Report of AAPM Task Group 235 Radiochromic Film Dosimetry: An Update to TG-55.

The use of radiochromic film (RCF) dosimetry in radiation therapy is extensive due to its high level of achievable accuracy for a wide range of dose values and its suitability under a variety of measurement conditions. However, since the publication of the 1998 AAPM Task Group 55, Report No. 63 on RCF dosimetry, the chemistry, composition, and readout systems for RCFs have evolved steadily. There are several challenges in using the new RCFs, readout systems and validation of the results depending on their applications. Accurate RCF dosimetry requires understanding of RCF selection, handling and calibration methods, calibration curves, dose conversion methods, correction methodologies as well as selection, operation and quality assurance (QA) programs of the readout systems. Acquiring this level of knowledge is not straight forward, even for some experienced users. This Task Group report addresses these issues and provides a basic understanding of available RCF models, dosimetric characteristics and properties, advantages and limitations, configurations, and overall elemental compositions of the RCFs that have changed over the past 20 yr. In addition, this report provides specific guidelines for data processing and analysis schemes and correction methodologies for clinical applications in radiation therapy.

Dosimetry for 131Cs and 125I seeds in solid water phantom using radiochromic EBT film.

PURPOSE: To measure the 2D dose distributions with submillimeter resolution for (131)Cs (model CS-1 Rev2) and (125)I (model 6711) seeds in a Solid Water phantom using radiochromic EBT film for radial distances from 0.06cm to 5cm. To determine the TG-43 dosimetry parameters in water by applying Solid Water to liquid water correction factors generated from Monte Carlo simulations. METHODS: Each film piece was positioned horizontally above and in close contact with a (131)Cs or (125)I seed oriented horizontally in a machined groove at the center of a Solid Water phantom, one film at a time. A total of 74 and 50 films were exposed to the (131)Cs and (125)I seeds, respectively. Different film sizes were utilized to gather data in different distance ranges. The exposure time varied according to the seed air-kerma strength and film size in order to deliver doses in the range covered by the film calibration curve. Small films were exposed for shorter times to assess the near field, while larger films were exposed for longer times in order to assess the far field. For calibration, films were exposed to either 40kV (M40) or 50kV (M50) x-rays in air at 100.0cm SSD with doses ranging from 0.2Gy to 40Gy. All experimental, calibration and background films were scanned at a 0.02cmpixel resolution using a CCD camera-based microdensitometer with a green light source. Data acquisition and scanner uniformity correction were achieved with Microd3 software. Data analysis was performed using ImageJ, FV, IDL and Excel software packages. 2D dose distributions were based on the calibration curve established for 50kV x-rays. The Solid Water to liquid water medium correction was calculated using the MCNP5 Monte Carlo code. Subsequently, the TG-43 dosimetry parameters in liquid water medium were determined. RESULTS: Values for the dose-rate constants using EBT film were 1.069±0.036 and 0.923±0.031cGyU(-1)h(-1) for (131)Cs and (125)I seed, respectively. The corresponding values determined using the Monte Carlo method were 1.053±0.014 and 0.924±0.016cGyU(-1)h(-1) for (131)Cs and (125)I seed, respectively. The radial dose functions obtained with EBT film measurements and Monte Carlo simulations were plotted for radial distances up to 5cm, and agreed within the uncertainty of the two methods. The 2D anisotropy functions obtained with both methods also agreed within their uncertainties. CONCLUSION: EBT film dosimetry in a Solid Water phantom is a viable method for measuring (131)Cs (model CS-1 Rev2) and (125)I (model 6711) brachytherapy seed dose distributions with submillimeter resolution. With the Solid Water to liquid water correction factors generated from Monte Carlo simulations, the measured TG-43 dosimetry parameters in liquid water for these two seed models were found to be in good agreement with those in the literature.

Evaluation of material heterogeneity dosimetric effects using radiochromic film for COMS eye plaques loaded with (125)I seeds (model I25.S16).

