Validation of a precision radiochromic film dosimetry system for quantitative two-dimensional imaging of acute exposure dose distributions

We present an evaluation of the precision and accuracy of image-based radiochromic film (RCF) dosimetry performed using a commercial RCF product (Gafchromic MD-55-2, Nuclear Associates, Inc.) and a commercial high-spatial resolution (100 microm pixel size) He-Ne scanning-laser film-digitizer (Personal Densitometer, Molecular Dynamics, Inc.) as an optical density (OD) imaging system. The precision and accuracy of this dosimetry system are evaluated by performing RCF imaging dosimetry in well characterized conformal external beam and brachytherapy high dose-rate (HDR) radiation fields. Benchmarking of image-based RCF dosimetry is necessary due to many potential errors inherent to RCF dosimetry including: a temperature-dependent time evolution of RCF dose response; nonuniform response of RCF; and optical-polarization artifacts. In addition, laser-densitometer imaging artifacts can produce systematic OD measurement errors as large as 35% in the presence of high OD gradients. We present a RCF exposure and readout protocol that was developed for the accurate dosimetry of high dose rate (HDR) radiation sources. This protocol follows and expands upon the guidelines set forth by the American Association of Physicists in Medicine (AAPM) Task Group 55 report. Particular attention is focused on the OD imaging system, a scanning-laser film digitizer, modified to eliminate OD artifacts that were not addressed in the AAPM Task Group 55 report. RCF precision using this technique was evaluated with films given uniform 6 MV x-ray doses between 1 and 200 Gy. RCF absolute dose accuracy using this technique was evaluated by comparing RCF measurements to small volume ionization chamber measurements for conformal external-beam sources and an experimentally validated Monte Carlo photon-transport simulation code for a 192Ir brachytherapy source. Pixel-to-pixel standard deviations of uniformly irradiated films were less than 1% for doses between 10 and 150 Gy; between 1% and 5% for lower doses down to 1 Gy and 1% and 1.5% for higher doses up to 200 Gy. Pixel averaging to form 200-800 microm pixels reduces these standard deviations by a factor of 2 to 5. Comparisons of absolute dose show agreement within 1.5%-4% of dose benchmarks, consistent with a highly accurate dosimeter limited by its observed precision and the precision of the dose standards to which it is compared. These results provide a comprehensive benchmarking of RCF, enabling its use in the commissioning of novel HDR therapy sources.     

Comparison of the epson expression 1680 flatbed and the vidar VXR-16 dosimetry PRO film scanners for use in IMRT dosimetry using gafchromic and radiographic film

Intensity-modulated radiotherapy (IMRT) treatment plan verification is often done using Kodak EDR2 film and a Vidar Dosimetry PRO film digitizer. However, since many hospitals are moving towards a filmless environment, access to a film processor may not be available. Therefore, we have investigated a newly available Gafchromic EBT film for IMRT dosimetry. Planar IMRT dose distributions are delivered to both EBT and EDR2 film and scanned with the Vidar VXR-16 as well as an Epson Expression 1680 flatbed scanner. The measured dose distributions are then compared to those calculated with a Pinnacle treatment planning system. The IMRT treatments consisted of 7-9 6 MV beams for treatment of prostate, head and neck, and a few other sites. The films were analyzed using FilmQATM (3cognition LLC) software. Comparisons between measured and calculated dose distributions are reported as dose difference (DD) (pixels within +/-5%), distance to agreement (DTA) (3 mm), as well as gamma values (y) (dose= +/-3%, dist. =2 mm). Using EDR2 with the Vidar scanner is an established technique and agreement between calculated and measured dose distributions was better than 90% in all indices (DD, DTA, and gamma). However, agreement with calculations deteriorated reaching the lower 80% for EBT film scans with the Vidar scanner in logarithmic mode. The EBT Vidar scans obtained in linear mode showed an improved agreement to the upper 80% range, but artifacts were still observed across the scan. These artifacts were very distinct in all EBT scans and can be attributed to the way the film is transported through the scanner. In the Epson scanner both films are rigidly immobilized and the light source scans over the film. It was found that the Epson scanner performed equally well with both types of film giving agreement to better than 90% in all indices.     

