Precise radiochromic film dosimetry using a flat-bed document scanner

In this study, a measurement protocol is presented that improves the precision of dose measurements using a flat-bed document scanner in conjunction with two new GafChromic film models, HS and Prototype A EBT exposed to 6 MV photon beams. We established two sources of uncertainties in dose measurements, governed by measurement and calibration curve fit parameters contributions. We have quantitatively assessed the influence of different steps in the protocol on the overall dose measurement uncertainty. Applying the protocol described in this paper on the Agfa Arcus II flat-bed document scanner, the overall one-sigma dose measurement uncertainty for an uniform field amounts to 2% or less for doses above around 0.4 Gy in the case of the EBT (Prototype A), and for doses above 5 Gy in the case of the HS model GafChromic film using a region of interest 2 X 2 mm2 in size.     

EBT3胶片数字化的横向响应伪影消除方法

目的：消除EBT3胶片数字化过程中的横向响应伪影,优化EBT3胶片的治疗计划验证结果。方法：覆盖双面镀膜的减反射玻璃进行胶片数字化,通过净光密度与扫描仪不同位置间的抛物线拟合关系来消除横向响应伪影后,借助剂量刻度曲线将胶片净光密度转换为剂量。使用辐照面积较大的治疗计划进行验证,对胶片与计划剂量分布进行γ分析。结果：在3%/3 mm标准下对不小于0.1 Gy的剂量点进行γ分析,消除横向响应伪影后胶片与计划剂量分布的γ通过率为95%,相比未消除横向响应伪影的剂量分布提升了3%的通过率。结论：利用该方法可以有效消除横向响应伪影,提高EBT3胶片治疗计划验证的γ通过率。     

GafChromic EBT film dosimetry with flatbed CCD scanner: a novel background correction method and full dose uncertainty analysis

Film dosimetry using radiochromic EBT film in combination with a flatbed charge coupled device scanner is a useful method both for two-dimensional verification of intensity-modulated radiation treatment plans and for general quality assurance of treatment planning systems and linear accelerators. Unfortunately, the response over the scanner area is nonuniform, and when not corrected for, this results in a systematic error in the measured dose which is both dose and position dependent. In this study a novel method for background correction is presented. The method is based on the subtraction of a correction matrix, a matrix that is based on scans of films that are irradiated to nine dose levels in the range 0.08-2.93 Gy. Because the response of the film is dependent on the film's orientation with respect to the scanner, correction matrices for both landscape oriented and portrait oriented scans were made. In addition to the background correction method, a full dose uncertainty analysis of the film dosimetry procedure was performed. This analysis takes into account the fit uncertainty of the calibration curve, the variation in response for different film sheets, the nonuniformity after background correction, and the noise in the scanned films. The film analysis was performed for film pieces of size 16 x 16 cm, all with the same lot number, and all irradiations were done perpendicular onto the films. The results show that the 2-sigma dose uncertainty at 2 Gy is about 5% and 3.5% for landscape and portrait scans, respectively. The uncertainty gradually increases as the dose decreases, but at 1 Gy the 2-sigma dose uncertainty is still as good as 6% and 4% for landscape and portrait scans, respectively. The study shows that film dosimetry using GafChromic EBT film, an Epson Expression 1680 Professional scanner and a dedicated background correction technique gives precise and accurate results. For the purpose of dosimetric verification, the calculated dose distribution can be compared with the film-measured dose distribution using a dose constraint of 4% (relative to the measured dose) for doses between 1 and 3 Gy. At lower doses, the dose constraint must be relaxed.     

GafChromic RTQA film for routine quality assurance of high-energy photon beams

Self-developing film offers many advantages over conventional radiographic verification film for routine radiotherapy quality assurance (QA). This paper presents results from an initial evaluation of a beam measurement system using GafChromic RTQA film and a flatbed scanner. Variability and energy dependence of the film calibration and accuracy of scanner readout are investigated in the context of QA measurements. For exposures of film between 2 and 4 Gy, the system is adequate for measurement of beam dimensions, as in multi-leaf collimator (MLC) offsets and secondary jaw calibrations, where agreement with conventional film measurements is within 0.5 mm. However, the measurement of absolute dose is subject to errors of about 25 cGy.     