PURPOSE: (1) To measure absolute dose distributions in eye phantom for COMS eye plaques with (125)I seeds (model I25.S16) using radiochromic EBT film dosimetry. (2) To determine the dose correction function for calculations involving the TG-43 formalism to account for the presence of the COMS eye plaque using Monte Carlo (MC) method specific to this seed model. (3) To test the heterogeneous dose calculation accuracy of the new version of Plaque Simulator (v5.3.9) against the EBT film data for this seed model. METHODS: Using EBT film, absolute doses were measured for (125)I seeds (model I25.S16) in COMS eye plaques (1) along the plaque's central axis for (a) uniformly loaded plaques (14-20 mm in diameter) and (b) a 20 mm plaque with single seed, and (2) in off-axis direction at depths of 5 and 12 mm for all four plaque sizes. The EBT film calibration was performed at (125)I photon energy. MC calculations using MCNP5 code for a single seed at the center of a 20 mm plaque in homogeneous water and polystyrene medium were performed. The heterogeneity dose correction function was determined from the MC calculations. These function values at various depths were entered into PS software (v5.3.9) to calculate the heterogeneous dose distributions for the uniformly loaded plaques (of all four sizes). The dose distributions with homogeneous water assumptions were also calculated using PS for comparison. The EBT film measured absolute dose rate values (film) were compared with those calculated using PS with homogeneous assumption (PS Homo) and heterogeneity correction (PS Hetero). The values of dose ratio (film∕PS Homo) and (film∕PS Hetero) were obtained. RESULTS: The central axis depth dose rate values for a single seed in 20 mm plaque measured using EBT film and calculated with MCNP5 code (both in ploystyrene phantom) were compared, and agreement within 9% was found. The dose ratio (film∕PS Homo) values were substantially lower than unity (mostly between 0.8 and 0.9) for all four plaque sizes, indicating dose reduction by COMS plaque compared with homogeneous assumption. The dose ratio (film∕PS Hetero) values were close to unity, indicating the PS Hetero calculations agree with those from the film study. CONCLUSIONS: Substantial heterogeneity effect on the (125)I dose distributions in an eye phantom for COMS plaques was verified using radiochromic EBT film dosimetry. The calculated doses for uniformly loaded plaques using PS with heterogeneity correction option enabled were corroborated by the EBT film measurement data. Radiochromic EBT film dosimetry is feasible in measuring absolute dose distributions in eye phantom for COMS eye plaques loaded with single or multiple (125)I seeds. Plaque Simulator is a viable tool for the calculation of dose distributions if one understands its limitations and uses the proper heterogeneity correction feature.

Dosimetry of (125)I and (103)Pd COMS eye plaques for intraocular tumors: report of Task Group 129 by the AAPM and ABS.

Dosimetry of eye plaques for ocular tumors presents unique challenges in brachytherapy. The challenges in accurate dosimetry are in part related to the steep dose gradient in the tumor and critical structures that are within millimeters of radioactive sources. In most clinical applications, calculations of dose distributions around eye plaques assume a homogenous water medium and full scatter conditions. Recent Monte Carlo (MC)-based eye-plaque dosimetry simulations have demonstrated that the perturbation effects of heterogeneous materials in eye plaques, including the gold-alloy backing and Silastic insert, can be calculated with reasonable accuracy. Even additional levels of complexity introduced through the use of gold foil "seed-guides" and custom-designed plaques can be calculated accurately using modern MC techniques. Simulations accounting for the aforementioned complexities indicate dose discrepancies exceeding a factor of ten to selected critical structures compared to conventional dose calculations. Task Group 129 was formed to review the literature; re-examine the current dosimetry calculation formalism; and make recommendations for eye-plaque dosimetry, including evaluation of brachytherapy source dosimetry parameters and heterogeneity correction factors. A literature review identified modern assessments of dose calculations for Collaborative Ocular Melanoma Study (COMS) design plaques, including MC analyses and an intercomparison of treatment planning systems (TPS) detailing differences between homogeneous and heterogeneous plaque calculations using the American Association of Physicists in Medicine (AAPM) TG-43U1 brachytherapy dosimetry formalism and MC techniques. This review identified that a commonly used prescription dose of 85 Gy at 5 mm depth in homogeneous medium delivers about 75 Gy and 69 Gy at the same 5 mm depth for specific (125)I and (103)Pd sources, respectively, when accounting for COMS plaque heterogeneities. Thus, the adoption of heterogeneous dose calculation methods in clinical practice would result in dose differences >10% and warrant a careful evaluation of the corresponding changes in prescription doses. Doses to normal ocular structures vary with choice of radionuclide, plaque location, and prescription depth, such that further dosimetric evaluations of the adoption of MC-based dosimetry methods are needed. The AAPM and American Brachytherapy Society (ABS) recommend that clinical medical physicists should make concurrent estimates of heterogeneity-corrected delivered dose using the information in this report's tables to prepare for brachytherapy TPS that can account for material heterogeneities and for a transition to heterogeneity-corrected prescriptive goals. It is recommended that brachytherapy TPS vendors include material heterogeneity corrections in their systems and take steps to integrate planned plaque localization and image guidance. In the interim, before the availability of commercial MC-based brachytherapy TPS, it is recommended that clinical medical physicists use the line-source approximation in homogeneous water medium and the 2D AAPM TG-43U1 dosimetry formalism and brachytherapy source dosimetry parameter datasets for treatment planning calculations. Furthermore, this report includes quality management program recommendations for eye-plaque brachytherapy.

Confirmation of skin doses resulting from bolus effect of intervening alpha-cradle and carbon fiber couch in radiotherapy.