Predicting energy response of radiographic film in a 6 MV x-ray beam using Monte Carlo calculated fluence spectra and absorbed dose

The advantage of radiographic film is that it allows two-dimensional, high-resolution dose measurement. While there is concern over its photon energy dependence, these problems are considered acceptable within small fields, where the scatter component is small. The application of film dosimetry to intensity modulated radiotherapy (IMRT) raises additional concern since the primary fluence may vary significantly within the field. The varying primary fluence in combination with a large scatter fraction, present for large fields and large depths, causes the spectrum at various points within the IMRT field to differ from the spectrum in the uniform fields typically used for calibrating the film. As a result, significant artifacts are introduced in the measured dose distribution. The purpose of this work is to quantify and develop a method to correct for these artifacts. Two approaches based on Monte Carlo (MC) simulations are examined. In the first method, the film artifact, as quantified by film and ion chamber output measurements in uniform square fields, is derived from the MC calculated ratio of absorbed doses to film and to water. In the second method, the measured film artifact is correlated with MC calculated photon spectra, revealing a strong correlation between the measured artifact and the "scatter"-to-"primary" ratio, defined by the ratio of the number of photons below to the number of photons above 0.1 MeV, independent of field size and depth. These methods are evaluated in high- and low-dose regions of a large intensity-modulated field created with a central block. The spectral approach is also tested with a clinical IMRT field. The absorbed dose method accurately corrects the measured film dose in the open part of the field and in points under the block and outside the field. The dose error is reduced from as much as 16% of the open field dose to less than 1%, as verified with an ion chamber. The spectral method accurately corrects the measured film dose in the open region of the centrally blocked field, but does not fully correct for the film artifact for points under the block and outside the field, where the spectrum is substantially different. Applied to the clinical field, the corrected film measurement shows good agreement with data obtained with a two-dimensional diode array.     

Interinstitutional variations of sensitometric curves of radiographic dosimetric films

Depth and field size dependence of the sensitometric curves of radiographic films have been studied by various groups. Limited information is, however, available on the magnitude of the variations in sensitometric curves applied in clinical practice in different institutions. In this study we assessed in a systematic way the effect of the various parameters influencing the shape of the sensitometric curve: batch composition, irradiation conditions, film processing, and film scanning. Two types of film, Kodak X-Omat V and CEA TVS, were irradiated, processed, and analyzed in three different institutions. The interinstitutional variation of the sensitometric curves, expressed as the OD variation at 50 cGy, can be up to 32% and is mainly caused by differences in film processing and to a lesser degree to differences in batch composition, film scanning, and irradiation conditions. For the Kodak films, the average OD difference at 50 cGy between the three institutions is 17% as a result of differences in batch composition and 25% due to differences in processing conditions. For the CEA films these data are 6% and 24%, respectively. The long-term variation of the sensitometric curves of KODAK films in one institution was smaller than the differences in batch composition between the three institutions. The sensitometric curves of CEA films showed in one institution a large variation with time; the shape gradually varied from sigmoidal to quasilinear. By using relative OD values rather than absolute OD values, variations in sensitometric curves of KODAK films can be reduced to 2%. Consequently, one sensitometric curve is sufficient to derive relative dose values. If processing conditions are well controlled, it might therefore be advantageous to determine the absolute OD only at one or two dose values, in combination with a "universal" relative sensitometric curve.     