Dosimetric properties of improved GafChromic films for seven different digitizers

Two recently introduced GafChromic film models, HS and XR-T, have been developed as more sensitive and uniform alternatives to GafChromic MD-55-2 film. The HS model has been specifically designed for measurement of absorbed dose in high-energy photon beams (above 1 MeV), while the XR-T model has been introduced for dose measurements of low energy (0.1 MeV) photons. The goal of this study is to compare the sensitometric curves and estimated dosimetric uncertainties associated with seven different GafChromic film dosimetry systems for the two new film models. The densitometers tested are: LKB Pharmacia UltroScan XL, Molecular Dynamics Personal Densitometer, Nuclear Associates Radiochromic Densitometer Model 37-443, Photoelectron Corporation CMR-604, Laser Pro 16, Vidar VXR-16, and AGFA Arcus II document scanner. Pieces of film were exposed to different doses in a dose range from 0.5 to 50 Gy using 6 MV photon beam. Functional forms for dose vs net optical density have been determined for each of the GafChromic film-dosimetry systems used in this comparison. Two sources of uncertainties in dose measurements, governed by the experimental measurement and calibration curve fit procedure, have been compared for the densitometers used. Among the densitometers tested, it is found that for the HS film type the uncertainty caused by the experimental measurement varies from 1% to 3% while the calibration fit uncertainty ranges from 2% to 4% for doses above 5 Gy. Corresponding uncertainties for XR-T film model are somewhat higher and range from 1% to 5% for experimental and from 2% to 7% for the fit uncertainty estimates. Notwithstanding the significant variations in sensitivity, the studied densitometers exhibit very similar precision for GafChromic film based dose measurements above 5 Gy.     

Energy dependence (75 kVp to 18 MV) of radiochromic films assessed using a real-time optical dosimeter

The response of radiochromic film, GafChromic EBT, was investigated for dependence on x-ray beam energy using a previously reported real-time optical readout approach. X-ray beams of energy from 75 kVp to 18 MV were employed. The dose-induced change in optical density for the EBT film was compared to values obtained for GafChromic HS and MD-55 films, exposed under the same conditions. All responses were normalized to that obtained for 60Co irradiation. While change in optical density for 1 Gy of applied dose as measured with HS and MD-55 films decreased by approximately 40% at low energies, the mean change in optical density of EBT film remained within 3% of that in the 60Co beam over the entire energy range.     

Dosimetric characterization of GafChromic EBT film and its implication on film dosimetry quality assurance

The suitability of radiochromic EBT film was studied for high-precision clinical quality assurance (QA) by identifying the dose response for a wide range of irradiation parameters typically modified in highly-conformal treatment techniques. In addition, uncertainties associated with varying irradiation conditions were determined. EBT can be used for dose assessment of absorbed dose levels as well as relative dosimetry when compared to absolute absorbed dose calibrated using ionization chamber results. For comparison, a silver halide film (Kodak EDR-2) representing the current standard in film dosimetry was included. As an initial step a measurement protocol yielding accurate and precise results was established for a flatbed transparency scanner (Epson Expression 1680 Pro) that was utilized as a film reading instrument. The light transmission measured by the scanner was found to depend on the position of the film on the scanner plate. For three film pieces irradiated with doses of 0 Gy, approximately 1 Gy and approximately 7 Gy, the pixel values measured in portrait or landscape mode differed by 4.7%, 6.2% and 10.0%, respectively. A study of 200 film pieces revealed an excellent sheet-to-sheet uniformity. On a long time scale, the optical development of irradiated EBT film consisted of a slow but steady increase of absorbance which was not observed to cease during 4 months. Sensitometric curves of EBT films obtained under reference conditions (SSD = 95 cm, FS = 5 x 5 cm(2), d = 5 cm) for 6, 10 and 25 MV photon beams did not show any energy dependence. The average separation between all curves was only 0.7%. The variation of the depth d (range 2-25 cm) in the phantom did not affect the dose response of EBT film. Also the influence of the radiation field size (range 3 x 3-40 x 40 cm(2)) on the sensitometric curve was not significant. For EDR-2 films maximum differences between the calibration curves reached 7-8% for X6MV and X25MV. Radiochromic EBT film, in combination with a flatbed scanner, presents a versatile system for high-precision dosimetry in two dimensions, provided that the intrinsic behaviour of the film reading device is taken into account. EBT film itself presents substantial improvements on formerly available models of radiographic and a radiochromic film and its dosimetric characteristics allow us to measure absorbed dose levels in a large variety of situations with a single calibration curve.     