In this study, we verified the treatment planning calculations of skin doses with the incorporation of the bolus effect due to the intervening alpha-cradle (AC) and carbon fiber couch (CFC) using radiochromic EBT2 films. A polystyrene phantom (25 × 25 × 15 cm(3)) with six EBT2 films separated by polystyrene slabs, at depths of 0, 0.1, 0.2, 0.5, 1, 1.4 cm, was positioned above an AC, which was ~1 cm thick. The phantom and AC assembly were CT scanned and the CT-images were transferred to the treatment planning system (TPS) for calculations in three scenarios: (A) ignoring AC and CFC, (B) accounting for AC only, (C) accounting for both AC and CFC. A single posterior 10 × 10 cm(2) field, a pair of posterior-oblique 10 × 10 cm(2) fields, and a posterior IMRT field (6 MV photons from a Varian Trilogy linac) were planned. For each radiation field configuration, the same MU were used in all three scenarios in the TPS. Each plan for scenario C was delivered to expose a stack of EBT2 films in the phantom through AC and CFC. In addition, in vivo EBT2 film measurement on a lung cancer patient immobilized with AC undergoing IMRT was also included in this study. Point doses and planar distributions generated from the TPS for the three scenarios were compared with the data from the EBT2 film measurements. For all the field arrangements, the EBT2 film data including the in vivo measurement agreed with the doses calculated for scenario (C), within the uncertainty of the EBT2 measurements (~4%). For the single posterior field (a pair of posterior-oblique fields), the TPS generated doses were lower than the EBT2 doses by 34%, 33%, 31%, 13% (34%, 31%, 31%, 11%) for scenario A and by 27%, 25%, 22%, 8% (25%, 21%, 21%, 6%) for scenario B at the depths of 0, 0.1, 0.2, 0.5 cm, respectively. For the IMRT field, the 2D dose distributions at each depth calculated in scenario C agree with those measured data. When comparing the central axis doses for the IMRT field, we found the TPS generated doses for scenario A (B) were lower than the EBT2 data by 35%, 34%, 31%, 16% (29%, 26%, 23%, 10%) at the depths of 0, 0.1, 0.2, 0.5 cm, respectively. There were no significant differences for the depths of 1.0 and 1.4 cm for all the radiation fields studied. TPS calculation of doses in the skin layers accounting for AC and CFC was verified by EBT2 film data. Ignoring the presence of AC and/or CFC in TPS calculation would significantly underestimate the doses in the skin layers. For the clinicians, as more hypofractionated regimens and stereotactic regimens are being used, this information will be useful to avoid potential serious skin toxicities, and also assist in clinical decisions and report these doses accurately to relevant clinical trials/cooperative groups, such as RTOG.

Comparison of dose calculation methods for brachytherapy of intraocular tumors.

PURPOSE: To investigate dosimetric differences among several clinical treatment planning systems (TPS) and Monte Carlo (MC) codes for brachytherapy of intraocular tumors using 125I or 103Pd plaques, and to evaluate the impact on the prescription dose of the adoption of MC codes and certain versions of a TPS (Plaque Simulator with optional modules). METHODS: Three clinical brachytherapy TPS capable of intraocular brachytherapy treatment planning and two MC codes were compared. The TPS investigated were Pinnacle v8.0dp1, BrachyVision v8.1, and Plaque Simulator v5.3.9, all of which use the AAPM TG-43 formalism in water. The Plaque Simulator software can also handle some correction factors from MC simulations. The MC codes used are MCNP5 v1.40 and BrachyDose/EGSnrc. Using these TPS and MC codes, three types of calculations were performed: homogeneous medium with point sources (for the TPS only, using the 1D TG-43 dose calculation formalism); homogeneous medium with line sources (TPS with 2D TG-43 dose calculation formalism and MC codes); and plaque heterogeneity-corrected line sources (Plaque Simulator with modified 2D TG-43 dose calculation formalism and MC codes). Comparisons were made of doses calculated at points-of-interest on the plaque central-axis and at off-axis points of clinical interest within a standardized model of the right eye. RESULTS: For the homogeneous water medium case, agreement was within approximately 2% for the point- and line-source models when comparing between TPS and between TPS and MC codes, respectively. For the heterogeneous medium case, dose differences (as calculated using the MC codes and Plaque Simulator) differ by up to 37% on the central-axis in comparison to the homogeneous water calculations. A prescription dose of 85 Gy at 5 mm depth based on calculations in a homogeneous medium delivers 76 Gy and 67 Gy for specific 125I and 103Pd sources, respectively, when accounting for COMS-plaque heterogeneities. For off-axis points-of-interest, dose differences approached factors of 7 and 12 at some positions for 125I and 103Pd, respectively. There was good agreement (approximately 3%) among MC codes and Plaque Simulator results when appropriate parameters calculated using MC codes were input into Plaque Simulator. Plaque Simulator and MC users are perhaps at risk of overdosing patients up to 20% if heterogeneity corrections are used and the prescribed dose is not modified appropriately. CONCLUSIONS: Agreement within 2% was observed among conventional brachytherapy TPS and MC codes for intraocular brachytherapy dose calculations in a homogeneous water environment. In general, the magnitude of dose errors incurred by ignoring the effect of the plaque backing and Silastic insert (i.e., by using the TG-43 approach) increased with distance from the plaque's central-axis. Considering the presence of material heterogeneities in a typical eye plaque, the best method in this study for dose calculations is a verified MC simulation.