Comparison of three high-resolution digitizers for radiochromic film dosimetry

A recently introduced radiographic film scanner from Howtek is evaluated and compared to two other commercially available densitometry systems for its use in radiochromic (RC) film dosimetry in the clinical dose range 0-100 Gy. It has a high-intensity red LED light-source centered at 662 nm (near the major absorption peak for RC film), and is coupled to a CCD linear array detector. This new densitometry system is directly compared to two high-resolution film scanners commonly employed in RC film dosimetry, namely the Lumiscan 75 digitizer (He-Ne laser light source) and the Vidar VXR-16 digitizer (fluorescent light source). A spot densitometer (Nuclear Associates Radiochromic Densitometer) with a filtered 671 nm laser-diode light source is also included as a reference for comparison. The response of the spot densitometer and three high-resolution digitizers is characterized by the dose required to reach a net optical density of 1 (DNOD1), and is 16.0, 37.3, and 46.4 Gy for the Nuclear Associates Radiochromic Densitometer, Howtek MultiRAD 460 and Lumiscan 75 digitizer, respectively. The Vidar VXR-16 does not reach a net optical density of 1. The minimum usable dose at which a 2% level of uncertainty can be achieved (MUD2%) on the three digitizers are 2.6, 6.0, and 38.5 Gy for the Howtek MultiRAD 460, Lumiscan 75, and Vidar VXR-16, respectively. The Howtek MultiRAD 460 shows the greatest sensitivity, lowest MUD2% and best signal-to-noise ratio in the clinical dose range 0-100 Gy. Furthermore, it has no apparent interference (moiré) artifacts that severely limit the low optical density region of the He-Ne laser digitizer. For high-resolution radiochromic dosimetry in the clinical dose range 0-100 Gy, the high-intensity red LED light-source digitizer proves to be the superior modality.     

Useful optical density range in film dosimetry: limitations due to noise and saturation

The optical density (OD) range for the scanners used in film dosimetry is limited due to saturation and noise. As the OD increases, saturation causes the rate of change of the output with respect to the input to become smaller, while at the same time noise remains fairly constant or increases. The combined effect leads to a degradation of the signal-to-noise ratio (SNR) at high optical densities. In this study, the uncertainty in the OD measurement, d(m), is expressed as a function of the optical density d. The functional relationship obtained gives the amplitude w of an interval around d in which d(m) will be found with a given probability p. The relationship w = w(d, p) is later used to determine which OD ranges fulfil a set of requirements on w and p. As an application of the procedure, the noise and saturation characteristics of a commercial film digitizer system are measured. Their contribution to the uncertainties of the dosimetric procedure is reported, and the data are used to provide an optical density range for a given uncertainty and confidence level associated with the digitizer. These data can be further combined with the data from other sources of noise such as film noise in order to estimate the final uncertainty of the dosimetric process.     

Effect of processing time delay on the dose response of Kodak EDR2 film

Kodak EDR2 film is a widely used two-dimensional dosimeter for intensity modulated radiotherapy (IMRT) measurements. Our clinical use of EDR2 film for IMRT verifications revealed variations and uncertainties in dose response that were larger than expected, given that we perform film calibrations for every experimental measurement. We found that the length of time between film exposure and processing can affect the absolute dose response of EDR2 film by as much as 4%-6%. EDR2 films were exposed to 300 cGy using 6 and 18 MV 10 x 10 cm2 fields and then processed after time delays ranging from 2 min to 24 h. An ion chamber measured the relative dose for these film exposures. The ratio of optical density (OD) to dose stabilized after 3 h. Compared to its stable value, the film response was 4%-6% lower at 2 min and 1% lower at 1 h. The results of the 4 min and 1 h processing time delays were verified with a total of four different EDR2 film batches. The OD/dose response for XV2 films was consistent for time periods of 4 min and 1 h between exposure and processing. To investigate possible interactions of the processing time delay effect with dose, single EDR2 films were irradiated to eight different dose levels between 45 and 330 cGy using smaller 3 x 3 cm2 areas. These films were processed after time delays of 1, 3, and 6 h, using 6 and 18 MV photon qualities. The results at all dose levels were consistent, indicating that there is no change in the processing time delay effect for different doses. The difference in the time delay effect between the 6 and 18 MV measurements was negligible for all experiments. To rule out bias in selecting film regions for OD measurement, we compared the use of a specialized algorithm that systematically determines regions of interest inside the 10 x 10 cm2 exposure areas to manually selected regions of interest. There was a maximum difference of only 0.07% between the manually and automatically selected regions, indicating that the use of a systematic algorithm to determine regions of interest in large and fairly uniform areas is not necessary. Based on these results, we recommend a minimum time of 1 h between exposure and processing for all EDR2 film measurements.     