Measurements and Monte Carlo calculations to determine the absolute detector response of radiochromic film for brachytherapy dosimetry

GafChromic (MD-55-2) radiochromic film has become increasingly popular for medical applications and has proven to be useful for brachytherapy dosimetry. To measure the absolute dose near a brachytherapy source, the response of the proposed detector in the measurement conditions relative to the response of the detector in calibration conditions must be known. MD-55-2 radiochromic film has been exposed in four different photon beams, a 30 and 40 kVp tungsten anode x-ray beam, a 75 kVp orthovoltage therapy beam, and a 60Co teletherapy beam to measure the relative detector response. These measurements were combined with coupled photon/electron Monte Carlo transport calculations to determine the absolute detector response. The Los Alamos National Laboratory Monte Carlo transport code MCNP4B2 was used. The measured relative response of this batch of MD-55-2 film varies from 8.79 mOD/Gy, measured for the 60Co beam, by as much as 42% for the low-energy x-ray beams. However, the absolute detector response varies from 4.32 mOD/Gy for the 60Co beam by, at most, only 6.3%. In this work we demonstrate that the absolute detector response of MD-55-2 radiochromic film is a constant and independent of beam quality. Further, this work shows that MCNP4B2 accurately simulates the energy response and geometry artifacts of the radiochromic film.     

A wide dynamic range, high-spatial-resolution scanning system for radiochromic dye films

This paper describes the evaluation of an inexpensive, commercially available 35 mm transparency slide scanner as a potential alternative scanning device for GafChromic HD-810 radiochromic dye film. Besides its low cost, the principal advantages of this type of scanner are high spatial resolution and high speed (a typical scan taking less than 1 min). With broad-band illumination the useful dose range using grey-scale imaging of GafChromic HD-810 is limited to about 50-800 Gy. By using the colour-scale imaging capability of the scanner we have been able to achieve a significant extension covering a similar range (15-2000 Gy) to that attainable using monochromatic illumination. The short-term reproducibility of the system is good, with a coefficient of variation of doses estimated from repeat scanning of uniformly exposed calibration films of less than 2%. Long-term stability is ensured by the scanning of a manufacturer-supplied test slide. The slide scanner system has been used in the determination of depth dose distributions from a model 'hot particle' source containing 106Ru/Rh. GafChromic dye film stacks irradiated by the source were read out on both the slide scanner and a conventional Joyce Loebl MDM6 scanning stage microdensitometer. The overall agreement between the dose estimates provided by the two systems was within 10%.     

An application of GafChromic MD-55 film for 67.5 MeV clinical proton beam dosimetry

The purpose of this study is to explore the use of GafChromic MD-55 (RC) film for 67.5 MeV clinical proton beam dosimetry at the Crocker Nuclear Laboratory, University of California, Davis. Several strips of RC film 6 cm x 6 cm in dimension were irradiated at a depth of 18.2 mm corresponding to the middle of a 24 mm spread-out Bragg peak (SOBP). The films were irradiated to a proton dose in the range of 0.5 Gy to 100 Gy. The beam profiles were also measured at the middle of the 24 mm SOBP. The Bragg peak was measured by using a wedge shaped phantom made of Lucite. The Bragg peak measured with RC film was compared with diode and ionization chamber measurements. After background subtraction, the calibration of the dose response of RC film showed, to a maximum deviation of 10%, a linear increase of optical density (OD) with dose from 0.5 to 100 Gy. The uniformity of OD over a single sheet of film showed a variation of +/-6%. The distal-fall off between 90% and 20% measured with GafChromic film for the Bragg peak was 1.3 mm as compared to 1.1 mm for a diode measurement and 1.4 mm for an ionization chamber measurement. The FWHM of the Bragg peak was 7.5 mm when measured with GafChromic film, 5.3 mm when measured with a diode and 8.1 mm as measured by an ionization chamber. The peak/plateau ratio with GafChromic film was 3.3 as compared to 3.7 with a diode and 3.2 with an ionization chamber. In conclusion, GafChromic MD-55 film may be a useful and convenient detector for dose measurement and quality assurance programmes of proton beams.     

