate was 90.0%. The independent calculation software had average agreements (max) with the patient and QA plan calculation of 0.2% (2.2%) and -0.1% (2.0%), respectively.

CONCLUSION

We successfully implemented the first GammaPod PSQA program. These results show that the GammaPod can be used to calculate and deliver the predicted dose precisely and accurately. For routine PSQA performed prior to treatments, the independent calculation is recommended as it verifies the accuracy of the planned dose without increasing the risk of losing vacuum due to prolonged waiting times.

Development of a PMMA phantom as a practical alternative for quality control of gamma knife® dosimetry

BACKGROUND

To measure the absorbed dose rate to water and penumbra of a Gamma Knife® (GK) using a polymethyl metacrylate (PMMA) phantom.

METHODS

A multi-purpose PMMA phantom was developed to measure the absorbed dose rate to water and the dose distribution of a GK. The phantom consists of a hemispherical outer phantom, one exchangeable cylindrical chamber-hosting inner phantom, and two film-hosting inner phantoms. The radius of the phantom was determined considering the electron density of the PMMA such that it corresponds to 8 g/cm2 water depth, which is the reference depth of the absorbed dose measurement of GK. The absorbed dose rate to water was measured with a PTW TN31010 chamber, and the dose distributions were measured with radiochromic films at the calibration center of a patient positioning system of a GK Perfexion. A spherical water-filled phantom with the same water equivalent depth was constructed as a reference phantom. The dose rate to water and dose distributions at the center of a circular field delimited by a 16-mm collimator were measured with the PMMA phantom at six GK Perfexion sites.

RESULTS

The radius of the PMMA phantom was determined to be 6.93 cm, corresponding to equivalent water depth of 8 g/cm2. The absorbed dose rate to water was measured with the PMMA phantom, the spherical water-filled phantom and a commercial solid water phantom. The measured dose rate with the PMMA phantom was 1.2% and 1.8% higher than those measured with the spherical water-filled phantom and the solid water phantom, respectively. These differences can be explained by the scattered photon contribution of PMMA off incoming 60Co gamma-rays to the dose rate. The average full width half maximum and penumbra values measured with the PMMA phantom showed reasonable agreement with two calculated values, one at the center of the PMMA phantom (LGP6.93) and other at the center of a water sphere with a radius of 8 cm (LGP8.0) given by Leksell Gamma Plan using the TMR10 algorithm.

CONCLUSIONS

A PMMA phantom constructed in this study to measure the absorbed dose rates to water and dose distributions of a GK represents an acceptable and practical alternative for GK dosimetry considering its cost-effectiveness and ease of handling.

Investigation of a source model for a new electronic brachytherapy tandem by film measurement

PURPOSE

To investigate the accuracy of a vendor-supplied source model for a new Xoft Axxent 0-degree titanium tandem by film measurement.

METHODS

We measured the anisotropy factors at varying distances and angles from the tandem in water using radiochromic film (Gafchromic EBT3) and an Epson Perfection v750 desktop flatbed scanner (US Epson, Long Beach, CA). A 0-degree tandem was placed vertically in a water phantom. Four pieces of film, each at varying depths, were positioned orthogonal to the longitudinal axis of the tandem for azimuthal anisotropy measurements. Polar anisotropy measurements were taken with the film aligned parallel to the tandem. An absolute dose calibration for the film was verified with a PTW 34013 Soft X-Ray Chamber. The film measurements were analyzed using different color channels. The measured polar anisotropy for varying source positions was compared to the vendor's data. Azimuthal anisotropy was measured as a function of the radius and angle, and normalized to the mean value over all angles at the specified radius.

RESULTS

The azimuthal anisotropy of the tandem and source was found to be consistent for different positions along the tandem's longitudinal axis and at varying distances from the tandem. Absolute dose using a calibrated parallel plate chamber showed agreement to within 2% of expected TPS values. The custom tandem, which has a thicker tip than the wall, was attenuating the 50 kV photons more than expected, at the angles where the photons had more wall material to traverse. This discrepancy was verified at different distances from the tandem and with different measurement techniques. As distance increased, anisotropy values had better agreement.

CONCLUSIONS

We quantified the agreement between the measured and provided anisotropy factors for a new Xoft Axxent 0-degree titanium tandem. Radiochromic film response at low kV energy was also investigated. Our results showed that vendor-supplied TG-43 values were appropriate for clinical use at majority of the angles. A rigorous quality assurance method for new electronic brachytherapy sources and applicators, along with complete knowledge of all dosimetric measuring tools, should be implemented for all parts of the verification and commissioning process.

