Sensitometric Curve Research Articles

Experimental dosimetry of EDR2 films in scanning carbon-ion irradiation.

PurposeTo investigate the dose‐sensitometric response of extended dose range (EDR2) films to scanning carbon‐ion beams and to evaluate the applications of the obtained response curves to carbon‐ion dose distributions.MethodsEDR2 films were irradiated by mono‐energetic scanning carbon‐ion beams with different doses to obtain sensitometric curves at different integrated depth doses (DDDs). Six different DDDs were generated by using a proper buildup for each mono‐energetic beam and were used to investigate the energy dependence. The sensitometric curves were obtained by fitting the net optical density (netOD) to dose at different DDDs. The dose difference between the value converted from the netOD and that calculated in the treatment planning system (TPS) was investigated to evaluate the application scope of the sensitometric curve.ResultsDigitizing the EDR2 film with a resolution of 0.36 (72 dpi) provided a good signal‐to‐noise ratio, and the sensitometric curve was linear at all DDDs of clinically relevant incident kinetic energies in the netOD range of 0.02–1.70 for carbon‐ion film dosimetry. The factors used to convert the netOD to absorbed dose were expressed as a linear function of DDDs, with which the depth dose difference between converted and TPS was less than 3% in the proximal area for incident kinetic energies lower than 307.5 MeV/u.ConclusionThe EDR2 film is a feasible tool for scanning carbon‐ion beam profile measurements by directly evaluating the netOD distribution with proper digitizing resolution and netOD range. By applying the conversion factors, the EDR2 film can also be employed to perform the percentage depth dose consistency checking and linear energy transfer comparison of carbon‐ion lower than 307.5 MeV/u.

Microdosimetric modeling of the sensitometric curve of GafChromic films in the photon fields

The sensitometric curve of EBT3 GafChromic film located at a depth 5 cm of RW3 water-equivalent phantom exposed to 6 MV X-rays is investigated. Variation of optical density for absorbed doses less than 2 Gy is determined by the experimental measurement together with the microdosimetric one-hit detector model. It is found that this model needs two fitting parameters, a maximum optical density and a saturation parameter. Both of them depend on the film structure as well as the photon spectrum. Meanwhile, the saturation parameter is a function of the microdosimetric single-event distribution of specific energy, i.e., f1(z). To calculate this distribution, irradiation of the films is simulated by the Geant4 toolkit. A sample of EBT3 film with 2 mm × 2 mm area is simulated. Active layer of the film is considered to contain 5000 cylindrical sensitive targets (SV) with 9.4 μm length and 1.62 μm diameter, located in random positions with different axial directions. The results obtained show that below an absorbed dose 1 Gy the maximum difference between the measured and the calculated optical densities is about 11%, while for the doses above 1 Gy the discrepancy is at most 3%. Eventually, it can be concluded that the sensitometric curve of EBT3 GafChromic film can satisfactorily be determined by the microdosimetric one-hit detector model, especially in the dose range above 1 Gy.

The physics of radiochromic process: one calibration equation for all film types

A unique calibration equation for all radiochromic film types is proposed. The determination of this equation comes from an analogy that the authors made between the radiochromic and photographic processes. Photography dynamics is ruled by a differential equation, the logistic equation, describing phenomena of growth versus competition. A solution of the logistic equation is the Green-Saunders equation which was used for the first time by the same authors in a previous referred work for the calibration of a set of EBT3 Gafchromic films. Since this equation has a fundamental physical interpretation, the fitting parameters also represent important physical quantities and two of them in particular fix the film dynamics; one being the contrast and the other the abscissa of the inflection point of the sensitometric curve. In order to show the validity of this approach for more than one radiochromic film type, two sets of radiochromic films, the B3 and HD-V2 Gafchromic films were exposed to several known doses and the Green-Saunders equation was found to perfectly fit the experimental data. A comparison with the same procedure used for EBT3 Gafchromic films is proposed. The findings of this work are an important step forward in the understanding of an interesting and widely used dosimetry technique.

Sensitometric curve of radiographic films by X-ray fluorescence

Radiographic film exposure is traditionally measured by the transmittance of a beam of light through the film. There are many mathematical and computational models to characterize the curve behaviour and its properties, but almost none of them considers the limitations caused by the equipment used. As long as exposure in film increases, light intensity measured after the film decreases in a way that from a certain exposure, light couldn’t be distinguished from any kind of noise. This work aims to propose x-ray fluorescence as a solution for better measure high exposed films and show how it could be modelled mathematically.