Evaluation of two-dimensional bolus effect of immobilization/support devices on skin doses: a radiochromic EBT film dosimetry study in phantom.

PURPOSE: In this study, the authors have quantified the two-dimensional (2D) perspective of skin dose increase using EBT film dosimetry in phantom in the presence of patient immobilization devices during conventional and IMRT treatments. METHODS: For 6 MV conventional photon field, the authors evaluated and quantified the 2D bolus effect on skin doses for six different common patient immobilization/support devices, including carbon fiber grid with Mylar sheet, Orfit carbon fiber base plate, balsa wood board, Styrofoam, perforated AquaPlast sheet, and alpha-cradle. For 6 and 15 MV IMRT fields, a stack of two film layers positioned above a solid phantom was exposed at the air interface or in the presence of a patient alpha-cradle. All the films were scanned and the pixel values were converted to doses based on an established calibration curve. The authors determined the 2D skin dose distributions, isodose curves, and cross-sectional profiles at the surface layers with or without the immobilization/support device. The authors also generated and compared the dose area histograms (DAHs) and dose area products from the 2D skin dose distributions. RESULTS: In contrast with 20% relative dose [(RD) dose relative to dmax on central axis] at 0.0153 cm in the film layer for 6 MV 10 x 10 cm2 open field, the average RDs at the same depth in the film layer were 71%, 69%, 55%, and 57% for Orfit, balsa wood, Styrofoam, and alpha-cradle, respectively. At the same depth, the RDs were 54% under a strut and 26% between neighboring struts of a carbon fiber grid with Mylar sheet, and between 34% and 56% for stretched perforated AquaPlast sheet. In the presence of the alpha-cradle for the 6 MV (15 MV) IMRT fields, the hot spot doses at the effective measurement depths of 0.0153 and 0.0459 cm were 140% and 150%, (83% and 89%), respectively, of the isocenter dose. The enhancement factor was defined as the ratio of a given DAH parameter (minimum dose received in a given area) with and without the support device. For 6 MV conventional 10 x 10 cm2 field, the enhancement factor was the highest (3.4) for the Orfit carbon fiber plate. As for the IMRT field, the enhancement factors varied with the size of the area of interest and were as high as 3.8 (4.3) at the hot spot of 5 cm2 area in the top film layer (0.0153 cm) for 6 MV (15 MV) beams. CONCLUSIONS: Significant 2D bolus effect on skin dose in the presence of patient support and immobilization devices was confirmed and quantified with EBT film dosimetry. Furthermore, the EBT film has potential application for in vivo monitoring of the 2D skin dose distributions during patient treatments.

A comprehensive dosimetric comparison between (131)Cs and (125)I brachytherapy sources for COMS eye plaque implant.

PURPOSE: To verify the dosimetric characteristics of (131)Cs source in the Collaborative Ocular Melanoma Study (COMS) eye plaque brachytherapy, to compare (131)Cs with (125)I in a sample implant, and to examine the accuracy of treatment planning system in dose calculation. METHODS AND MATERIALS: Monte Carlo (MC) technique was used to generate three-dimensional dose distributions of a 16-mm COMS eye plaque loaded with (131)Cs and (125)I brachytherapy sources separately. A spherical eyeball, 24.6mm in diameter, and an ellipsoidal tumor, 6mm in height and 12mm in diameter, were used to evaluate the doses delivered. The simulations were carried out both with and without the gold and gold alloy plaque. A water-equivalent seed carrier was used instead of the silastic insert designed for the traditional COMS eye plaque. The 13 sources involved were also individually simulated to evaluate the intersource effect. In addition, a treatment planning system was used to calculate the doses at the central axis for comparison with MC data. RESULTS: The gold plaque had significantly reduced the dose in the tumor volume; at the prescription point of this study, that is, 6mm from the edge of inner sclera, the gold plaque reduced the dose by about 7% for both types of (131)Cs and (125)I sources, but the gold alloy plaque reduced the dose only by 4% for both types of sources. The intersource effect reduced the dose by 2% for both types of sources. At the same prescription dose, the treatment with the gold plaque applicator tended to create more hot regions for either type of sources than were seen with the homogeneous water phantom. The doses of TPS agree with the MC. CONCLUSION: The (131)Cs source is comparable to the (125)I source in the eye plaque brachytherapy. The TPS can provide accurate dose calculations for eye plaque implants with either type of sources.

Photon beam dosimetry in the superficial buildup region using radiochromic EBT film stack.