AN INTERNATIONAL COOPERATION BY USING AN ALL-ENCOMPASSING PASSIVE RADON MONITOR

The recently developed radon film-badge makes it possible to measure radon indoors, in soil, in water and/or in aqueous media (e.g. mud). As a result of its wide response linearity, this monitor has been successfully used to measure radon in-water with concentrations from 10 to ~10 000 Bq/L. By exploiting the unique characteristics of this badge, a mini-survey has been carried out by Health Canada in which radon in water was measured from 12 private wells, as well as in tap water originating from the Ottawa River. Due to the widespread interest of different laboratories in using these passive monitors, laboratories were provided with plastic films to construct their own badges by using in-house CR-39 detectors. Monitors were then irradiated by a known radon concentration at the National Institute of Radiation Metrology (ENEA)'s radon chamber and sent back to each laboratory for processing and counting. Even though these laboratories have been using different etching- and counting-procedures, the film-badge responses varied only within ~12%.

Dosimetric validation of a magnetic resonance image gated radiotherapy system using a motion phantom and radiochromic film

PURPOSE

Magnetic resonance image (MRI) guided radiotherapy enables gating directly on the target position. We present an evaluation of an MRI-guided radiotherapy system's gating performance using an MRI-compatible respiratory motion phantom and radiochromic film. Our evaluation is geared toward validation of our institution's clinical gating protocol which involves planning to a target volume formed by expanding 5 mm about the gross tumor volume (GTV) and gating based on a 3 mm window about the GTV.

METHODS

The motion phantom consisted of a target rod containing high-contrast target inserts which moved in the superior-inferior direction inside a body structure containing background contrast material. The target rod was equipped with a radiochromic film insert. Treatment plans were generated for a 3 cm diameter spherical planning target volume, and delivered to the phantom at rest and in motion with and without gating. Both sinusoidal trajectories and tumor trajectories measured during MRI-guided treatments were used. Similarity of the gated dose distribution to the planned, motion-frozen, distribution was quantified using the gamma technique.

RESULTS

Without gating, gamma pass rates using 4%/3 mm criteria were 22-59% depending on motion trajectory. Using our clinical standard of repeated breath holds and a gating window of 3 mm with 10% target allowed outside the gating boundary, the gamma pass rate was 97.8% with 3%/3 mm gamma criteria. Using a 3 mm window and 10% allowed excursion, all of the patient tumor motion trajectories at actual speed resulting in at least 95% gamma pass rate at 4%/3 mm.

CONCLUSIONS

Our results suggest that the device can be used to compensate respiratory motion using a 3 mm gating margin and 10% allowed excursion results in conjunction with repeated breath holds. Full clinical validation requires a comprehensive evaluation of tracking performance in actual patient images, outside the scope of this study.

Assessment of Patient's Peak Skin Dose Using Gafchromic Films During Interventional Cardiology Procedures: Routine Experience Feedback

To assess the interest of Gafchromic films in detection of patient's peak skin dose (PSD) in interventional cardiology. A prospective study of 112 patients was conducted (July-December 2015). Three diagnostic and therapeutic procedures were evaluated: coronary angiography (CA), coronary angiography and coronary angioplasty for one or two vessels disease (CA-PTCA) and coronary angioplasty of complex chronic total occlusion (CTO). Dosimetric indicators (DIs) were collected and PSD were measured with Gafchromic films. Dose distribution was evaluated within 10 'Thorax Body-zone' defined by the system. Correlations between PSD and DI or dose distribution were computed. Delivered dose increased in complex procedures. The PSD were 0.121 ± 0.063 Gy for CA, 0.256 ± 0.142 Gy for CA-PTCA and 1.116 ± 0.721 Gy for CTO. High correlations were observed for PSD and DI as well for dose distribution within the 'Thorax Body-zone'. Film dosimetry is suggested for CTO procedures since the threshold of 2 Gy for skin injuries is likely to be exceeded.

Estimation of Eye Lens Dose During Brain Scans Using Gafchromic Xr-QA2 Film in Various Multidetector CT Scanners

The purpose of this study was to estimate eye lens dose during brain scans in 16-, 64-, 128- and 256-slice multidetector computed tomography (CT) scanners in helical acquisition mode and to test the feasibility of using radiochromic film as eye lens dosemeter during CT scanning. Eye lens dose measurements were performed using Gafchromic XR-QA2 film on a polystyrene head phantom designed with outer dimensions equivalent to the head size of a reference Indian man. The response accuracy of XR-QA2 film was validated by using thermoluminescence dosemeters. The eye lens dose measured using XR-QA2 film on head phantom for plain brain scanning in helical mode ranged from 43.8 to 45.8 mGy. The XR-QA2 film measured dose values were in agreement with TLD measured dose values within a maximum variation of 8.9%. The good correlation between the two data sets confirms the viability of using XR-QA2 film for eye lens dosimetry.

Local Radiolesion in X-Ray Inspection Specialists

As a result of some deviations in the installation of the X-ray inspection plant 'Extravolt-225/1600' two workers of an X-ray inspection laboratory were exposed to the radiation in a dose enough to cause an acute local radiolesions (LRs). The first patient was diagnosed with an acute LR of the hands of severe and extremely severe degree. The second patient was diagnosed with a mild LR of her right hand. The first patient received a surgical treatment followed by subcutaneous introduction of the autologous mesenchymal stem cells. The second patient received only conservative treatment. The complete epithelization of the traumatic surface was achieved. Modeling the incident with the following EPR analysis of the compact substance of the ablated bone structures made it possible to specify the spatial-temporal properties of the exposure.