A calibration method for patient specific IMRT QA using a single therapy verification film

AimThe aim of the present study is to develop and verify the single film calibration procedure used in intensity-modulated radiation therapy (IMRT) quality assurance. BackgroundRadiographic films have been regularly used in routine commissioning of treatment modalities and verification of treatment planning system (TPS). The radiation dosimetery based on radiographic films has ability to give absolute two-dimension dose distribution and prefer for the IMRT quality assurance. However, the single therapy verification film gives a quick and significant reliable method for IMRT verification. Materials and methodsA single extended dose rate (EDR 2) film was used to generate the sensitometric curve of film optical density and radiation dose. EDR 2 film was exposed with nine 6cm×6cm fields of 6MV photon beam obtained from a medical linear accelerator at 5-cm depth in solid water phantom. The nine regions of single film were exposed with radiation doses raging from 10 to 362cGy. The actual dose measurements inside the field regions were performed using 0.6cm3 ionization chamber. The exposed film was processed after irradiation using a VIDAR film scanner and the value of optical density was noted for each region. Ten IMRT plans of head and neck carcinoma were used for verification using a dynamic IMRT technique, and evaluated using the gamma index method with TPS calculated dose distribution. ResultsSensitometric curve has been generated using a single film exposed at nine field region to check quantitative dose verifications of IMRT treatments. The radiation scattered factor was observed to decrease exponentially with the increase in the distance from the centre of each field region. The IMRT plans based on calibration curve were verified using the gamma index method and found to be within acceptable criteria. ConclusionThe single film method proved to be superior to the traditional calibration method and produce fast daily film calibration for highly accurate IMRT verification.

Gafchromic EBT2 film dosimetry in reflection mode with a novel plan‐based calibration method

A dosimetric system formed by Gafchromic EBT2 radiochromic film and Epson Expression 10000XL flatbed scanner was commissioned for dosimetry. In this paper, several open questions concerning the commissioning of radiochromic films for dosimetry were addressed: (a) is it possible to employ this dosimetric system in reflection mode; (b) if so, can the methods used in transmission mode also be used in reflection mode; (c) is it possible to obtain accurate absolute dose measurements with Gafchromic EBT2 films; (d) which calibration method should be followed; (e) which calibration models should be used; and (f) does three-color channel dosimetry offer a significant improvement over single channel dosimetry. The purpose of this paper is to help clarify these questions. In this study, films were scanned in reflection mode, the effect of surrounding film was evaluated and the feasibility of EBT2 film dosimetry in reflection mode was studied. EBT2's response homogeneity has been reported to lead to excessive dose uncertainties. To overcome this problem, a new plan-based calibration method was implemented. Plan-based calibration can use every pixel and each of the three color channels of the scanned film to obtain the parameters of the calibration model. A model selection analysis was conducted to select lateral correction and sensitometric curve models. The commonly used calibration with fragments was compared with red-channel plan-based calibration and with three-channel plan-based calibration. No effect of surrounding film was found in this study. The film response inhomogeneity in EBT2 films was found to be important not only due to differences in the fog but also due to differences in sensitivity. The best results for lateral corrections were obtained using absolute corrections independent of the dose. With respect to the sensitometric curves, an empirical polynomial fit of order 4 was found to obtain results equivalent to a gamma-distributed single hit model based on physical assumptions. Three-channel dosimetry was found to be substantially superior to red-channel dosimetry. Reflection mode with Gafchromic EBT2 radiochromic film was found to be a viable alternative to transmission mode. The same methods that are used in transmission mode can be followed in reflection mode. A novel plan-based method was developed for calibration and multichannel dosimetry. This novel method offers increased robustness against film response inhomogeneities and reduces considerably the time required for calibration.

SU-GG-T-266: Impacts of the Sensitomertic Curve Depending on Scanning Parameters and Post-Operative Corrections on Radiochromic Film Dosimetry

