Ion Chamber Data Research Articles

Sci—Wed PM: Delivery—11: Dosimetric Properties of an EPID for Real‐Time Dose Verification

PURPOSE: Dosimetric properties of an amorphous‐silicon electronic portal imaging device (EPID) operated in a real‐time acquisition mode were investigated. This mode will be essential for time‐resolved dose verification of dynamic‐IMRT and arc‐IMRT. METHODS: The EPID was used in continuous acquisition mode, where individual sequential image frames are acquired in real‐time. Properties studied include dose linearity and reproducibility. Summed continuous acquisition mode results were also compared to dose results using the well‐studied integrated acquisition mode, for example treatment deliveries including dynamic‐IMRT and single‐arc‐IMRT. Comparison was made using percentage dose difference of in‐field pixels (pixels >10% of maximum signal). Temporally‐resolved EPID response was also compared to that of ion‐chamber data for selected points in the deliveries. RESULTS: Using continuous acquisition mode, EPID response was not linear with dose, with response approximately corresponding to 1–1.5 missed images per irradiation. Reproducibility of EPID response improved with increasing MU. Analysis of the example irradiations revealed summed continuous acquisition mode compared well to integrated acquisition mode, within 2% of maximum dose for more than 95% of in‐field pixels. Time resolved EPID data compared well to ion chamber data, with dose increases/decreases overlying each other. CONCLUSION: Continuous acquisition mode is suited for time‐resolved dosimetry applications including single‐arc‐IMRT and dynamic IMRT, giving comparable dose results to the integrated acquisition mode. Linearity and reproducibility should be adequate for clinical applications although caution should be used in low MU work. Time‐resolved EPID dose information also compared well to time‐resolved ion‐chamber measurements.

Dosimetric properties of an amorphous‐silicon EPID used in continuous acquisition mode for application to dynamic and arc IMRT

Dosimetric properties of an amorphous-silicon electronic portal imaging device (EPID) operated in a real-time acquisition mode were investigated. This mode will be essential for time-resolved dose verification of dynamic (sliding window) intensity modulated radiation therapy (IMRT) and intensity modulated arc radiation therapy (arc-IMRT). The EPID was used in continuous acquisition mode (i.e., "cine" mode) where individual sequential image frames are acquired in real time. The properties studied include dose linearity, reproducibility of response, and image stability. Results of using the continuous acquisition mode with several example treatments including dynamic IMRT, arc treatment, and single-arc-IMRT are compared to results using the well-studied integrated acquisition mode (i.e., "frame averaging" or "IMRT" mode). Real-time EPID response was also compared to real-time ion-chamber data for selected points in the deliveries. The example treatment deliveries in both continuous and integrated acquisition modes were converted to arbitrary EPID dose units via a calibration field. The summation of all acquired continuous mode images was compared using percentage dose difference to the single image acquired in the integrated mode using in-field pixels only (defined as those pixels > 10% of maximum, in-field signal). Using the continuous acquisition mode, the EPID response was not linear with dose. It was found that the continuous mode dose response corresponded approximately to dropping one image per acquisition session. Reproducibility of EPID response to low monitor units (MUs) was found to be poor but greatly improved with increasing MU. Open field profiles were found to be stable in the cross-plane direction but required several frames to become stable in the in-plane direction. However, both of these issues are clinically insignificant due to arc-IMRT deliveries requiring relatively large monitor units (> 100 MU). Analysis of the five IMRT, arc, and arc-IMRT tests revealed that all examples compared to within 2% of maximum dose for more than 95% of in-field pixels. The continuous acquisition mode is suited to time-resolved dosimetry applications including arc-IMRT and dynamic IMRT, giving comparable dose results to the well-studied integrated acquisition mode, although caution should be used in low MU applications. Time-resolved EPID dose information also compared well to time-resolved ion-chamber measurements.