Effect of Alanine Energy Response and Phantom Material on Depth Dose Measurements in Ocular Proton Beams

The usefulness of the alanine dosimeter for proton beams has been demonstrated before. In this paper a literature overview is given of the application of alanine in proton beam dosimetry. The energy dependence of the dosimeter has been investigated and Monte Carlo calculations were performed to investigate the influence of this energy dependence on a Bragg curve as well as to explain some phenomena related to the use of alanine in a solid phantom that were reported in the literature.

SKIN DOSE, EFFECTIVE DOSE AND RELATED RISK IN TRANSCATHETER AORTIC VALVE IMPLANTATION (TAVI) PROCEDURES: IS THE CANCER RISK ACCEPTABLE FOR YOUNGER PATIENTS?

The aim of this study was to estimate conversion coefficients for maximum entrance skin dose (MESD) and effective dose (E) for patients undergoing transcatheter aortic valve implantation (TAVI) and to evaluate the risk of exposure-induced cancer death (REID) for prospectively younger patients. Effective doses and risks were estimated for 22 patients using PCXMC whereas MESDs were estimated for a sub-group of 15 patients using Gafchromic film. The estimated conversion coefficients for skin dose [CCS = MESD/dose-area product (DAP)] and E (CCE = E/DAP) were 9.7±1.5 and 0.24±0.02 mSv/Gy cm(2), respectively. The REID ranged from 1:9900 to 1:1400 and by decreasing the age of examination to 40-50 y of age, the REID increased with a factor of 2 for females and 1.5 for males. The organ at risk was the lung. Currently, the patient population is elderly with radiation-induced skin injuries as the main risk. The risk of cancer induction should additionally be considered if younger patient populations are to be treated.

Implementation of full/half bowtie filter models in a commercial treatment planning system for kilovoltage cone-beam CT dose estimations

The purpose of this study was to implement full/half bowtie filter models in a com-mercial treatment planning system (TPS) to calculate kilovoltage (kV) cone-beam CT (CBCT) doses of Varian On-Board Imager (OBI) kV X-ray imaging system. The full/half bowtie filter models were created as compensators in Pinnacle TPS using MATLAB software. The physical profiles of both bowtie filters were imported and hard-coded in the MATLAB system. Pinnacle scripts were written to import bowtie filter models into Pinnacle treatment plans. Bowtie filter-free kV X-ray beam models were commissioned and the bowtie filter models were validated by analyzing the lateral and percent-depth-dose (PDD) profiles of anterior/posterior X-ray beams in water phantoms. A CT dose index (CTDI) phantom was employed to calculate CTDI and weighted CTDI values for pelvis and pelvis-spotlight CBCT protocols. A five-year-old pediatric anthropomorphic phantom was utilized to evaluate absorbed and effective doses (ED) for standard and low-dose head CBCT protocols. The CBCT dose calculation results were compared to ion chamber (IC) and Monte Carlo (MC) data for the CTDI phantom and MOSFET and MC results for the pediatric phantom, respectively. The differences of lateral and PDD profiles between TPS calculations and IC measurements were within 6%. The CTDI and weighted CTDI values of the TPS were respectively within 0.25 cGy and 0.08 cGy compared to IC measurements. The absorbed doses ranged from 0 to 7.22 cGy for the standard dose CBCT and 0 to 1.56 cGy for the low-dose CBCT. The ED values were found to be 36-38 mSv and 7-8 mSv for the standard and low-dose CBCT protocols, respectively. This study demonstrated that the established full/half bowtie filter beam models can produce reasonable dose calculation results. Further study is to be performed to evaluate the models in clinical situations.

Radiation dose levels for conventional chest and abdominal X-ray procedures in elected hospitals in Sudan

This study aimed to assess patient entrance surface air kerma (ESAK) during chest and abdominal X-ray procedures in screen film radiography (SFR) and computed radiography (CR) to establish dose reference levels. Patients' doses were measured in five hospitals for a total of 196 patients. ESAK was calculated from exposure parameters using DosCal software. The X-ray tube output (mGy mAs(-1)), accuracy of exposure factors, linearity and reproducibility were measured using an Unfors Xi dosimeter. The overall mean and range of ESAK during chest X-ray were 0.6 ± 0.3 (0.1-1.3) mGy, while for abdominal X-rays they were 4.0 ± 3.2 (1.3-9.2) mGy. Hospital with a CR system was found to use relatively higher doses. Dose values for abdominal X-ray procedures were comparable with previous studies. The dose for chest X-ray procedure was higher by a factor of 2-3 compared with the current international reference levels.