Purpose: Impacts of sensitometric curve depending on scanning parameters and post‐operative processes to correct active layer and background effects on radiochromic film dosimetry were investigated to determine the key factors for more accurate dose verification. Method and Materials: Dose discrepancies were evaluated in the 60°‐enhanced dynamic wedged field and intensity‐modulated beams using the GafChromic EBT2 films and the software developed in‐house. Dosimetric effects were analyzed as the following conditions were applied: adjustment of the scanning contrast and brightness, background corrections in relative and absolute mode using non‐irradiated or irradiated film corresponding delivered dose levels, and active layer correction to lessen the thickness difference of the response material. Maximum dose and pass ratio applied the gamma criteria (distance‐to‐agreement of 3 mm and dose difference of 3%) were compared with planning system results in the wedged and the modulated field, respectively. Results: Absolute dose discrepancies of 3–6 %, pass ratio difference of 1–9% were generated by the effect of the applied sensitometric curve. Dose discrepancies were lower than 1% according to the correction methods of background or active layer correction. Slope variations of sensitometric curves led to different dose differences at each dose range. Thus under the same scanning conditions, lower pass ratio was represented in modulated beams, although the discrepancy at the point of maximum dose was decreased in wedged field. When the exposed film was used for background correction, dose differences in the portion of irradiated field were reduced, but the pass ratio was not improved because of the non‐uniform film response according to the dose levels. Conclusion: Determination of the sensitometric curve influenced by the scanning parameters was the primary factor to the dose differences. Under the selection of the sensitometric curve and post‐operative correction methods, dose discrepancy can be minimized for required radiochromic film dosimetry.

SU‐EE‐A2‐03: Evaluating AAPM TG‐43 In‐Water HDR 192Ir Brachytherapy Reference Dosimetry: A Comparison Study

Purpose: To investigate the accuracy of AAPM TG‐43 in HDR 192Ir brachytherapy in water reference dosimetry by comparing the protocol against ionometric and Gafchromic film calibration procedures introduced as well as a water calorimetry‐based primary standard. Methods and Materials: Dose to water Dwater was measured directly in water using an Exradin A1SL farmer‐type chamber and EBT‐1 Gafchromic films. The chamber had a NIST‐traceable 60Co calibration factor while the films were calibrated under 6 MV photons. Accurate Monte Carlo modeling and simulation of the chamber (egs++) and EBT Gafchromic films (DOSRZnrc) were performed to convert calibration factors of the two detectors from their respective conditions into 192Ir brachytherapy. The Dwater results were compared to measurements made using a Standard Imaging well‐type chamber following AAPM TG‐43 protocol and water calorimetry primary standard measurements. Results: By calculating the ratio of dose‐to‐water to dose‐to‐gas for the A1SL chamber under reference 60Co conditions and 192Ir setup conditions, the ionization measurements in 192Ir were converted to dose to water. The Monte Carlo calculations in film dosimetry revealed that if the intrinsic energy dependence of the film is negligible, a sensitometric curve obtained with 6 MV can be used in 192Ir measurements, with the energy dependence correction being 0.9971 (1σ=0.1%). The overall one‐sigma uncertainty on ionization chamber, Gafchromic film, and water calorimetry dose rate measurement amounts to 1.44%, 1.78%, and 1.96%, respectively. The indirect Dwater measurements from TG‐43 agreed to within 1.4% with ionometric measurements, 0.3% with Gafchromic measurements, and 0.6% with Calorimetric absolute dose measurements. Conclusions: Accurate ionometric and Gafchromic film based calibration protocols are introduced. For 192Ir brachytherapy, the 1‐sigma uncertainty of TG‐43 reference dosimetry was found to be better than 1.4%.

Using the “STEP Test” to evaluate processing speed in Moroccan hospitals

The STEP Test is a simple method that can be used to quickly evaluate processing speed of automatic processors. It allows the identification of large deviations (>20%) in the processor-chemicals-film system. STEP means Sensitometric Test for the Evaluation of Processing. The performance of the processors taking part in the survey are calculated and compared to a standard processor where the film is processed under ideal conditions, according to manufacturer's recommendations. For each processor, in the dark room, a film is exposed to a sensitometer light, processed and the sensitometric curve is obtained. This survey was performed in 8 Moroccan hospitals in the cities of Casablanca and Rabat and included 11 processors. It was concluded that 50% of the processors present processing problems and are functioning under inadequate conditions since they present deviations larger than 20%.