SU‐FF‐T‐443: Measurement of Lateral Dose Distributions Using GafChromic EBT Films and PTW Starcheck 2‐D Array

Purpose: One‐dimensional in‐air scans have been proposed for accurate Monte Carlo commissioning of clinically linac beams. The purpose of this work was to extend the measurement of such electron and photon dose distributions to two dimensions, in order to confirm the circular symmetry assumption of the 1‐D case and investigate differences previously reported between ion chamber scans and EGSnrc calculations. Method and Materials: Two systems were investigated ‐ GafChromic EBT film and the PTW Stacheck device ‐ and dose profiles were obtained for electron and photon beams from a Vickers research accelerator. A range of scattering foils (electron beams) and target materials (photon beams) were investigated for a 20 MeV beam. Prior to the investigation, both systems were accurately commissioned in beams from an Elekta Precise clinical linac. All results were compared with previously published 1‐D ion chamber and Monte Carlo data. Results: The EBT film measurements confirmed the circular symmetry of both electron and photon beams for all scatter foils and targets. Electron relative dose profiles measured using film were consistently narrower than Monte Carlo simulations, while ion chamber scans were broader than simulation. For X‐rays, relative dose profiles measured using film generally agree better with Monte Carlo than ion chamber profiles. X‐ray profiles measured by the Starcheck also show better agreement with Monte Carlo than 1‐D ion chamber scan. In comparing the two 2‐D systems, EBT film shows a lower uncertainty and smoother uniformity response than the Starcheck. Absolute X‐ray dose measurements using film and Starcheck showed significant target‐dependent differences of up to 5%. Conclusion: Both EBT film and Starcheck can be used to obtain relative dose profiles with uncertainties at the 2% level. The systematic difference between absolute dose measurement with the two systems suggests that the Starcheck is more sensitive to changes in X‐ray spectra.

Dosimetric measurements of a new electronic brachytherapy surface applicator

Dosimetric measurements of a new electronic brachytherapy surface applicator

Simulating oblique incident irradiation using the BEAMnrc Monte Carlo code

A new source for the simulation of oblique incident irradiation has been developed for the BEAMnrc Monte Carlo code. In this work, we describe a method for the simulation of any component that is rotated at some angle relative to the central axis of the modelled radiation unit. The performance of the new BEAMnrc source was validated against experimental measurements. The comparison with ion chamber data showed very good agreement between experiments and calculation for a number of oblique irradiation angles ranging from 0° to 30°. The routine was also cross-validated, in geometrically equivalent conditions, against a different radiation source available in the DOSXYZnrc code. The test showed excellent consistency between the two routines. The new radiation source can be particularly useful for the Monte Carlo simulation of radiation units in which the radiation beam is tilted with respect to the unit's central axis. To highlight this, a modern cone-beam CT unit is modelled using this new source and validated against measurement.

Use of a two‐dimensional ionization chamber array for proton therapy beam quality assurance

Two-dimensional ion chamber arrays are primarily used for conventional and intensity modulated radiotherapy quality assurance. There is no commercial device of such type available on the market that is offered for proton therapy quality assurance. We have investigated suitability of the MatriXX, a commercial two-dimensional ion chamber array detector for proton therapy QA. This device is designed to be used for photon and electron therapy QA. The device is equipped with 32 x 32 parallel plate ion chambers, each with 4.5 mm diam and 7.62 mm center-to-center separation. A 250 MeV proton beam was used to calibrate the dose measured by this device. The water equivalent thickness of the buildup material was determined to be 3.9 mm using a 160 MeV proton beam. Proton beams of different energies were used to measure the reproducibility of dose output and to evaluate the consistency in the beam flatness and symmetry measured by MatriXX. The output measurement results were compared with the clinical commissioning beam data that were obtained using a 0.6 cc Farmer chamber. The agreement was consistently found to be within 1%. The profiles were compared with film dosimetry and also with ion chamber data in water with an excellent agreement. The device is found to be well suited for quality assurance of proton therapy beams. It provides fast two-dimensional dose distribution information in real time with the accuracy comparable to that of ion chamber measurements and film dosimetry.

Sci‐Fri PM: Planning‐07: A low diffusion radiochromic gel dosimeter for three‐dimensional radiation dosimetry

To develop a low diffusion radiochromic leuco crystal violet (LCV) hydrogel utilizing micelles, for three-dimensional (3-D) radiation dosimetry. Concentrations of LCV dye, Triton X-100 and trichloroacetic acid were varied to determine the optimal gel sensitivity for optical computed tomography (CT). Using a laser optical CT scanner at λ = 594 nm, diffusion rate measurements were performed on half-irradiated (6 MV x-rays) cuvette gel samples made with and without surfactant, respectively. A cylindrical 1 L gel volume was irradiated with a 12 MeV electron beam (Varian Clinac 2100C) to a dose of 30 Gy and scanned with cone- beam optical CT at λ ∼ 590 nm (Vista™, Modus Medical Devices Inc.). The most radiation sensitive gel formulation was found to be: 1 mM LCV, 4 mM Triton X-100, 30 mM trichloroacetic acid and 4% gelatin. The diffusion rates of a LCV gel without and with surfactant present were about 2 and 20 times lower than the Fricke xylenol-orange gel system, respectively. Comparison of the central axis gel attenuation coefficients normalized at depth of maximum dose (dmax ) with TG21-corrected ion chamber data, were in agreement, thus, indicating energy and dose-rate independence. Radiochromic LCV micelle gels show minimal diffusion effects and a dose response that is linear, energy and dose-rate independent. Optical CT scanned LCV micelle gels are a promising system for 3-D dose verification. Two of the authors (JB, KJ) have a licensing agreement with Modus Medical Devices Inc. concerning the commercialization of Vista™.