Evaluation of lens dose from anterior electron beams: comparison of Pinnacle and Gafchromic EBT3 film

Lens dose is a concern during the treatment of facial lesions with anterior electron beams. Lead shielding is routinely employed to reduce lens dose and minimize late complications. The purpose of this work is twofold: 1) to measure dose pro-files under large-area lead shielding at the lens depth for clinical electron energies via film dosimetry; and 2) to assess the accuracy of the Pinnacle treatment planning system in calculating doses under lead shields. First, to simulate the clinical geometry, EBT3 film and 4 cm wide lead shields were incorporated into a Solid Water phantom. With the lead shield inside the phantom, the film was positioned at a depth of 0.7 cm below the lead, while a variable thickness of solid water, simulating bolus, was placed on top. This geometry was reproduced in Pinnacle to calculate dose profiles using the pencil beam electron algorithm. The measured and calculated dose profiles were normalized to the central-axis dose maximum in a homogeneous phantom with no lead shielding. The resulting measured profiles, functions of bolus thickness and incident electron energy, can be used to estimate the lens dose under various clinical scenarios. These profiles showed a minimum lead margin of 0.5 cm beyond the lens boundary is required to shield the lens to ≤ 10% of the dose maximum. Comparisons with Pinnacle showed a consistent overestimation of dose under the lead shield with discrepancies of ~ 25% occur-ring near the shield edge. This discrepancy was found to increase with electron energy and bolus thickness and decrease with distance from the lead edge. Thus, the Pinnacle electron algorithm is not recommended for estimating lens dose in this situation. The film measurements, however, allow for a reasonable estimate of lens dose from electron beams and for clinicians to assess the lead margin required to reduce the lens dose to an acceptable level.

Dose reconstruction for the million worker study: status and guidelines

The primary aim of the epidemiologic study of one million U.S. radiation workers and veterans [the Million Worker Study (MWS)] is to provide scientifically valid information on the level of radiation risk when exposures are received gradually over time and not within seconds, as was the case for Japanese atomic bomb survivors. The primary outcome of the epidemiologic study is cancer mortality, but other causes of death such as cardiovascular disease and cerebrovascular disease will be evaluated. The success of the study is tied to the validity of the dose reconstruction approaches to provide realistic estimates of organ-specific radiation absorbed doses that are as accurate and precise as possible and to properly evaluate their accompanying uncertainties. The dosimetry aspects for the MWS are challenging in that they address diverse exposure scenarios for diverse occupational groups being studied over a period of up to 70 y. The dosimetric issues differ among the varied exposed populations that are considered: atomic veterans, U.S. Department of Energy workers exposed to both penetrating radiation and intakes of radionuclides, nuclear power plant workers, medical radiation workers, and industrial radiographers. While a major source of radiation exposure to the study population comes from external gamma- or x-ray sources, for some of the study groups, there is a meaningful component of radionuclide intakes that requires internal radiation dosimetry assessments. Scientific Committee 6-9 has been established by the National Council on Radiation Protection and Measurements (NCRP) to produce a report on the comprehensive organ dose assessment (including uncertainty analysis) for the MWS. The NCRP dosimetry report will cover the specifics of practical dose reconstruction for the ongoing epidemiologic studies with uncertainty analysis discussions and will be a specific application of the guidance provided in NCRP Report Nos. 158, 163, 164, and 171. The main role of the Committee is to provide guidelines to the various groups of dosimetrists involved in the MWS to ensure that certain dosimetry criteria are considered: calculation of annual absorbed doses in the organs of interest, separation of low and high linear-energy transfer components, evaluation of uncertainties, and quality assurance and quality control. It is recognized that the MWS and its approaches to dosimetry are a work in progress and that there will be flexibility and changes in direction as new information is obtained with regard to both dosimetry and the epidemiologic features of the study components. This paper focuses on the description of the various components of the MWS, the available dosimetry results, and the challenges that have been encountered. It is expected that the Committee will complete its report in 2016.

Patient dose map indications on interventional X-ray systems and validation with Gafchromic XR-RV3 film

To help avoiding secondary effects of interventional procedures like skin damage, a dose map method has been developed to provide an indication of the local dose on a surface representative of individual patient shapes. To minimise user interactions, patient envelope shapes are automatically determined depending on simple patient data information. Local doses are calculated in 1-cm² areas depending on the estimated air kerma, table and gantry positions and system settings, taking into account the table and mattress attenuations and estimated backscatter from the patient. These local doses are cumulated for each location of the patient envelope during the clinical procedure. To assess the accuracy of the method, Gafchromic XR-RV3 films have been used in several operating configurations. Good visual agreements on cumulated dose localisation were obtained within the 1-cm² precision of the map and the dose values agreed within 24.9 % accuracy. The resulting dose map method has been integrated into GE Healthcare X-Ray angiographic systems and should help in the management of the dose by the users during the procedure.