From the limits of the classical model of sensitometric curves to a realistic model based on the percolation theory for GafChromic™ EBT films

Modern radiotherapy uses complex treatments that necessitate more complex quality assurance procedures. As a continuous medium, GafChromic EBT films offer suitable features for such verification. However, its sensitometric curve is not fully understood in terms of classical theoretical models. In fact, measured optical densities and those predicted by the classical models differ significantly. This difference increases systematically with wider dose ranges. Thus, achieving the accuracy required for intensity-modulated radiotherapy (IMRT) by classical methods is not possible, plecluding their use. As a result, experimental parametrizations, such as polynomial fits, are replacing phenomenological expressions in modern investigations. This article focuses on identifying new theoretical ways to describe sensitometric curves and on evaluating the quality of fit for experimental data based on four proposed models. A whole mathematical formalism starting with a geometrical version of the classical theory is used to develop new expressions for the sensitometric curves. General results from the percolation theory are also used. A flat-bed-scanner-based method was chosen for the film analysis. Different tests were performed, such as consistency of the numeric results for the proposed model and double examination using data from independent researchers. Results show that the percolation-theory-based model provides the best theoretical explanation for the sensitometric behavior of GafChromic films. The different sizes of active centers or monomer crystals of the film are the basis of this model, allowing acquisition of information about the internal structure of the films. Values for the mean size of the active centers were obtained in accordance with technical specifications. In this model, the dynamics of the interaction between the active centers of GafChromic film and radiation is also characterized by means of its interaction cross-section value. The percolation model fulfills the accuracy requirements for quality-control procedures when large ranges of doses are used and offers a physical explanation for the film response.

SU-FF-J-148: Comparison of the Intratumoral Distribution of Targeted and Non-Targeted Cu-67-Carrying Composite Nanodevices (CNDs)

Purpose: To investigate the tumor distribution of angiogenic microvascular targeted and non‐targeted beta‐emitting Cu‐67‐carrying composite nanodevices (CNDs) using an in vivo mouse tumormodel.Method and Materials:Composite nanodevices (CNDs) are third‐generation “smart” nanoparticles synthesized from nanomaterials. We synthesized gold/dendrimer CND's (cRGD‐AuCND) with certain peptides molecules, which target the proteins expressed in tumor microvasculature. The same constructs, but without targeting molecules (AuCND) were used as control. Labeling was achieved by complexing known activities of the beta emitter Cu‐67 ions with the targeting and non‐targeting CNDs in solution. The solutions were introduced into a group of subcutaneous prostate‐tumor‐bearing mice via tail‐vein injections. Sections of the tumors were resected and exposed on autoradiographic film. Autoradiographs were also produced of an agarose gel standard containing various concentrations of the same Cu‐67 sample. These calibration films were digitized and analyzed using a threshold technique, and provided 32 regions of interest. Average pixel values of these regions were plotted versus the calculated number of beta events, and fit to a power law, to produce a sensitometric curve. Results: The regions of interest of the calibration films varied between a high signal of 57 and a low signal of 177, based on 8‐bit digital images.Images of tumor sections containing the targeted and non‐targeted CND were analyzed using the sensitometric curve. The average relative activity of beta decays corresponding to the targeted cRGD‐AuCND was 3.4 times higher than that of the non‐targeted AuCND. Conclusion: Based on analysis of autoradiographs, we have shown that targeted CND's are preferentially deposited in tumors. The relative activity of Cu‐67 in tumor tissue was a factor of 3–4 greater for the targeted cRGD‐AuCND than the non‐targeted AuCND. In combination with other tumorimage analysis techniques, these results will permit detailed quantitative intratumoral biodistribution and then microdosimetry of the nanodevices.

On the characterization and uncertainty analysis of radiochromic film dosimetry

Radiochromic film is a dosimeter of choice in applications requiring high spatial resolution, two dimensional measurements, or minimum perturbation of the beam fluence. Since the measurement uncertainty in Gafchromic film dosimetry is thought to be significant compared to that of ionization chambers, a rigorous method to evaluate measurement uncertainties is desired. This article provides a method that takes into account the correlation between fit parameters as well as single dose values in order to obtain accurate uncertainties in absolute and relative measurements. A complete portrait of all sources of uncertainty in Gafchromic film dosimetry is given. The parametrization of variance as a function of the number of averaged pixels is obtained in order to accurately predict the uncertainty as a function of the size of the region of interest. The choice of functional form for the sensitometric curve is based on four criteria and a convergence of global net optical density uncertainty to 0.0013 is demonstrated. A minimum number of 12 points is recommended to characterize the sensitometric curve to a sufficient precision on the uncertainty estimation. Uncertainty levels of 0.9% on absolute dose measurements and 0.45% on relative measurements are achieved using a 12-point calibration curve with 220 cGy and repeating measurements five times. Uncertainties of 0.8% and 0.4% are achievable when using 35 points during film characterization. Ignoring covariance terms is shown to lead to errors in the estimation of uncertainty.