TH‐C‐350‐05: Performance of a Beam Tracking System for Treatment of Moving Targets with Scanned Ion Beams

Purpose: Scanned ion beam therapy of intra‐fractionally moving targets requires motion mitigation to prevent deteriorations of the delivered doses due to interplay. A beam tracking system was developed and experiments with various detectors were performed to assess system performance. Method and Materials: Tracking of moving targets with scanned ion beams requires simultaneous lateral and longitudinal range adaptation of pencil beam positions. Lateral beam tracking is achieved directly with the beam scanning system. Longitudinal tracking is performed with a dedicated energy modulation system consisting of fast, motorized absorber wedges. In experiments with radiographic films, a range telescope, and an array of pin‐point ionization chambers in a water phantom, the precision of lateral, longitudinal, and 3D motion compensation was measured. Motion was induced laterally by a sliding table and longitudinally by ramp‐shaped absorbers. Motion detection was performed with a laser‐triangulation sensor. Experimental tracking data were compared to results of measurements with stationary detector systems. Films were analyzed with respect to geometrical shape as well as mean, minimum, and maximum response level within the target region. Ionization chamber data were analyzed for different volumes of the setup. Analysis of range telescope data focused on Bragg‐peak shapes and ranges. Results: Film analysis yielded conservation of geometrical shape and responses (mean±SD, minimum, maximum) of (0.276±0.010, 0.241, 0.288) and (0.275±0.007, 0.252, 0.284) for stationary irradiation and tracking respectively. Within the target volume, ionization chamber data with beam tracking deviated from stationary irradiation by 0.3 ± 1.5% with −2.7% minimum and 3.7% maximum deviation. The Bragg peak range for tracking deviated by less than 0.25 mm water‐equivalent from the stationary reference. Conclusion: The tracking system for scanned ion beams successfully restored dose distributions for moving targets by adapting individual Bragg peaks during irradiations in quasi real time.Research sponsored in part by Siemens Healthcare, Particle Therapy.

SU-GG-T-190: In-Vivo Surface Dosimetry With An Optically Stimulated Luminescence Dosimeter

Purpose: We investigate the use of a new optically stimulated luminescencedosimeter (OSLD) for surface dosimetry.Method and Materials: OSLDs are small plastic disks (5mm diameter × 0.2mm thick) infused with aluminum oxide doped with carbon ( Al 2 O 3 : C ) . Their operation is similar to TLDs; the irradiated crystals yield a signal proportional to absorbed dose upon optical stimulation. The detectors are housed in a light‐tight plastic casing measuring 24 × 10 × 1‐mm. A dose‐response curve for 6MV photons from Varian 21Ex was established. OSLDs were next used to measure surface/buildup doses in a plastic water phantom. Three field sizes were considered: 10×10cm, 5×5cm and 2×2cm and results compared with a parallel plate ion chamber. Clinical performance of OSLD was next evaluated by measuring surface doses in head and neck patients undergoing 3DCRT/IMRT. All measurements were repeated with a MOSFETdosimeter currently used in our clinic. Results: The detectors are easy to use, require no preparation/annealing and can be read 10 minutes post‐irradiation. The effective depth of measurement for OSLD was found to be 0.4 mm inside the plastic casing. Surface/buildup region doses with OSLD were in excellent agreement with ion chamber data. When these detectors were used to measure patient surface dose, they compared favorably with MOSFET detectors. Due to smaller intrinsic buildup, OSLD measured surface doses were slightly lower with a mean dose ratio of 0.968 ± 0.011. Conclusion: A new commercially available Al 2 O 3 detector from Landauer is a convenient tool for measuring patient surface dosimetry. Detector performance compares well with existing dosimeters in radiotherapy. Unlike other dosimeters, however, the new detector shows no field size, energy or angular dependence.