Calibration of EBT2 film by the PDD method with scanner non-uniformity correction

The EBT2 film together with a flatbed scanner is a convenient dosimetry QA tool for verification of clinical radiotherapy treatments. However, it suffers from a relatively high degree of uncertainty and a tedious film calibration process for every new lot of films, including cutting the films into several small pieces, exposing with different doses, restoring them back and selecting the?proper region of interest (ROI) for each piece for curve fitting. In this work, we present a percentage depth dose (PDD) method that can accurately calibrate the EBT2 film together with the scanner non-uniformity correction and provide an easy way to perform film dosimetry. All films were scanned before and after the irradiation in one of the two homemade 2?mm thick acrylic frames (one portrait and the other landscape), which was located at a fixed position on the scan bed of an Epson 10?000XL scanner. After the pre-irradiated scan, the film was placed parallel to the beam central axis and sandwiched between six polystyrene plates (5?cm thick each), followed by irradiation of a 20?×?20?cm(2)?6 MV photon beam. Two different beams on times were used on two different films to deliver a dose to the film ranging from 32 to 320 cGy. After the post-irradiated scan, the net optical densities for a total of 235 points on the beam central axis on the films were auto-extracted and compared with the corresponding depth doses that were calculated through the measurement of a 0.6 cc farmer chamber and the related PDD table to perform the curve fitting. The portrait film location was selected for routine calibration, since the central beam axis on the film is parallel to the scanning direction, where non-uniformity correction is not needed (Ferreira et al 2009 Phys. Med. Biol. 54 1073-85). To perform the scanner non-uniformity calibration, the cross-beam profiles of the film were analysed by referencing the measured profiles from a Profiler?. Finally, to verify our method, the films were exposed to 60° physical wedge fields and the compositive fields, and their relative dose profiles were compared with those from the water phantom measurement. The fitting uncertainty was less than 0.5% due to the many calibration points, and the overall calibration uncertainty was within 3% for doses above 50 cGy, when the average of four films were used for the calibration. According to our study, the non-uniformity calibration factor was found to be independent of the given dose for the EBT2 film and the relative dose differences between the profiles measured by the film and the Profiler were within 1.5% after applying the non-uniformity correction. For the verification tests, the relative dose differences between the measurements by films and in the water phantom, when the average of three films were used, were generally within 3% for the 60° wedge fields and compositive fields, respectively. In conclusion, our method is convenient, time-saving and cost-effective, since no film cutting is needed and only two films with two exposures are needed.     

Characterization and real-time optical measurements of the ionizing radiation dose response for a new radiochromic medium

A new radiochromic film, GafChromic EBT, was investigated for use in a real-time radiation dosimetry system. It was found to be approximately eight times more sensitive to ionizing radiation dose, exhibited less postexposure development and achieved stable readout faster than one of its predecessors, GafChromic MD-55. A clear distinction in change in optical density between exposure and postexposure was observed, but the measurements obtained during exposure were not linear with time or dose. This could not be explained by a shift in wavelength of maximum change in absorbance, as it was stable at approximately 636 nm during the entire exposure range (up to 9.52 Gy). Increasing the spectral window of interest over which calculations were performed did little to correct the nonlinearity. The radiochromic film exhibited small dose rate dependence in real-time measurements, with an increase in standard deviation of change in optical density measurements from 0.9% to 1.8% over a sixfold variation in dose rate. Overall, GafChromic EBT has increased sensitivity and decreased postexposure darkening, and this bodes well for its potential role as a radiation dosimeter, including real-time applications.     

Radiochromic film dosimetry with flatbed scanners: a fast and accurate method for dose calibration and uniformity correction with single film exposure

Film dosimetry is an attractive tool for dose distribution verification in intensity modulated radiotherapy (IMRT). A critical aspect of radiochromic film dosimetry is the scanner used for the readout of the film: the output needs to be calibrated in dose response and corrected for pixel value and spatial dependent nonuniformity caused by light scattering; these procedures can take a long time. A method for a fast and accurate calibration and uniformity correction for radiochromic film dosimetry is presented: a single film exposure is used to do both calibration and correction. Gafchromic EBT films were read with two flatbed charge coupled device scanners (Epson V750 and 1680Pro). The accuracy of the method is investigated with specific dose patterns and an IMRT beam. The comparisons with a two-dimensional array of ionization chambers using a 18 x 18 cm2 open field and an inverse pyramid dose pattern show an increment in the percentage of points which pass the gamma analysis (tolerance parameters of 3% and 3 mm), passing from 55% and 64% for the 1680Pro and V750 scanners, respectively, to 94% for both scanners for the 18 x 18 open field, and from 76% and 75% to 91% for the inverse pyramid pattern. Application to an IMRT beam also shows better gamma index results, passing from 88% and 86% for the two scanners, respectively, to 94% for both. The number of points and dose range considered for correction and calibration appears to be appropriate for use in IMRT verification. The method showed to be fast and to correct properly the nonuniformity and has been adopted for routine clinical IMRT dose verification.     