Important considerations for radiochromic film dosimetry with flatbed CCD scanners and EBT GAFCHROMIC film

In this study, we present three significant artifacts that have the potential to negatively impact the accuracy and precision of film dosimetry measurements made using GAFCHROMIC EBT radiochromic film when read out with CCD flatbed scanners. Films were scanned using three commonly employed instruments: a Macbeth TD932 spot densitometer, an Epson Expression 1680 CCD array scanner, and a Microtek ScanMaker i900 CCD array scanner. For the two scanners we assessed the variation in optical density (OD) of GAFCHROMIC EBT film with scanning bed position, angular rotation of the film with respect to the scan line direction, and temperature inside the scanner due to repeated scanning. Scanning uniform radiochromic films demonstrated a distinct bowing effect in profiles in the direction of the CCD array with a nonuniformity of up to 17%. Profiles along a direction orthogonal to the CCD array demonstrated a 7% variation. A strong angular dependence was found in measurements made with the flatbed scanners; the effect could not be reproduced with the spot densitometer. An IMRT quality assurance film was scanned twice rotating the film 90' between the scans. For films scanned on the Epson scanner, up to 12% variation was observed in unirradiated EBT films rotated between 0 degrees and 90 degrees, which decreased to approximately 8% for EBT films irradiated to 300 cGy. Variations of up to 80% were observed for films scanned with the Microtek scanner. The scanners were found to significantly increase the film temperature with repeated scanning. Film temperature between 18 and 33 degrees C caused OD changes of approximately 7%. Considering these effects, we recommend adherence to a strict scanning protocol that includes: maintaining the orientation of films scanned on flatbed scanners, limiting scanning to the central portion of the scanner bed, and limiting the number of consecutive scans to minimize changes in OD caused by film heating.     

Pencil beam approach for correcting the energy dependence artifact in film dosimetry for IMRT verification

The higher sensitivity to low-energy scattered photons of radiographic film compared to water can lead to significant dosimetric error when the beam quality varies significantly within a field. Correcting for this artifact will provide greater accuracy for intensity modulated radiation therapy (IMRT) verification dosimetry. A procedure is developed for correction of the film energy-dependent response by creating a pencil beam kernel within our treatment planning system to model the film response specifically. Film kernels are obtained from EGSnrc Monte Carlo simulations of the dose distribution from a 1 mm diameter narrow beam in a model of the film placed at six depths from 1.5 to 40 cm in polystyrene and solid water phantoms. Kernels for different area phantoms (50 x 50 cm2 and 25 x 25 cm2 polystyrene and 30 x 30 cm2 solid water) are produced. The Monte Carlo calculated kernel is experimentally verified with film, ion chamber and thermoluminescent dosimetry (TLD) measurements in polystyrene irradiated by a narrow beam. The kernel is then used in convolution calculations to, predict the film response in open and IMRT fields. A 6 MV photon beam and Kodak XV2 film in a polystyrene phantom are selected to test the method as they are often used in practice and can result in large energy-dependent artifacts. The difference in dose distributions calculated with the film kernel and the water kernel is subtracted from film measurements to obtain a practically film artifact free IMRT dose distribution for the Kodak XV2 film. For the points with dose exceeding 5 cGy (11% of the peak dose) in a large modulated field and a film measurement inside a large polystyrene phantom at depth of 10 cm, the correction reduces the fraction of pixels for which the film dose deviates from dose to water by more than 5% of the mean film dose from 44% to 6%.     