SU‐GG‐T‐253: Interface Dosimetry in Heterogeneous Water Phantom Using EBT Film

Purpose: Interface dosimetry measurements in heterogeneous phantoms are subject to additional uncertainties when using solid materials due to the presence of air gaps between the materials and the detector. The use of solid materials also limits the location of dosimeters. In this study, we investigate performing interface dosimetry using EBT film in a liquid water phantom with lung‐equivalent media inside the phantom. Method and Materials: A 6 MV 10×10 cm2 static field was measured at 90 cm SSD with an ionization chamber and EBT film placed in water. Then, interface measurements were made using EBT film or an ion chamber for a 6 MV beam of a 3×3 cm2 static field at 95 cm SSD. The heterogenous phantom consisted of a 4×4×10 cm3 lung equivalent material suspended 5 cm below the water surface in 40×40×40 cm3 water phantom. The phantom was CT scanned and the images were used for dose calculations. EBT film was placed perpendicular to the central axis at multiple depths and in a parallel orientation at central axis and at ±1 cm off axis. The film extended from the water surface to 12.5 cm depth. Data were averaged over multiple measurements. Film data were compared to Monte Carlo simulations and convolution/superposition calculations. Results: EBT film data obtained in both orientations in water agree, to within 2%, with ion chamber data. In the heterogeneous water phantom, a 3% agreement with Monte Carlo simulations is achieved. Large deviations with convolution and superposition calculations was observed. Conclusion: EBT film is a reliable dosimeter in water with and without the presence of heterogeneity when the film is irradiated parallel or perpendicular to central axis. The film can be used in a variety of phantoms to obtain more reliable measurements for commissioning of sophisticated dose calculations algorithms.

SU‐GG‐T‐263: IMRT Beamlet Match‐Line Dose Heterogeneities Associated with Varian 120 MLCs

Purpose: To measure the dosimetric consequences of a range of MLC leaf offsets, examining effects of leaf tip transmission and “scallop edge” overshine from rounded Varian MLC leaves. In particular, this investigation focuses on how these effects influence the dose heterogeneity of the match‐line between abutted beamlet fields that are often present in step‐and‐shoot IMRT. Method and Materials: Offset values from 0.0 to 1.0 mm were evaluated. For each offset, two abutted 2×2cm2 MLC‐shaped fields were delivered sequentially using 6 MV photons from a Varian Platinum EX accelerator with a 120 leaf MLC. Four different techniques—Kodak EDR‐2 film, GAFChromic EBT film, CC01 Wellhofer ion chamber (1 mm radius), and 1 mm3 LiF TLD chips—were used to measure delivered dose at depths of 5 to 15 cm. Film data was taken at 100 SAD. TLD and ion chamber data were collected at 400 cm SSD to increase their effective resolution. Results: Increasing the offset from 0.0 to 0.5 mm decreased the average overdose from 30 to 10% in the abutment region. For offsets greater than 0.5 mm, dose at the center of the match line decreased, but dose inhomogeneities became more pronounced due to effects of overshine, scatter, and leaf tip penumbra. At larger offsets, both over‐ and underdoses of 5 to 10% were present in the abutment region. Conclusion: The measured dose distributions suggest that an optimum offset provides, at best, a balance of over‐ and underdosing in abutment regions. Our data show that the choice of offset significantly changes the dose distribution in a region approximately 6 mm wide at the match line. The width of this high‐gradient area is sufficient to influence clinical IMRT doses delivered.

SU‐GG‐T‐178: End Effect Influence On Segment Precision

Purpose: To study the influence of end effect on segment precision as a guide to minimum MU per segment guidelines for IMRT. Method and Materials: Two methods of end effect measurement (linear curve fit and split exposure method) were correlated with measured variation in segment dose ranging from 0.5 to 128 MU per segment. End effect measurements were based on ionization chamber data. Segment dose variations were based on MapCheck results from a segmented field consisting of 4 equal segments, each exposing a quadrant of a 10×10 field. Results: There was considerable variation in the value of end effect determined by the split exposure technique depending on the value of the single exposure and the number of fractions into which it was split, i.e. α=((Rn‐R1)/(nR1−Rn))t was not consistent as t and n were varied. The segmented exposures tended to show overdose in the first and third quadrant and under dose in the second and fourth quadrant, again with some inconsistency. Variations as high as 40% dose difference in a segment compared to a non‐segmented field were noted. Maximum deviations were equivalent to 0.4 MU. Conclusion: The results suggest a reaction time equivalent to about 0.4 MU. This represents the time between reaching a desired dose and a circuit response. This is consistent with the variation in both segment precision and end effect measurements. To make 0.4 MU insignificant requires a high number of MU/segment and other considerations such as the percentage of dose delivered by a segment need to be included in developing final MU/segment guidelines.