Evaluation of PC-ISO for customized, 3D Printed, gynecologic 192-Ir HDR brachytherapy applicators

The purpose of this study was to evaluate the radiation attenuation properties of PC-ISO, a commercially available, biocompatible, sterilizable 3D printing material, and its suitability for customized, single-use gynecologic (GYN) brachytherapy applicators that have the potential for accurate guiding of seeds through linear and curved internal channels. A custom radiochromic film dosimetry apparatus was 3D-printed in PC-ISO with a single catheter channel and a slit to hold a film segment. The apparatus was designed specifically to test geometry pertinent for use of this material in a clinical setting. A brachytherapy dose plan was computed to deliver a cylindrical dose distribution to the film. The dose plan used an 192Ir source and was normalized to 1500 cGy at 1 cm from the channel. The material was evaluated by comparing the film exposure to an identical test done in water. The Hounsfield unit (HU) distributions were computed from a CT scan of the apparatus and compared to the HU distribution of water and the HU distribution of a commercial GYN cylinder applicator. The dose depth curve of PC-ISO as measured by the radiochromic film was within 1% of water between 1 cm and 6 cm from the channel. The mean HU was -10 for PC-ISO and -1 for water. As expected, the honeycombed structure of the PC-ISO 3D printing process created a moderate spread of HU values, but the mean was comparable to water. PC-ISO is sufficiently water-equivalent to be compatible with our HDR brachytherapy planning system and clinical workflow and, therefore, it is suitable for creating custom GYN brachytherapy applicators. Our current clinical practice includes the use of custom GYN applicators made of commercially available PC-ISO when doing so can improve the patient's treatment. 

Skin dose measurements using radiochromic films, TLDS and ionisation chamber and comparison with Monte Carlo simulation

Estimation of the surface dose is very important for patients undergoing radiation therapy. The purpose of this study is to investigate the dose at the surface of a water phantom at a depth of 0.007 cm as recommended by the International Commission on Radiological Protection and International Commission on Radiation Units and Measurement with radiochromic films (RFs), thermoluminescent dosemeters and an ionisation chamber in a 6-MV photon beam. The results were compared with the theoretical calculation using Monte Carlo (MC) simulation software (MCNP5, BEAMnrc and DOSXYZnrc). The RF was calibrated by placing the films at a depth of maximum dose (d(max)) in a solid water phantom and exposing it to doses from 0 to 500 cGy. The films were scanned using a transmission high-resolution HP scanner. The optical density of the film was obtained from the red component of the RGB images using ImageJ software. The per cent surface dose (PSD) and percentage depth dose (PDD) curve were obtained by placing film pieces at the surface and at different depths in the solid water phantom. TLDs were placed at a depth of 10 cm in a solid water phantom for calibration. Then the TLDs were placed at different depths in the water phantom and were exposed to obtain the PDD. The obtained PSD and PDD values were compared with those obtained using a cylindrical ionisation chamber. The PSD was also determined using Monte Carlo simulation of a LINAC 6-MV photon beam. The extrapolation method was used to determine the PSD for all measurements. The PSD was 15.0±3.6% for RF. The TLD measurement of the PSD was 16.0±5.0%. The (0.6 cm(3)) cylindrical ionisation chamber measurement of the PSD was 50.0±3.0%. The theoretical calculation using MCNP5 and DOSXYZnrc yielded a PSD of 15.0±2.0% and 15.7±2.2%. In this study, good agreement between PSD measurements was observed using RF and TLDs with the Monte Carlo calculation. However, the cylindrical chamber measurement yielded an overestimate of the PSD. This is probably due to the ionisation chamber calibration factor that is only valid in charged particle equilibrium condition, which is not achieved at the surface in the build-up region.

Monte Carlo simulation and film dosimetry for electron therapy in vicinity of a titanium mesh

Titanium (Ti) mesh plates are used as a bone replacement in brain tumor surgeries. In the case of radiotherapy, these plates might interfere with the beam path. The purpose of this study is to evaluate the effect of titanium mesh on the dose distribution of electron fields. Simulations were performed using Monte Carlo BEAMnrc and DOSXYZnrc codes for 6 and 10 MeV electron beams. In Monte Carlo simulation, the shape of the titanium mesh was simulated. The simulated titanium mesh was considered as the one which is used in head and neck surgery with a thickness of 0.055 cm. First, by simulation, the percentage depth dose was obtained while the titanium mesh was present, and these values were then compared with the depth dose of homogeneous phantom with no titanium mesh. In the experimental measurements, the values of depth dose with titanium mesh and without titanium mesh in various depths were measured. The experiments were performed using a RW3 phantom with GAFCHROMIC EBT2 film. The results of experimental measurements were compared with values of depth dose obtained by simulation. In Monte Carlo simulation, as well as experimental measurements, for the voxels immediately beyond the titanium mesh, the change of the dose were evaluated. For this purpose the ratio of the dose for the case with titanium to the case without titanium was calculated as a function of titanium depth. For the voxels before the titanium mesh there was always an increase of the dose up to 13% with respect to the same voxel with no titanium mesh. This is because of the increased back scattering effect of the titanium mesh. The results also showed that for the voxel right beyond the titanium mesh, there is an increased or decreased dose to soft tissues, depending on the depth of the titanium mesh. For the regions before the depth of maximum dose, there is an increase of the dose up to 10% compared to the dose of the same depth in homogeneous phantom. Beyond the depth of maximum dose, there was a 16% decrease in dose. For both 6 and 10 MeV, before the titanium mesh, there was always an increase in dose. If titanium mesh is placed in buildup region, it causes an increase of the dose and could lead to overdose of the adjacent tissue, whereas if titanium mesh is placed beyond the buildup region, it would lead to a decrease in dose compared to the homogenous tissue.