It has been a challenge to perform accurate 2D or 3D dosimetry in the surface region with steep dose gradient for megavoltage photon beams. We developed a dosimetry method in the superficial buildup region for the 6 and 15 MV photon beams using a radiochromic EBT film stack. Eight radiochromic EBT film strips (3 x 20 x 0.024 cm3) stacked together formed a 3D dosimeter. The film stack was positioned above a polystyrene phantom and surrounded by Solid Water slabs (0.2 cm) with the top film layer at 100 cm SSD. A 10 x 10 cm2 open field was used to irradiate the film stack with 1000 MU. All films were scanned using Epson 4870 flatbed scanner with transmission mode, 48 bit color, and 150 dpi (0.017 cm pixel resolution). The pixel values were converted to doses using an established calibration curve. This method allowed dose measurement for depths from 0.012 to 0.18 cm with fine spatial resolution (0.017 cm horizontally and 0.024 cm vertically). For each energy modality, we obtained both the central axis percent depth doses and the beam profiles along the central line covering the primary field and peripheral region at each depth. The primary field doses varied steeply with depth, while those in the peripheral region were weakly dependent on depth. For the 6 MV and 15 MV photon beams, (1) the central axis percent depth doses in the eight film layers ranged from 22% to 66% and from 15% to 44%, respectively; (2) the extrapolated percent depth doses at d = 0 were 15% and 14%, respectively. Agreement with the previously reported central axis percent depth doses in this region using parallel plate thin window ion chamber and ultrathin TLD was observed. The percent depth doses and beam profiles data can be incorporated in the treatment planning system for more accurate assessment of the doses to skin and shallow tumors to accomplish more accurate calculation results in the clinical usage.

The use of new GAFCHROMIC EBT film for 125I seed dosimetry in Solid Water phantom.

Radiochromic film dosimetry has been extensively used for intravascular brachytherapy applications for near field within 1 cm from the sources. With the recent introduction of new model of radiochromic films, GAFCHROMIC EBT, with higher sensitivity than earlier models, it is promising to extend the distances out to 5 cm for low dose rate (LDR) source dosimetry. In this study, the use of new model GAFCHROMIC EBT film for 125I seed dosimetry in Solid Water was evaluated for radial distances from 0.06 cm out to 5 cm. A multiple film technique was employed for four 125I seeds (Implant Sciences model 3500) with NIST traceable air kerma strengths. Each experimental film was positioned in contact with a 125I seed in a Solid Water phantom. The products of the air kerma strength and exposure time ranged from 8 to 3158 U-h, with the initial air kerma strength of 6 U in a series of 25 experiments. A set of 25 calibration films each was sequentially exposed to one 125I seed at about 0.58 cm distance for doses from 0.1 to 33 Gy. A CCD camera based microdensitometer, with interchangeable green (520 nm) and red (665 nm) light boxes, was used to scan all the films with 0.2 mm pixel resolution. The dose to each 125I calibration film center was calculated using the air kerma strength of the seed (incorporating decay), exposure time, distance from seed center to film center, and TG43U1S1 recommended dosimetric parameters. Based on the established calibration curve, dose conversion from net optical density was achieved for each light source. The dose rate constant was determined as 0.991 cGy U(-1)h(-1) (+/-6.9%) and 1.014 cGy U(-1)h(-1) (+/-6.8%) from films scanned using green and red light sources, respectively. The difference between these two values was within the uncertainty of the measurement. Radial dose function and 2D anisotropy function were also determined. The results obtained using the two light sources corroborated each other. We found good agreement with the TG43U1S1 recommended values of radial dose function and 2D anisotropy function, to within the uncertainty of the measurement. We also verified the dosimetric parameters in the near field calculated by Rivard using Monte Carlo method. The radial dose function values in Solid Water were lower than those in water recommended by TG43U1S1, by about 2%, 3%, 7%, and 14% at 2, 3, 4, and 5 cm, respectively, partially due to the difference in the phantom material composition. Radiochromic film dosimetry using GAFCHROMIC EBT model is feasible in determining 2D dose distributions around low dose rate 125I seed. It is a viable alternative to TLD dosimetry for 125I seed dose characterization.

Dose calculation formalisms and consensus dosimetry parameters for intravascular brachytherapy dosimetry: recommendations of the AAPM Therapy Physics Committee Task Group No. 149.