WE-D-AUD B-04: Dosimetric Evaluation of a Delivery Verification and Dose Reconstruction Method for Helical Tomotherapy

Purpose: To determine the dosimetric accuracy of a dose reconstruction method used for verification of helical tomotherapy delivery for four different clinical sites. Method and Materials: A delivery verification and dose reconstruction method has been applied to helical tomotherapy treatment plans of four different treatment sites (prostate, head and neck, lung and total scalp). Treatment plans were generated on a cylindrical measurement phantom (TomoPhantom) using contours, prescriptions and planning objectives taken from clinical patient plans from the four sites. Several film and ion chambermeasurements were made for each plan to simulate the dose variation with multiple deliveries. Additional measurements mere made with intentional changes made to the machine output and leaf open times. A TomoTherapy delivery verification tool uses pulse‐by‐pulse machine CT detector and transmission ion chamber data, extracted at the conclusion of each delivery, to determine the incident energy fluence delivered for each projection. Dose reconstruction with the delivered energy fluence was calculated on the planning CT and compared with both the measured dose distributions and those calculated from the original treatment plan. Differences were compared using dose difference plots. Results:Measured(ion chamber) dose variations in the center of the PTV for repeated daily deliveries of a given treatment plan were small, typically within 2%. Greatest differences in the measured doses occurred with intentional changes in leaf open times. The measured dose variations were well predicted by the dose reconstruction method, which demonstrated agreement with the measured dose within 2%. The dose reconstruction method also demonstrated acceptable agreement with the film dose measurements, for each of the clinical plans. Conclusion: Variations in treatment delivery due to machine output or leaf opening time changes can be determined using the investigated method. Conflict of Interest: Supported in part by a research agreement with TomoTherapy, Inc.

Testing the portal imager GLAaS algorithm for machine quality assurance

BackgroundTo report about enhancements introduced in the GLAaS calibration method to convert raw portal imaging images into absolute dose matrices and to report about application of GLAaS to routine radiation tests for linac quality assurance procedures programmes.MethodsTwo characteristic effects limiting the general applicability of portal imaging based dosimetry are the over-flattening of images (eliminating the "horns" and "holes" in the beam profiles induced by the presence of flattening filters) and the excess of backscattered radiation originated by the detector robotic arm supports. These two effects were corrected for in the new version of GLAaS formalism and results are presented to prove the improvements for different beams, detectors and support arms. GLAaS was also tested for independence from dose rate (fundamental to measure dynamic wedges).With the new corrections, it is possible to use GLAaS to perform standard tasks of linac quality assurance. Data were acquired to analyse open and wedged fields (mechanical and dynamic) in terms of output factors, MU/Gy, wedge factors, profile penumbrae, symmetry and homogeneity. In addition also 2D Gamma Evaluation was applied to measurement to expand the standard QA methods. GLAaS based data were compared against calculations on the treatment planning system (the Varian Eclipse) and against ion chamber measurements as consolidated benchmark. Measurements were performed mostly on 6 MV beams from Varian linacs. Detectors were the PV-as500/IAS2 and the PV-as1000/IAS3 equipped with either the robotic R- or Exact- arms.ResultsCorrections for flattening filter and arm backscattering were successfully tested. Percentage difference between PV-GLAaS measurements and Eclipse calculations relative doses at the 80% of the field size, for square and rectangular fields larger than 5 × 5 cm2 showed a maximum range variation of -1.4%, + 1.7% with a mean variation of <0.5%. For output factors, average percentage difference between GLAaS and Eclipse (or ion chamber) data was -0.4 ± 0.7 (-0.2 ± 0.4) respectively on square fields. Minimum, maximum and average percentage difference between GLAaS and Eclipse (or ion chamber) data in the flattened field region were: 0.1 ± 1.0, 0.7 ± 0.8, 0.1 ± 0.4 (1.0 ± 1.4, -0.3 ± 0.2, -0.1 ± 0.2) respectively. Similar minimal deviations were observed for flatness and symmetry.For Dynamic wedges, percentage difference of MU/Gy between GLAaS and Eclipse (or ion chamber) was: -1.1 ± 1.6 (0.4 ± 0.7). Minimum, maximum and average percentage difference between GLAaS and Eclipse (or ion chamber) data in the flattened field region were: 0.4 ± 1.6, -1.5 ± 1.8, -0.1 ± 0.3 (-2.2 ± 2.3, 2.3 ± 1.2, 0.8 ± 0.3) respectively.For mechanical wedges differences of transmission factors were <1.6% (Eclipse) and <1.1% (ion chamber) for all wedges. Minimum, maximum and average percentage difference between GLAaS and Eclipse (or ion chamber) data in the flattened field region were: -1.3 ± 0.7, -0.7 ± 0.7, -0.2 ± 0.2 (-0.8 ± 0.8, 0.7 ± 1.1, 0.2 ± 0.3) respectively.ConclusionGLAaS includes now efficient methods to correct for missing "horns" and "holes" induced by flattening filter in the beam and to compensate for excessive backscattering from the support arm. These enhancements allowed to use GLAaS based dosimetric measurement to perform standard tasks of Linac quality assurance with reliable and consistent results. This fast method could be applied to routine practice being also fast in usage and because it allows the introduction of new analysis tools in routine QA by means, e.g., of the Gamma Index analysis.