Pilot study for compact microbeam radiation therapy using a carbon nanotube field emission micro-CT scanner

PURPOSE

Microbeam radiation therapy (MRT) is defined as the use of parallel, microplanar x-ray beams with an energy spectrum between 50 and 300 keV for cancer treatment and brain radiosurgery. Up until now, the possibilities of MRT have mainly been studied using synchrotron sources due to their high flux (100s Gy/s) and approximately parallel x-ray paths. The authors have proposed a compact x-ray based MRT system capable of delivering MRT dose distributions at a high dose rate. This system would employ carbon nanotube (CNT) field emission technology to create an x-ray source array that surrounds the target of irradiation. Using such a geometry, multiple collimators would shape the irradiation from this array into multiple microbeams that would then overlap or interlace in the target region. This pilot study demonstrates the feasibility of attaining a high dose rate and parallel microbeam beams using such a system.

METHODS

The microbeam dose distribution was generated by our CNT micro-CT scanner (100 μm focal spot) and a custom-made microbeam collimator. An alignment assembly was fabricated and attached to the scanner in order to collimate and superimpose beams coming from different gantry positions. The MRT dose distribution was measured using two orthogonal radiochromic films embedded inside a cylindrical phantom. This target was irradiated with microbeams incident from 44 different gantry angles to simulate an array of x-ray sources as in the proposed compact CNT-based MRT system. Finally, phantom translation in a direction perpendicular to the microplanar beams was used to simulate the use of multiple parallel microbeams.

RESULTS

Microbeams delivered from 44 gantry angles were superimposed to form a single microbeam dose distribution in the phantom with a FWHM of 300 μm (calculated value was 290 μm). Also, during the multiple beam simulation, a peak to valley dose ratio of ~10 was found when the phantom translation distance was roughly 4x the beam width. The first prototype CNT-based x-ray tube dedicated to the development of compact MRT technology development was proposed and planned based on the preliminary experimental results presented here and the previous corresponding Monte Carlo simulations.

CONCLUSIONS

The authors have demonstrated the feasibility of creating microbeam dose distributions at a high dose rate using a proposed compact MRT system. The flexibility of CNT field emission x-ray sources could possibly bring compact and low cost MRT devices to the larger research community and assist in the translational research of this promising new approach to radiation therapy.

A liquid crystal-based passive badge for personal monitoring of exposure to hydrogen sulfide

A new liquid crystal (LC)-based passive dosimeter badge for personal monitoring of exposure to hydrogen sulfide (H2S) gas is reported. When a thin film of LC supported on a surface functionalized with lead perchlorate Pb(ClO4)2 (the LC sensor) is exposed to H2S, the orientation of LC molecules in the film changes from perpendicular to parallel. This reorientation induces a change in the appearance of the LC film when viewed between crossed polarizers. A H2S dosimeter was fabricated by pairing a LC sensor with a glass substrate forming a headspace between the two surfaces, to control diffusion of H2S across the LC film. When the dosimeter is exposed to H2S, a bright front appears as a function of exposure time. An algorithm has been developed to correlate this response length and exposure dose. The dosimeters are functionally stable when subjected to extreme temperature and humidity fluctuations, and are immune to a number of potentially interfering chemicals, except mercaptans. These dosimeters detect H2S at 0.2 ppm TWA (8 hr) with ±20% overall accuracy. The dosimeters were used to monitor the personal exposure of personnel working in an oil refinery. The TWA concentrations measured by the LC-based dosimeters correlate strongly with the NIOSH 1063 method that uses a sorbent tube and a pump followed by laboratory analysis. Thus, the LC-based dosimeters can provide a sensitive tool for on-site assessment of personal exposure to H2S in different environments.

[A Survey Towards Standardization of Dosimetric Verification in Intensity-modulated Radiation Therapy]

PURPOSE

The purpose of this study was to investigate the status of the implementation of quality assurance (QA) for intensity-modulated radiation therapy (IMRT) in Japan using a questionnaire survey.

METHODS

The questionnaire consisted of seven sections: (1) clinical uses of IMRT, (2) treatment planning systems, treatment machines, phantoms for verification and CT scanning, (3) absorbed dose verification, (4) dose distribution verification, (5) fluence map verification, (6) acceptance criteria for each verification, and (7) comments.