Since the publication of AAPM Task Group 60 report in 1999, a considerable amount of dosimetry data for the three coronary brachytherapy systems in use in the United States has been reported. A subgroup, Task Group 149, of the AAPM working group on Special Brachytherapy Modalities (Bruce Thomadsen, Chair) was charged to develop recommendations for dose calculation formalisms and the related consensus dosimetry parameters. The recommendations of this group are presented here. For the Cordis 192Ir and Novoste 90Sr/90Y systems, the original TG-43 formalism in spherical coordinates should be used along with the consensus values of the dose rate constant, geometry function, radial dose function, and anisotropy function for the single seeds. Contributions from the single seeds should be added linearly for the calculation of dose distributions from a source train. For the Guidant 32P wire system, the modified TG-43 formalism in cylindrical coordinates along with the recommended data for the 20 and 27 mm wires should be used. Data tables for the 6, 10, 14, 18, and 22 seed trains of the Cordis system, 30, 40, and 60 mm seed trains of the Novoste system, and the 20 and 27 mm wires of the Guidant system are presented along with our rationale and methodology for selecting the consensus data. Briefly, all available datasets were compared with each other and the consensus dataset was either an average of available data or the one obtained from the most densely populated study; in most cases this was a Monte Carlo calculation.

Energy dependence of response of new high sensitivity radiochromic films for megavoltage and kilovoltage radiation energies.

The purpose of this paper is to evaluate the energy dependence of the response of two new high sensitivity models of radiochromic films EBT and XR-QA. We determined the dose response curves of these films for four different radiation sources, namely, 6 MV photon beams (6 MVX), Ir-192, I-125, and Pd-103. The first type (EBT) is designed for intensity modulated radiation therapy (IMRT) dosimetry, and the second type (XR-QA) is designed for kilovoltage dosimetry. All films were scanned using red (665 nm) and green (520 nm) light sources in a charge-coupled device-based densitometer. The dose response curves [net optical density (NOD) versus dose] were plotted and compared for different radiation energies and light sources. Contrary to the early GAFCHROMIC film types (such as models XR, HS, MD55-2, and HD810), the net optical densities of both EBT and XR-QA were higher with a green (520 nm) than those with a red (665 nm) light source due to the different absorption spectrum of the new radiochromic emulsion. Both film types yield measurable optical densities for doses below 2 Gy. EBT film response is nearly independent of radiation energy, within the uncertainty of measurement. The NOD values of EBT film at 1 and 2 Gy are 0.13 and 0.25 for green, and 0.1 and 0.17 for red, respectively. In contrast, the XR-QA film sensitivity varies with radiation energy. The doses required to produce NOD of 0.5 are 6.9, 5.4, 0.7, and 0.9 Gy with green light and 19, 13, 1.7, and 1.5 Gy with red light, for 6 MVX, Ir-192, I -125, and Pd-103, respectively. EBT film was found to have minimal photon energy dependence of response for the energies tested and is suitable for dosimetry of radiation with a wide energy spectrum, including primary and scattered radiation. XR-QA film is promising for kilovoltage sources with a narrow energy spectra. The new high sensitivity radiochromic films are promising tools in radiation dosimetry.

Comparison of uniformity of dose response of double layer radiochromic films (MD-55-2) measured at 5 institutions.

In recent years, double-layer radiochromic films (MD-55-2) also known as new improved Gafchromic Films (NMD-55), have been used for measuring dose distributions of radiation fields. It is reported that the response of radiochromic film is affected by the scanning densitometry systems used to read the film. To quantify the response of MD-55-2 films, two sheets (12.5 cm x 12.5 cm) from different batches, were irradiated to 900 cGy and 2000 cGy, using uniform flat photon beams. The films were sent to 5 institutions for response evaluation using 5 different densitometry systems with narrow band light source centered at nominal 633 to 665 nm, which is near the major absorption peaks of the film sensitive emulsion. The dose response curve was established for each dens tometer. The one-dimensional film responses were obtained for specified directions. A set of fiducial marks, located at approximately 1.5 cm from the film edges, was used for identification of scan direction and alignment. The local fluctuations were assessed by comparing the film response with the mean response and its relative (percentage) standard deviation (RSD) in the region of interest. The regional non-uniformity was measured by examining the difference between the maximum and minimum responses within the region of interest. Our data indicates that the RSD, as obtained by the 5 institutions, varied from 2.4% to 5.8% for the film irradiated at low ( approximately 900 cGy) dose and from 1.2% to 4.3% for the film irradiated at the higher dose ( approximately 2000 cGy). The regional non-uniformity was also improved with increased dose and was less in longitudinal direction of the film that is parallel to the direction of coating application. Data comparison for regional non-uniformity indicates that the film responses were affected not only by the wavelength of analyzing source, but also by other instrumentation factors such as step size and sampling size. High-resolution scanners may also have more noise that should not be attributed to film non-uniformity.

Dose response characteristics of new models of GAFCHROMIC films: dependence on densitometer light source and radiation energy.