Optimal sensitometric curves of Kodak EDR2 film for dynamic intensity modulated radiation therapy verification

Purpose:To investigate the optimal sensitometric curves of extended dose range (EDR2) radiographic film in terms of depth, field size, dose range and processing conditions for dynamic intensity modulated radiation therapy (IMRT) dosimetry verification with 6 MV X-ray beams.Materials and methods:A Varian Clinac 23 EX linear accelerator with 6 MV X-ray beam was used to study the response of Kodak EDR2 film. Measurements were performed at depths of 5, 10 and 15 cm in MedTec virtual water phantom and with field sizes of 2x2, 3x3, 10x10 and 15x15 cm2. Doses ranging from 20 to 450 cGy were used. The film was developed with the Kodak RP X-OMAT Model M6B automatic film processor. Film response was measured with the Vidar model VXR-16 scanner. Sensitometric curves were applied to the dose profiles measured with film at 5 cm in the virtual water phantom with field sizes of 2x2 and 10x10 cm2 and compared with ion chamber data. Scanditronix/Wellhofer OmniProTM IMRT software was used for the evaluation of the IMRT plan calculated by Eclipse treatment planning.Results:Investigation of the reproducibility and accuracy of the film responses, which depend mainly on the film processor, was carried out by irradiating one film nine times with doses of 20 to 450 cGy. A maximum standard deviation of 4.9% was found which decreased to 1.9% for doses between 20 and 200 cGy. The sensitometric curves for various field sizes at fixed depth showed a maximum difference of 4.2% between 2x2 and 15x15 cm2 at 5 cm depth with a dose of 450 cGy. The shallow depth tended to show a greater effect of field size responses than the deeper depths. The sensitometric curves for various depths at fixed field size showed slightly different film responses; the difference due to depth was within 1.8% for all field sizes studied. Both field size and depth effect were reduced when the doses were lower than 450 cGy. The difference was within 2.5% in the dose range from 20 to 300 cGy for all field sizes and depths studied. Dose profiles measured with EDR2 film were consistent with those measured with an ion chamber. The optimal sensitometric curve was acquired by irradiating film at a depth of 5 cm with doses ranging from 20 to 450 cGy with a 3×3 cm2 multileaf collimator. The optimal sensitometric curve allowed accurate determination of the absolute dose distribution. In almost 200 cases of dynamic IMRT plan verification with EDR2 film, the difference between measured and calculated dose was generally less than 3% and with 3 mm distance to agreement when using gamma value verification.Conclusion:EDR2 film can be used for accurate verification of composite isodose distributions of dynamic IMRT when the optimal sensitometric curve has been established.