Characterization and use of EBT radiochromic film for IMRT dose verification

We present an evaluation of a new and improved radiochromic film, type EBT, for its implementation to IMRT dose verification. Using a characterized flat bed color CCD scanner, the film's dose sensitivity, uniformity, and speed of development post exposure were shown to be superior to previous types of radiochromic films. The film's dose response was found to be very similar to ion chamber scans in water through comparisons of depth dose and lateral dose profiles. The effect of EBT film polarization with delivered dose and film scan orientation was shown to have a significant effect on the scanner's OD readout. In addition, the film's large size, flexibility, and the ability to submerge it in water for relatively short periods of time allowed for its use in both water and solid water phantoms to verify TomoTherapy IMRT dose distributions in flat and curved dose planes. Dose verification in 2D was performed on ten IMRT plans (five head and neck and five prostate) by comparing measured EBT dose distributions to TomoTherapy treatment planning system calculated dose. The quality of agreement was quantified by the gamma index for four sets of dose difference and distance to agreement criteria. Based on this study, we show that EBT film has several favorable features that allow for its use in routine IMRT patient-specific QA.     

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.     

Dose area product evaluations with Gafchromic XR-R films and a flat-bed scanner

Gafchromic XR-R films are a useful tool to evaluate entrance skin dose in interventional radiology. Another dosimetric quantity of interest in diagnostic and interventional radiology is the dose area product (DAP). In this study, a method to evaluate DAP using Gafchromic XR-R films and a flat-bed scanner was developed and tested. Film samples were exposed to an x-ray beam of 80 kVp over a dose range of 0-10 Gy. DAP measurements with films were obtained from the digitalization of a film sample positioned over the x-ray beam window during the exposure. DAP values obtained with this method were compared for 23 cardiological interventional procedures with DAP values displayed by the equipment. The overall one-sigma dose measurement uncertainty depended on the absorbed dose, with values below 6% for doses above 1 Gy. A maximum discrepancy of 16% was found, which is of the order of the differences in the DAP measurements that may occur with different calibration procedures. Based on the results presented, after an accurate calibration procedure and a thorough inspection of the relationship between the actual dose and the direct measured quantity (net optical density or net pixel value variation), Gafchromic XR-R films can be used to assess the DAP.     

Accurate dosimetry with GafChromic EBT film of a 6 MV photon beam in water: what level is achievable?

This paper focuses on the accuracy, in absolute dose measurements, with GafChromicTM EBT film achievable in water for a 6 MV photon beam up to a dose of 2.3 Gy. Motivation is to get an absolute dose detection system to measure up dose distributions in a (water) phantom, to check dose calculations. An Epson 1680 color (red green blue) transmission flatbed scanner has been used as film scanning system, where the response in the red color channel has been extracted and used for the analyses. The influence of the flatbed film scanner on the film based dose detection process was investigated. The scan procedure has been optimized; i.e. for instance a lateral correction curve was derived to correct the scan value, up to 10%, as a function of optical density and lateral position. Sensitometric curves of different film batches were evaluated in portrait and landscape scan mode. Between various batches important variations in sensitometric curve were observed. Energy dependence of the film is negligible, while a slight variation in dose response is observed for very large angles between film surface and incident photon beam. Improved accuracy in absolute dose detection can be obtained by repetition of a film measurement to tackle at least the inherent presence of film inhomogeneous construction. We state that the overall uncertainty is random in absolute EBT film dose detection and of the order of 1.3% (1 SD) under the condition that the film is scanned in a limited centered area on the scanner and at least two films have been applied. At last we advise to check a new film batch on its characteristics compared to available information, before using that batch for absolute dose measurements.     