This paper presents a systematic study of the dose response characteristics of two new models and one commonly used model of GAFCHROMIC film: HS, XR-T, and MD55-2, respectively. We irradiated these film models with three different radiation sources: I-125, Ir-192, and 6 MV photon beam (6 MVX). We scanned the films with three different densitometers: a He-Ne laser with a wavelength of 633 nm, a spot densitometer with a wavelength of 671 nm, and a CCD camera densitometer with interchangeable LED boxes with wavelengths of 665 nm (red), 520 nm (green), and 465 nm (blue). We compared the film sensitivities in terms of net optical density (NOD) per unit dose in Gy. The sensitivity of each film model depends on radiation energy and the densitometer light source. Using He-Ne laser based densitometer as a reference standard, we found the sensitivities (NOD/Gy) for the red lights of wavelengths, 671 nm and 665 nm, are higher by factors of about 2.5 and 2, respectively. The sensitivities for green (520 nm) and blue (465 nm) lights are lower than that for He-Ne laser (633 nm) by factors of about 2 and 4, respectively. The energy dependence of the sensitivity varies with the film model, but is similar for all densitometer light sources. Comparing I-125 to Ir-192 and 6MVX, we note that (a) model XR-T is about eight times more sensitive, and (b) models HS and MD55-2 are about 40% less sensitive. Relative to MD55-2, XR-T is 12 times more sensitive for I-125 but comparable for Ir-192 and 6MVX, whereas HS is 2 to 3 times more sensitive in all cases. This set of results can serve as useful information for making decisions in selecting the film model and compatible densitometer to achieve the best accuracy of dosimetry in the appropriate dose range.

Treatment planning dosimetric parameters for 192Ir seed at short distances: effects of air channels and neighboring seeds based on Monte Carlo study.

The dose distributions around two different arrangements of a single radioactive 192Ir seed in water, (1) with air channels at the ends, and (2) surrounded by two nonactive ("dummy") seeds on both longitudinal ends, were calculated using MCNP4C Monte Carlo simulations at distances up to 1 cm. The contributions from beta particles and electrons emitted by 192Ir were included in the calculations. The effects of (a) the air channels at the seed ends and (b) the interference effect of the dummy seeds on the dose distribution were quantified and compared. It was found that the dummy seeds do not cause significant dose reduction for radial distances beyond 0.05 cm from the seed center. It is decided to report the dose rate values and the dosimetric parameters in TG43 format for a single seed with air channels for use in treatment planning computer systems. The dose rate constant (at 1 cm) of 192Ir seed, lambda, is 1.108 cGyU(-1) h(-1). The values of radial dose function, g(r), are within 1% from the TG43 recommended polynomial fit, except for distances within 0.08 cm. The anisotropy function, F(r, theta), attains large values near the seed ends and shallow angles (up to 8), as well as many values greater than 2 at the 20 degrees polar angle. Treatment planning systems involving intravascular brachytherapy do not compromise the accuracy for dosimetry of multiple seed trains by summing single seed values in water.

Verification of Ir-192 near source dosimetry using GAFCHROMIC film.

Intravascular brachytherapy treatments of in-stent restenosis have been performed extensively using Ir-192 ribbon. Task Group 60 of the American Association of Physicists in Medicine (AAPM) recommends a dose reference point at 2 mm from the source center for these treatments. However, it is known that the source can be as close as 0.5 mm to the arterial wall if not centered in the lumen. Therefore, the source dosimetry needs to be characterized at these close distances to accurately determine the amount of dose delivered for noncentered cases. In this paper, we report the verification of the dose distributions around Ir-192 seed sources at radial distances from 0.5 mm to 6 mm using GAFCHROMIC film. We evaluated an Ir-192 single seed source and a train of 6 seeds spaced 1 mm apart enclosed in a nylon ribbon. Each source was placed in a homogeneous solid water phantom directly below a stack of GAFCHROMIC films (MD-55-2). The calibration curve of the lot of films used in the experiment was established for Ir-192 by exposing a set of calibration films, one at a time, to an Ir-192 high dose rate (HDR) source. All films were scanned 5 or more days after exposure with a Lumisys Model 150 microdensitometer. The data were acquired and evaluated using RIT113 (Radiological Imaging Technology) software and analyzed using Excel and IDL (Interactive Data Language) software. Isodose curve plots in the plane containing the source's longitudinal axis and dose rate plots in the radial direction were obtained. For both configurations, the dose rates along the transverse axes agree to within the margin of error with previous Monte Carlo results. The isodose curve plots display hot spots near the seed ends, which is consistent with the leakage of beta particles and electrons from the unsealed seed ends as predicted with Monte Carlo calculations.

Thermoluminescent dosimetry of the SourceTech Medical model STM1251 125I seed.