Long‐term changes in the cosmic ray intensity at Earth, 1428–2005

The instrumental cosmic ray data recorded in the interval 1933–1965 by S. E. Forbush and H. V. Neher and cosmogenic 10Be data are merged with the neutron monitor data since 1951 to study the long‐ and short‐term variations in the galactic cosmic radiation intensity for the interval 1428–2005. It is shown that the ionization chamber data published by S. E. Forbush were deliberately detrended to remove long‐term changes. The high‐altitude ionization chambers used by H. V. Neher during this interval were well calibrated and their data exhibit a substantial decrease between 1933 and 1954 that is consistent with the long‐term trends in the cosmogenic 10Be data. Using the specific yield functions appropriate to neutron monitors, ionization chambers, and 10Be, the nonlinear relationships between these data types are determined. It is shown that the nonlinearities are large and will introduce serious errors if ignored. An intercalibrated record (the “pseudo‐Climax neutron monitor record”) is developed for the interval 1428–2005. It is used to study several features of the long‐term periodicities in the cosmic radiation, after discussion of residual effects due to meteorological effects, and the production of 10Be by solar cosmic rays. It is shown that (1) the average intensity in the neutron monitor energy range for the interval 1954–1996 is ∼16% less than the average for the period 1428–1944 and that it shows a consistency and depth of modulation that had not occurred in the previous 580 years. (2) The residual cosmic ray modulation was low throughout the Gleissberg cycle 1540–1645, considerably higher for the next two Gleissberg Cycles, and highest of all since 1944. (3) The cosmogenic data imply that solar activity was anomalously low throughout the whole interval 1428–1715, the amplitude of the solar activity during the Gleissberg cycle 1540–1645 being ∼50% of that during the following two Gleissberg cycles and ∼25% of that in the post‐1954 era. (4) It is proposed that the steadily increasing cosmic ray modulation since 1428 constitutes a quarter cycle of the previously identified 2300 year periodicity in the cosmogenic data. (5) The cosmic ray intensity decreased in two steps between 1889 and 1901 and 1944 and 1954, in broad agreement with the two‐step increase in heliomagnetic field strength determined by Schrijver et al. (2002). It is proposed that the “pseudo‐Climax neutron record” will be of benefit in the normalization of other cosmogenic records to the neutron monitor record starting in 1951.

SU‐FF‐T‐62: Alternative IMRT QA Technique During Prostate Patient Treatment

Purpose: The objective of this study is to propose an alternative QA technique that addresses concerns of conventional IMRT QA. Methods and Materials: During IMRT patient treatments, delivered doses were measured using film and MOSFET detectors. Film was placed under the patient sandwiched between the patient and 1‐cm bolus; MOSFET detectors were placed on the anterior skin near the central‐axis under 0.5‐cm bolus. IMRT QA was also performed by exporting the patient plan into a 30×30×30 cm3 cubic phantom. In addition to the central axis dose, relative dose distributions were measured using XV films placed near the anterior and posterior surface of the QA phantom. Results: The results of in‐vivo measurement with film placed under the patient indicate good agreement between planned and measured doses with gamma index of 0.8. Anterior surface doses measured with MOSFET detectors however deviated from planning dose around 10–15%. For comparison, the same patient plan was also exported into the phantom for IMRT QA. The ion chamber measurement at the phantom's center showed good agreement with TPS (measured dose = 197±1 cGy). Results of surface doses measured with film in phantom agree with planned doses. This is consistent with in‐vivo measurement of dose distribution during patient treatment. The surface dose measured with film placed under the patient samples doses at different depths from various beams. The quality of agreement seen with films is consistent with the ion chamber data at the central axis. Thus, film based surface dose measurement is a good manifestation of central axis dose. Conclusion: An in‐vivo IMRT dose measurement is proposed as an alternative QA procedure. Based upon our measured data, proposed method is an accurate and reliable verification of patient dose. The technique is quick, efficient, and directly measures actual dose received by patient during IMRT delivery.

SU-FF-T-356: Preliminary Investigations of a Leucomalachite Green Hydrogel for Three-Dimensional Radiation Dosimetry

Purpose: To develop a radiochromic hydrogel dosimeter based on the conversion reaction of leuco‐triphenyl‐methane dye to coloured dye. Method and Materials: A systematic examination of surfactants revealed that Triton X‐100 provided adequate solubility to keep leucomalachite green dissolved in 4% gelatin hydrogels. The gel dose response was increased by lowering the pH and by adding trichloroacetic acid. A cylindrical one litre volume sample was irradiated with a 12 MeV electron beam (Varian Clinac 2100C) to a dose of 30 Gy and scanned with a commercial optical cone beam CT scanner (Vista™, Modus Medical Devices Inc.). Results: The most radiation sensitive gel formulation contained: 4% gelatin, 6 mM Triton X‐100, 15mM trichloroacetic acid and 0.1 mM leucomalachite green. A dose response of 0.005 cm−1Gy−1 was determined from irradiation of gel‐filled 1 cm pathlength polymethymethacrylate cuvettes and measurement of the optical attenuation improved at a wavelength equal to 633 nm. Comparison of the central axis gelattenuation coefficients normalized at depth of maximum dose (dmax) with TG21‐corrected ion chamber data, demonstrated agreement in the buildup region up to dmax. However, the gel over‐responded at greater depths. Conclusion: Leuco dyes can be dissolved to provide transparent, colourless hydrogels by employing surfactants such as Triton X‐100. The current gel formulation has a dose response approximately 20 times lower than the ferrous xylenol orange system. It is suggested that photochemistry arising from the UV light generated by the Triton X‐100 scintillation fluorescence and the Cerenkov process are possible causes of a radiochromic artifact measured with electron beams. Conflict of Interest: Two of the authors (KJ, JB) have a licensing agreement with Modus Medical Devices Inc. concerning the commercialization of Vista™.