Angiographic film substraction using a laser digitizer and computer processing

Digital subtraction angiography has been accepted as an invaluable clinical tool over the past decade; however, film-screen-based angiography is still performed routinely when high-resolution or large field-of-view angiograms are needed. A technique is presented whereby two films from an angiographic sequence are digitized using a high-resolution laser digitizer, and the digitized images are aligned, subtracted, and displayed using the computer. To accommodate for some types of patient motion, an image warping algorithm is presented and discussed in detail. The warping algorithm is piecewise linear, using triangular regions for warping, resulting in a global nonlinear transform across triangle elements. An algorithm describing optimal triangle selection also is discussed. The results show that subtraction images of excellent quality can be produced by the proposed technique, and suggest that, in some settings, digitized subtraction films may be preferred over conventional film subtraction.     

Reduction of motion blurring artifacts using respiratory gated CT in sinogram space: a quantitative evaluation

Techniques have been developed for reducing motion blurring artifacts by using respiratory gated computed tomography (CT) in sinogram space and quantitatively evaluating the artifact reduction. A synthetic sinogram was built from multiple scans intercepting a respiratory gating window. A gated CT image was then reconstructed using the filtered back-projection algorithm. Wedge phantoms, developed for quantifying the motion artifact reduction, were scanned while being moved using a computer-controlled linear stage. The resulting artifacts appeared between the high and low density regions as an apparent feature with a Hounsfield value that was the average of the two regions. A CT profile through these regions was fit using two error functions, each modeling the partial-volume averaging characteristics for the unmoving phantom. The motion artifact was quantified by determining the apparent distance between the two functions. The blurring artifact had a linear relationship with both the speed and the tangent of the wedge angles. When gating was employed, the blurring artifact was reduced systematically at the air-phantom interface. The gated image of phantoms moving at 20 mm/s showed similar blurring artifacts as the nongated image of phantoms moving at 10 mm/s. Nine patients were also scanned using the synchronized respiratory motion technique. Image artifacts were evaluated in the diaphragm, where high contrast interfaces intercepted the imaging plane. For patients, this respiratory gating technique reduced the blurring artifacts by 9%-41% at the lung-diaphragm interface.     

A Comparative Study of Conventional Mammography Film Interpretations with Soft Copy Readings of the Same Examinations

An acceptable mammography film digitizer must provide high-quality images at a level of diagnostic accuracy comparable to reading conventional film examinations. The purpose of this study was to determine if there are significant differences between the interpretations of conventional film-screen mammography examinations and soft copy readings of the images produced by a mammography film digitizer. Eight radiologists interpreted 120 mammography examinations, half as original films and the other half as digital images on a soft copy work station. No radiologist read the same examination twice. The interpretations were recorded in accordance with the Breast Imaging Reporting and Data System and included other variables such as perceived image quality and diagnostic difficulty and confidence. The results provide support for the hypothesis that there are no significant differences between the interpretations of conventional film-screen mammography examinations and soft copy examinations produced by a mammography film digitizer. The study was conducted primarily at the Johns Hopkins Medical Institutions in Baltimore, MD where all of the authors except Dr. Chad Mitchell are located. He is a Naval Officer at the Uniformed Services University of the Health Sciences in Bethesda, MD.     

Normalized sensitometric curves for the verification of hybrid IMRT treatment plans with multiple energies