RESULTS

The questionnaire was completed by 129 institutions (response rate: 76.8%). IMRT was performed for prostate cancer in 125 institutions (96.9%), followed by head and neck cancer in 83 (64.3%), and brain tumors in 69 (53.5%). Although at least three individuals were engaged in IMRT QA in 77.5% of the institutions, the number of full-time persons involved in IMRT QA was one or less in 94 institutions (72.9%). This indicated that most institutions in Japan have a staff shortage. More than 90% of the institutions verified both the absorbed dose and dose distribution. The acceptance criterion for the absorbed dose verification was set to ±3% in at least 80% of the institutions. Gafchromic film was used for the majority of dose distribution verifications. The acceptance criteria for dose distribution verification mainly involved gamma analysis and a comparison of dose profiles; however, the judgment of acceptance did not depend on the results of the gamma analysis.

CONCLUSION

This survey increases our understanding of how institutions currently perform IMRT QA analysis. This understanding will help to move institutions toward more standardization of IMRT QA in Japan.

Experimental estimates of peak skin dose and its relationship to the CT dose index using the CTDI head phantom

A straightforward method is presented to estimate peak skin doses (PSDs) delivered by computed tomography (CT) scanners. The measured PSD values are related to the well-known volume CT dose index (CTDI(vol)), displayed on the console of CT scanners. PSD measurement estimates were obtained, in four CT units, by placing radiochromic film on the surface of a CTDI head phantom. Six different X-ray tube currents including the maximum allowed value were used to irradiate the phantom. PSD and CTDI(vol) were independently measured and later related to the CTDI(vol) value displayed on the console. A scanner-specific relationship was found between the measured PSD and the associated CTDI(vol) displayed on the console. The measured PSD values varied between 27 and 136 mGy among all scanners when the routine head scan parameters were used. The results of this work allow relating the widely used CTDI(vol) to an actual radiation dose delivered to the skin of a patient.

Beta response of LiMgPO4:Tb,B based OSL discs for personnel monitoring applications

Properties such as high optically stimulated luminescent (OSL) sensitivity, ease of preparation and dose linearity over nine decades (µGy-kGy) make LiMgPO4:Tb, B (LMP) a unique phosphor for dosimetry applications. This led to the investigation of the beta response of highly sensitive LMP-based Teflon-embedded OSL discs for personnel monitoring applications. A PTB beta secondary standard calibration setup (BSS2), which contains three beta sources viz. (147)Pm, (85)Kr and (90)Sr/(90)Y, was used. The relative response with respect to (137)Cs photons for 0.4-mm thick LMP discs was found to be ∼7.32, ∼53.5 and 100 % for (147)Pm, (85)Kr and (90)Sr/(90)Y beta energies, respectively. The response of LMP discs under various filter combinations viz. 0.18-mm thick mylar (25 mg cm(-2)), 0.625-mm thick Poly-allyl-diglycol carbonate (PADC, 81 mg cm(-2)), 1-mm thick polythene (95 mg cm(-2)), 1-mm thick Perspex (118 mg cm(-2)), 1.25-mm thick PADC (162 mg cm(-2)) and 1.6-mm thick (189 mg cm(-2)) Perspex filters was also studied and the ratio of the response of open disc to the response under filters (DOpen/DFilter) of different thicknesses (mg cm(-2)) was evaluated. Studies were also performed for the mixed field of low- ((85)Kr) and high-energy ((90)Sr/(90)Y) beta particles and the DOpen/DFilter ratio was evaluated. The angular dependence of the response of OSL discs to (85)Kr and (90)Sr/(90)Y beta sources was also studied. Studies were also carried out for (204)Tl, (32)P, natural uranium and (106)Ru/(106)Rh beta sources and the ratios of the response of open disc to that of under 1.6-mm thick Perspex (DOpen/DFilter) filter were measured. A study with various beta sources for the evaluation of the DOpen/DFilter ratio was necessary as these ratios are used to estimate the energy of beta particles and to apply the correction factor while evaluating the beta dose/design of dose estimation algorithms.

The gamma ray response of alanine film dosimeters at low temperatures

The response of alanine film EPR dosimeters was studied for low temperature gamma irradiation conditions (77-293 K) in the dose interval from 6.3 to 80 kGy. It was found that the response of the dosimeter decreases with decreased irradiation temperature and saturates at lower doses for lower irradiation temperatures. The analysis of the EPR signal suggests that the radical species formed at low temperature are the same as those used for dosimetry at room temperature, but with different concentrations. Their concentrations evolve as the temperature of the sample increases until the usual EPR signal used at room temperature is obtained.

Experimental validation of deterministic Acuros XB algorithm for IMRT and VMAT dose calculations with the Radiological Physics Center's head and neck phantom

PURPOSE

The purpose of this study was to verify the dosimetric performance of Acuros XB (AXB), a grid-based Boltzmann solver, in intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT).