SU‐FF‐T‐97: Characterization of Amorphous Silicon Flat Panel Imager for Dose Measurements

Purpose: The major objective of the work was to categorize the dosimetric response of a Varian aS500 type Electronic Portal Imaging Device (EPID) for dose measurements. Method and Materials: For a 6MV beam from a Varian 21EX linear accelerator, reproducibility of EPID pixel response, dose rate effect, and cumulative dose linearity were studied. In addition, scatter characteristics and depth doses were also determined using the EPID. A Capintec ion chamber in a polystyrene miniphantom and a Wellhöfer water phantom were used for comparative measurements. Results: The EPID response was found to be independent of dose rates from 100 MU/min to 600 MU/min at larger source to detector distances. However, pixel saturation was observed for dose rates beyond 300 MU/min with the EPID placed at the isocentre. Raw pixel response was reproducible to within 2 % when measured over a period of 6 weeks. Changes in scatter conditions with field size were also measured for the EPID. The EPID scatter was found to be in between 5 to 10 cm of a full water phantom. The gray scale response is linear with the dose to the portal imager. The validity of inverse square relationship was also tested to be within 0.5%, for the EPID at various distances from the source. Conclusions: The depth dose data measured with EPID showed good correspondence with that measured with a thimble type ionization chamber in a water phantom. Compared to ionization chamber data, open beam profiles measured with EPID show flat response in high dose region, rapid fall‐off in high dose gradients (&gt; 30%/cm), and higher penumbra tail. The agreement with the ion chamber measurements could be improved if the flood field correction is performed to restore off‐axis variation of beam profile. In general the EPID qualifies as a useful tool for planar dose measurements.

SU-FF-T-355: Preliminary Evaluation of Optically Stimulated Luminescence (OSL) Dosimetry for Small Field High Energy Photon Beam

Purpose: Small field high energy photon beams, frequently used in stereotactic radiosurgery and intensity modulated radiation therapy, are characterized by steep dose gradient and lack of lateral electronic equilibrium. Small field dosimetry requires special detectors, such as diamond detector, micro-ionization chambers, or diode detectors. In the present study the feasibility of using Optically Stimulated Luminescence (OSL) dosimetry in small field 6MV photon beam was investigated. Methods and Materials: The performance of the OSL in this study was evaluated using absolute and depth dose measurements for small fields of 1×1, 2×2 and 4×4cm2 with detectors placed in a water tank at the central axis perpendicular to the beam. OSL detectors disks with diameter of 3mm) and an #a16 Exradin microcylindrical ionization chamber (EMIC) were used to evaluate the depth dose distributions of smaller fields. The depth dose distributions for field sizes of 4×4 and 10×10cm2 measured by EMIC were cross-compared and verified by Scanditronix CC13 ionization chamber for the Varian 21EX linear accelerator. Depth dose distributions measured by OSL for small field sizes of 1×1, 2×2 and 4×4cm2 were then compared to those acquired by EMIC. Results: For the 4×4 and 10×10cm2 field sizes, the EMIC data are in excellent agreement with the CC13 ionization chamber data. When compared the depth dose curves obtained using the OSL techniques against the EMIC data, the difference was ⩽ 1.9%, 0.9% and 1.9% for 1×1, 2×2, 4×4cm2 respectively, for depths between 10 mm to 150 mm. The data are currently being analyzed and will be presented. Conclusion: This is the first time that OSL dosimeters were investigated for small field dosimetry. Preliminary results indicated that the OSL dosimeters can be used to provide accurate dose estimates and could be a promising alternative detector for small field dosimetry.