Many new models of 125I seeds are being introduced, mainly due to the increase in prostate seed implants. We have evaluated the SourceTech Medical (STM), model STM1251, 125I seed using thermoluminescent dosimeters (TLDs) in a solid water phantom. TLD cubes, LiF TLD-100, with dimension 1 mm on each edge, were irradiated at various distances, 1, 2, 3, and 5 cm, at angles ranging from 0 degrees to 90 degrees in 10 degrees increments. Sensitivity calibration of the TLDs was achieved by irradiation to 10 cGy with 6 MV x rays from a clinical linear accelerator, Clinac 600C. Concurrent with the 125I seed exposures, several TLDs were also exposed to 10 cGy with the 600C as a control set. Dose rates per unit air kerma strength were determined based on the 1999 NIST traceable standard for the STM1251 seed. They are presented as a function of distance r and angle theta. The TG-43 parameters, including the dose rate constant, lambda, anisotropy function, F(r,theta), radial dose function, g(r), anisotropy factor, phian(r), and anisotropy constant, phi, were obtained for use in radiation treatment planning software. The value of lambda was determined as 1.07 +/- 5.5% cGy U(-1) h(-1), which is comparable to model 6702 and to the value determined using the point extrapolation method by Kirov and Williamson. We also find agreement between our TLD data and their Monte Carlo results for g(r), F(r,theta), phian(r), and phi. Additionally, agreement is found with the TLD data of Li and Williamson for lambda and g(r).

High beta and electron dose from 192Ir: implications for "gamma" intravascular brachytherapy.

PURPOSE: Trains of multiple 192Ir seeds are used in many clinical trials for intravascular brachytherapy. 192Ir source is commonly considered as a gamma emitter, despite the understanding that this radionuclide also emits a wide range of electron and beta energies, with a similar range of energy. The high dose from betas and electrons in the submillimeter range due to unsealed ends of seed sources should be precisely quantified to fully understand the backdrop for complications associated with 192Ir coronary artery brachytherapy. METHODS AND MATERIALS: Monte Carlo simulations (MCNP4C code) were performed for a model 5-seed 192Ir train used in SCRIPPS, GAMMA, and the Washington Radiation for In-Stent Restenosis (WRIST) randomized clinical trials. A stack of radiochromic films was also used to measure the dose distributions for an actual 6-seed train. RESULTS: In the submillimeter range very close to the source, Monte Carlo results show that betas and electrons deposit a higher dose than 192Ir photons (gamma and X-rays) over the interseed gap. A high luminal dose from the combined effects of betas, electrons, and photons emitted from 192Ir can be deposited, particularly between seeds. When prescribing 15 Gy at 2 mm, the combined dose can be as high as 160 Gy at 0.5 mm. Different peak doses near the interseed gaps were noted, which may be due to variability of seed-end surfaces and nonuniformity of seed activity within a real multiseed train. Dose-volume histograms (DVH) of lumen surfaces were evaluated for an eccentric seed train. The DVH parameters indicating the extent of hot spots in the lumen wall, DV(10), DV(5), DV(2), and DV(1) (dose received by 10, 5, 2, 1% respectively of the total lumen surface), can be as high as 55, 76, 81, and 155 Gy for a lumen with 3-mm diameter, and 75, 80, 110, and 158 Gy for a narrow 2-mm lumen. CONCLUSION: 192Ir multiple seed trains used in the SCRIPPS, GAMMA, and WRIST trials can deposit a very high dose to the luminal wall. A particularly high electron and beta dose can be delivered near the interseed gap if the source is not centered in the catheter and lumen. The dose from 192Ir betas and electrons may partially explain adverse outcomes reported from 192Ir multiseed clinical trials. Improvement of the encapsulation design to filter out the betas and electrons should be seriously considered.

Dose heterogeneity may not affect the neointimal proliferation after gamma radiation for in-stent restenosis: a volumetric intravascular ultrasound dosimetric study.

OBJECTIVES: The goal of this study was to use serial (postirradiation and follow-up) volumetric intravascular ultrasound (IVUS): 1) to evaluate the actual distribution of gamma radiation in human in-stent restenosis (ISR) lesions, and 2) to analyze the relationship between neointimal regrowth and the delivered radiation dose. BACKGROUND: The relationship between the neointimal regrowth and delivered dose during the treatment of ISR remains unknown. METHODS: We analyzed 20 actively (gamma emitter) treated, native artery ISR patients from the Washington Radiation for In-Stent restenosis Trial (WRIST) that met the following criteria: on both postirradiation and six-month follow-up IVUS imaging, > or =80% of the external elastic membrane circumference could be identified throughout the treated length including the lesion and proximal and distal reference segments. Intravascular ultrasound images were digitized every 1 mm. Proximal and distal reference and stented segment luminal and adventitial contours were imported and reconstructed. The source was placed circumferentially at the site of the IVUS catheter and longitudinally according to the relationship between the radioactive seeds and stent edges. Using Monte Carlo simulations, dose volume histograms for the adventitia and intima were calculated. The relationship between the neointimal regrowth and calculated doses were evaluated. RESULTS: There was large dose heterogeneity at both the intimal and adventitial levels. Most of the sites (93%) received >4 Gy at the adventitia, and all of the sites received >4 Gy at the intima. There was no relationship between neointimal regrowth and radiation dose. CONCLUSIONS: Although there may be large dose heterogeneity, gamma irradiation (using a fixed dose prescription) appears to deliver a sufficient dose to prevent neointimal regrowth.