METHODS

The Radiological Physics Center (RPC) head and neck (H&N) phantom was used for all calculations and measurements in this study. Clinically equivalent IMRT and VMAT plans were created on the RPC H&N phantom in the Eclipse treatment planning system (version 10.0) by using RPC dose prescription specifications. The dose distributions were calculated with two different algorithms, AXB 11.0.03 and anisotropic analytical algorithm (AAA) 10.0.24. Two dose report modes of AXB were recorded: dose-to-medium in medium (D(m,m)) and dose-to-water in medium (D(w,m)). Each treatment plan was delivered to the RPC phantom three times for reproducibility by using a Varian Clinac iX linear accelerator. Absolute point dose and planar dose were measured with thermoluminescent dosimeters (TLDs) and GafChromic® EBT2 film, respectively. Profile comparison and 2D gamma analysis were used to quantify the agreement between the film measurements and the calculated dose distributions from both AXB and AAA. The computation times for AAA and AXB were also evaluated.

RESULTS

Good agreement was observed between measured doses and those calculated with AAA or AXB. Both AAA and AXB calculated doses within 5% of TLD measurements in both the IMRT and VMAT plans. Results of AXB_D(m,m) (0.1% to 3.6%) were slightly better than AAA (0.2% to 4.6%) or AXB_D(w,m) (0.3% to 5.1%). The gamma analysis for both AAA and AXB met the RPC 7%/4 mm criteria (over 90% passed), whereas AXB_D(m,m) met 5%/3 mm criteria in most cases. AAA was 2 to 3 times faster than AXB for IMRT, whereas AXB was 4-6 times faster than AAA for VMAT.

CONCLUSIONS

AXB was found to be satisfactorily accurate when compared to measurements in the RPC H&N phantom. Compared with AAA, AXB results were equal to or better than those obtained with film measurements for IMRT and VMAT plans. The AXB_D(m,m) reporting mode was found to be closer to TLD and film measurements than was the AXB_D(w,m) mode. AXB calculation time was found to be significantly shorter (× 4) than AAA for VMAT.

[A method for removing moiré artifacts from scanned images of radiochromic films: application to transmissive scanning]

Dose verification using radiochromic films may bring about moiré artifacts during the film scanning process using a flatbed scanner. A method for removing the moiré artifacts was already reported for reflective scanning. Recently, transmissive scanning using EBT2 films has played a major role for dose verification in intensity-modulated radiation therapy. It was found that placing a nonreflecting glass plate on the detector side could eliminate all the moiré artifacts for transmissive scanning.

A study of the relationship between peak skin dose and cumulative air kerma in interventional neuroradiology and cardiology

A study of peak skin doses (PSDs) during neuroradiology and cardiology interventional procedures has been carried out using Gafchromic XR-RV2 film. Use of mosaics made from squares held in cling film has allowed doses to the head to be mapped successfully. The displayed cumulative air kerma (CAK) has been calibrated in terms of cumulative entrance surface dose (CESD) and results indicate that this can provide a reliable indicator of the PSD in neuroradiology. Results linking PSD to CESD for interventional cardiology were variable, but CAK is still considered to provide the best option for use as an indicator of potential radiation-induced effects. A CESD exceeding 3 Gy is considered a suitable action level for triggering follow-up of patients in neuroradiology and cardiology for possible skin effects. Application of dose action levels defined in this way would affect 8% of neurological embolisation procedures and 5% of cardiology ablation and multiple stent procedures at the hospitals where the investigations were carried out. A close relationship was observed between CESD and dose-area product (DAP) for particular types of procedure, and DAPs of 200-300  Gy cm(2) could be used as trigger levels where CAK readings were not available. The DAP value would depend on the mean field size and would need to be determined for each application.

Radiation dose to patients and clinicians during fluoroscopically-guided biopsy of peripheral pulmonary lesions

BACKGROUND

Fluoroscopic guidance may be utilized in some bronchoscopic procedures, including ultrasound-guided bronchoscopy for investigation of peripheral pulmonary lesions. Some authors have suggested this procedure may be performed without fluoroscopy, to minimize risks due to radiation exposure. However, the radiation dose has never been quantified, so the risk remains unknown.

OBJECTIVE

To determine the patient and clinician radiation exposure from fluoroscopy during bronchoscopy.

METHODS

We recorded exposure parameters during 45 consecutive ultrasound bronchoscopies with fluoroscopic guidance with a mobile C-arm fluoroscopy system. We calculated the patient effective radiation dose with Monte Carlo computer simulations. Passive personal film dosimeters were placed on 4 sites on both the proceduralist and the primary nursing assistant.

RESULTS

The mean fluoroscopy screening time was 96 ± 55 s. Patients received a median effective radiation dose of 0.49 ± 0.37 milli-Sieverts (mSv) (range 0.16-1.3 mSv). Only the film dosimeters worn outside the clinicians' protective aprons recorded measurable radiation doses. Based on typical attenuation properties of the protective garments across the diagnostic x-ray energy range, we estimate that the effective radiation dose per procedure to the proceduralist was 0.4 micro-Sieverts (μSv) and to the assistant was 0.2 μSv.

CONCLUSIONS

Patients are exposed to relatively small amounts of radiation from fluoroscopy during bronchoscopy. Clinically indicated fluoroscopic guidance during bronchoscopy should not be precluded on the basis of radiation safety concerns. Adequate shielding of clinicians results in negligible radiation doses during ultrasound bronchoscopy.