Evaluation of a novel triple-channel radiochromic film analysis procedure using EBT2

A novel approach to read out radiochromic film was introduced recently by the manufacturer of GafChromic film. In this study, the performance of this triple-channel film dosimetry method was compared against the conventional single-red-channel film dosimetry procedure, with and without inclusion of a pre-irradiation (pre-IR) film scan, using EBT2 film and kilo- and megavoltage photon beams up to 10 Gy. When considering regions of interest averaged doses, the triple-channel method and both single-channel methods produced equivalent results. Absolute dose discrepancies between the triple-channel method, both single-channel methods and the treatment planning system calculated dose values, were no larger than 5 cGy for dose levels up to 2.2 Gy. Signal to noise in triple-channel dose images was found to be similar to signal to noise in single-channel dose images. The accuracy of resulting dose images from the triple- and single-channel methods with inclusion of pre-IR film scan was found to be similar. Results of a comparison of EBT2 data from a kilovoltage depth dose experiment to corresponding Monte Carlo depth dose data produced dose discrepancies of 9.5 ± 12 cGy and 7.6 ± 6 cGy for the single-channel method with inclusion of a pre-IR film scan and the triple-channel method, respectively. EBT2 showed to be energy sensitive at low kilovoltage energies with response differences of 11.9% and 15.6% in the red channel at 2 Gy between 50-225 kVp and 80-225 kVp photon spectra, respectively. We observed that the triple-channel method resulted in non-uniformity corrections of ±1% and consistency values of 0-3 cGy for the batches and dose levels studied. Results of this study indicate that the triple-channel radiochromic film read-out method performs at least as well as the single-channel method with inclusion of a pre-IR film scan, reduces film non-uniformity and saves time with elimination of a pre-IR film scan.     

Precautions and strategies in using a commercial flatbed scanner for radiochromic film dosimetry

The purpose of this study was to investigate the value of a commercially available flatbed scanner for film dosimetry with radiochromic film for external radiotherapy. The EPSON Pro 1680 Expression scanner was examined as a densitometer for two-dimensional film dosimetry with Gafchromic EBT film. An accurate and efficient scanning procedure was established. Possible drift and warm-up effects of the scanner were studied and the direct physical influence of the scanner light on the radiochromic film was assessed. Next, we investigated the scan field uniformity. Also, we examined if the accuracy of radiochromic film was improved by subtracting the optical density of the unirradiated blank film from the optical density of the irradiated film. To assess the accuracy of Gafchromic EBT film when the EPSON scanner was used as a densitometer, the depth dose of a 2 x 15 cm(2) field and the in-plane and cross-plane profiles of a 15 x 15 cm(2) field were measured and compared with diamond detector measurements. When taking consecutive scans, we found that the optical density taken from the first scan was about 1% higher than the optical density taken from subsequent scans. We attribute this to the warming up of the lamp of the scanner. Longer-term drift of the scanner was found to be absent. We found that the use of a correction matrix was necessary to correct for the non-uniform scanner response over the scan field. Subtracting the optical density of the unirradiated blank film from the irradiated film improves the precision of the Gafchromic EBT film. Depth dose and profile measurements with Gafchromic EBT film and the diamond detector are in agreement within 2.5%. The EPSON Pro 1680 Expression scanner is an excellent tool for accurate two-dimensional film dosimetry with Gafchromic EBT film provided that some precautions and corrections are taken into account.     

Clinical use of EBT model Gafchromic film in radiotherapy

The Gafchromic EBT was recently introduced in film dosimetry for external beam therapy (EBT). The high spatial resolution, weak energy dependence, and near-tissue equivalence of EBT films make them suitable for measurement of dose distributions in radiotherapy, especially intensity-modulated radiation therapy (IMRT). Starting with a sensitometric curve and dose uncertainty relative to the flatbed scanner, the goal of this study was to find an efficient method of correcting for light scattering, and to compare dose distribution supplied by Gafchromic EBT with the distribution obtained with a 2D ion-chamber detector system. Light scattering was analyzed for different levels of dose, and was found to depend on the red-scale value as well as the position of the pixel on the scanner. Many "uniform" films were exposed at different levels of dose to create a two-dimensional matrix correction to take this effect into account. The dose distribution obtained for three clinical beams (10 x 10, 15 x 15 cm open fields and 12 x 12 cm wedge 60 degrees field) were in agreement with those supplied by the 2D array. Gamma index <1 (using 5 mm distance and 5% dose as constraints) for the three fields considered was reached in an average of 98% of the points.