With the clinical implementation of time-variable dose patterns and intensity modulated radiotherapy (IMRT) film dosimetry has regained popularity. Films are currently the most frequently used dosimetric means for patient specific quality assurance in IMRT. A common method is to verify a so-called hybrid IMRT plan, which is the patient specific treatment plan with unmodified fluence patterns recalculated in a dedicated phantom. For such applications the sensitometric curve, i.e., the relation between optical density (OD) and absorbed dose, should not depend critically on beam energy, field size and depth, or film orientation. In order to minimize the influence of all these variables a normalization of sensitometric curves is performed at various photon beam energies (6 MV, 10 MV, 25 MV). By doing so one unique sensitometric curve can be used for these three beam qualities. This holds for both film types investigated: Kodak X-Omat V films and EDR-2 films. Additionally, the influence of field size, depth and film orientation on a normalized sensitometric curve is determined for both film types. For doses smaller than 0.8 Gy for X-Omat V and doses smaller than 3 Gy for EDR-2 films the field size variation of normalized sensitometric curves is much smaller than 3% for fields up to 20 x 20 cm2. For X-Omat V films all differences between sensitometric curves determined at depths of 5, 10, and 15 cm are smaller than 3%. For EDR-2 films deviations larger than 3% are only observed at low net OD smaller than 0.25. The dependence of film orientation (parallel versus perpendicular) on a normalized sensitometric curve is found to be not critical. However, processing conditions have the largest influence and can result in differences up to 20% for sensitometric curves derived from films of the same batch but using different film processors. When normalizing sensitometric curves to the dose value necessary to obtain a net OD=1 for that respective geometry and energy the large energy dependence of sensitometric curves can be almost eliminated. This becomes especially important for the verification of hybrid IMRT plans with multiple energies. Additionally, such a normalization minimizes other influences such as field size, depth, and film orientation. This method is generally applicable to both Kodak X-Omat V and EDR-2 films. In order to achieve the highest accuracy level an upper dose limit of 0.8 Gy for X-Omat V films and 3 Gy for EDR-2 films should be taken into account. However, these dose limits may vary with film reading instrument and film processor.     

Absorption spectra variations of EBT radiochromic film from radiation exposure

Gafchromic EBT radiochromic film is one of the newest radiation-induced auto-developing x-ray analysis films available for therapeutic radiation dosimetry in radiotherapy applications. The spectral absorption properties in the visible wavelengths have been investigated and results show two main peaks in absorption located at 636 nm and 585 nm. These absorption peaks are different to many other radiochromic film products such as Gafchromic MD-55 and HS film where two peaks were located at 676 nm and 617 nm respectively. The general shape of the absorption spectra is similar to older designs. A much higher sensitivity is found at high-energy x-rays with an average 0.6 OD per Gy variation in OD seen within the first Gy measured at 636 nm using 6 MV x-rays. This is compared to approximately 0.09 OD units for the first Gy at the 676 nm absorption peak for HS film at 6 MV x-ray energy. The film's blue colour is visually different from older varieties of Gafchromic film with a higher intensity of mid-range blue within the film. The film provides adequate relative absorbed dose measurement for clinical radiotherapy x-ray assessment in the 1-2 Gy dose range which with further investigation may be useful for fractionated radiotherapy dose assessment.     

Wavelet denoising of vaginal pulse amplitude

Vaginal pulse amplitude (VPA) has been the most commonly analyzed signal of the vaginal photoplethysmograph. Frequent, large, and variable‐morphology artifacts typically have crowded this signal. These artifacts usually were corrected by hand, which may have introduced large differences in outcomes across laboratories. VPA signals were collected from 22 women who viewed a neutral film and a sexual film. An automated, wavelet‐based, denoising algorithm was compared against the uncorrected signal and the signal corrected in the typical manner (by hand). The automated wavelet denoising resulted in the same pattern of results as the hand‐corrected signal. The wavelet procedure automated artifact reduction in the VPA, and this mathematical instantiation permits the comparison of competing methods to improve signal:noise in the future.     

Utilization of DICOM GSDF to Modify Lookup Tables for Images Acquired on Film Digitizers

Film digitizers are used to transfer hardcopy x-ray transmission films into a PACS environment. Variability between digitizers is primarily due to a lack of an acquisition standard. By utilizing the Digital Imaging and Communications in Medicine (DICOM) Part 14 Grayscale Standard Display Function (GSDF) as basis from which to judge and modify digitizer output, a methodology using Just Noticeable Differences (JND) can be established that will create custom lookup tables for film digitizers. These modified images can then be judged against the original films using the GSDF to determine if the original films' contrast is preserved. Results based on JND indicated that relative contrast of the original image is preserved.     

Comparison of small photon beams measured using radiochromic and silver-halide films in solid water phantoms

In this study, we compared the dosimetric properties of four of the most commonly used films for megavoltage photon-beam dosimetry when irradiated under identical conditions by small multileaf-collimator (MLC) defined beamlets. Two silver-halide films (SHFs), Kodak XV2 and EDR2, and two radiochromic films (RCFs), Gafchromic HS and MD55-2, were irradiated by MLC-defined 1 x 1 cm2 beamlets from a Varian 2100 C/D linac equipped with a 120-leaf MLC. The beamlets were delivered with the accelerator gantry set laterally (90 degrees rotation) upon a solid-water compression film phantom at 100 cm source-to-surface distance which was positioned with the films parallel to the beam axis. Beamlets were delivered at central axis, 5.0 cm, and 10.5 cm off-axis for both leaf-end and leaf-side defined beamlets. The film dosimetry was performed using a quantitative optical density (OD) imaging system that was validated in a previous study. No significant differences between SHF and RCF measurements were observed in percentage depth doses, horizontal depth profiles, or two-dimension spatial isodose distributions in both the central axis and off-axis measurements. We found that regardless of the type of film used, RCF or SHF, a consistent data set for small beam dose modeling was generated. Previous validation studies based on the use of RCF and OD imaging system would indicate that all film produce an accurate result for small beam characterization.     

Dependence of radiochromic film optical density post-exposure kinetics on dose and dose fractionation

Radiochromic film (RCF) has been shown to be a precise and accurate secondary planar dosimeter for acute exposure radiation fields. However, its application to low dose-rate brachytherapy has been questioned because of possible dose-rate effects. To address this concern, we have measured the optical density (OD) of Model 55-2 RCF as a function of time (interval between the completion of irradiation and densitometry using a 633 nm laser scanner) following exposure (from less than 1 hour to 90 days) for single and split doses from 1 Gy to 100 Gy. Our work demonstrates that film darkening as a function of post-irradiation time depends significantly on total dose, with films exposed to lower doses developing faster than films given higher doses. At 1 Gy, the OD 90 days after exposure is 200% larger than that measured 1 h after exposure compared to a 20% increase over 90 days for doses larger than 20 Gy. An empirical model with time-independent, fast and slow growth terms was used to fit single exposure data. The dependence of the resulting best-fit parameters on dose was investigated. Splitting the dose into two fractions (20 Gy followed by doses of 1-80 Gy 24 h later) results in modest post-irradiation time-dependent changes in the total optical density (at most 15% at small doses), which dissipates within 20 hours following the second exposure. This experimental finding is consistent with the predictions of a simple cumulative dose superposition model. Overall, both experimental and empirical modeling suggest that dose-rate effects may be relatively small despite the strong dependence of film darkening kinetics on total dose. However, more experimental evaluation of radiochromic film response dependence on dose rate and dose-time-fractionation patterns is needed.     

Rapid radiographic film calibration for IMRT verification using automated MLC fields

A method for measuring a film sensitometric curve using a single sheet of film exposed with a two field step-and-shoot MLC treatment was developed and tested with Kodak XV2 and EDR2 films. With this technique a film sensitometric curve can be completed in only 10 minutes, making it practical to generate new film calibrations daily. This method is applicable to film calibrations for all purposes, but is particularly useful in IMRT treatment verification due to the method's use of small fields. This method agrees with the traditional large-field multifilm calibration within 0.5% and will produce sensitometric curves with errors less than 1% throughout the dose range, including uncertainties in dose delivery, film response, and optical density measurements. OD values for XV2 and EDR2 films were consistent in the middle of exposure areas at high depths, but the XV2 film penumbra regions showed large amounts of over-response as the calibration depth increased. If XV2 film is used for IMRT treatment verification, it is necessary to reduce the fluence of low energy photons in areas around the film by using thin lead shields. EDR2 film was shown to have minimal energy dependence, as it accurately represented penumbra areas and yielded identical sensitometric curves generated with 6 and 18 MV photons. However, its darker tint may make it more sensitive to scanning laser film digitizers' horizontal nonuniformities. This single film method proved to be superior to the traditional calibration method and allows fast daily calibrations of films for highly accurate IMRT delivery verifications.     

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.