Accurate dosimetry with GafChromic™ EBT film of a 6MV 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.

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

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

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, ∼1 Gy and ∼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 × 5 cm2, 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 × 3–40 × 40 cm2) 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.

SU-FF-T-397: Temporal Variation of Characteristic Curves of EDR2 Film Used for IMRT Quality Assurance

Routine IMRT QA is often performed using radiographic film in combination with a film analysis software package. In addition to differences in film batches, various factors could affect the film developing process such as temperature, transport rate, or replenishment rate. The conversion of the degree of blackening of the film, as measured by optical density, to dose is achieved by applying a sensitometric, or characteristic curve, to the film; thus converting it to a dose map. These curves need to be constructed at the beginning of the start of film IMRT QA. In order to account for processing variations, and film batch differences, our practice is to expose a single reference film to a known dose at the time of each IMRT QA and adjust the “baseline” film sensitometric curve accordingly; in effect assuming that the shape of the curve remains unchanged and it is only shifted by a fixed amount to a higher or lower dose. The validity of this assumption and the degree to which film processing can affect it are investigated here. We have analyzed the sensitometric curves and reference films used for adjusting the curves at two institutions over a three-year period. Our data show that sensitometrically-derived doses from the reference films vary from the delivered doses by up to 17% in one institution and 12% in another. Analysis of curves generated for different film batches, or over time, shows that this method of scaling the dose to shift the sensitometric curve results in an acceptable match when curves for the same film type are compared, validating our simple QA technique. Other factors, such as a good processor QA, may affect the results.

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.

The curvature of sensitometric curves for Kodak XV‐2 film irradiated with photon and electron beams

Sensitometric curves of Kodak XV-2 film, obtained in a time period of ten years with various types of equipment, have been analyzed both for photon and electron beams. The sensitometric slope in the dataset varies more than a factor of 2, which is attributed mainly to variations in developer conditions. In the literature, the single hit equation has been proposed as a model for the sensitometric curve, as with the parameters of the sensitivity and maximum optical density. In this work, the single hit equation has been translated into a polynomial like function as with the parameters of the sensitometric slope and curvature. The model has been applied to fit the sensitometric data. If the dataset is fitted for each single sensitometric curve separately, a large variation is observed for both fit parameters. When sensitometric curves are fitted simultaneously it appears that all curves can be fitted adequately with a sensitometric curvature that is related to the sensitometric slope. When fitting each curve separately, apparently measurement uncertainty hides this relation. This relation appears to be dependent only on the type of densitometer used. No significant differences between beam energies or beam modalities are observed. Using the intrinsic relation between slope and curvature in fitting sensitometric data, e.g., for pretreatment verification of intensity-modulated radiotherapy, will increase the accuracy of the sensitometric curve. A calibration at a single dose point, together with a predetermined densitometer-dependent parameter ODmax will be adequate to find the actual relation between optical density and dose.

Designing Camera Origination Films for Scan-Only Applications in Television and Digital Intermediate

With the growing popularity of digital intermediate in the feature film market and the continuing development of improved telecine technologies for television, the notion of traditional camera origination film design optimized around direct optical or contact printing is radically challenged. For scan-only film image capture systems where the camera film will be scanned exclusively to video or data and never directly printed, a very different approach is taken to sensitometric curve design. Rather than focusing on tonal characteristics, which yield acceptable and preferred results for overall contrast, grayscale neutrality, and shadow and highlight detail in a traditional printing paradigm, the film curves must instead be drawn to optimize the quality of signal provided to the digitization step in the film scanner. Further, standard constructs of color reproduction are also open for modification as the film's spectral sensitivity and color-enhancing chemistries are changed to better mesh with digital color-processing capabilities.

Dosimetric accuracy of Kodak EDR2 film for IMRT verifications

Patient-specific intensity-modulated radiotherapy (IMRT) verifications require an accurate two-dimensional dosimeter that is not labor-intensive. We assessed the precision and reproducibility of film calibrations over time, measured the elemental composition of the film, measured the intermittency effect, and measured the dosimetric accuracy and reproducibility of calibrated Kodak EDR2 film for single-beam verifications in a solid water phantom and for full-plan verifications in a Rexolite phantom. Repeated measurements of the film sensitometric curve in a single experiment yielded overall uncertainties in dose of 2.1% local and 0.8% relative to 300 cGy. 547 film calibrations over an 18-month period, exposed to a range of doses from 0 to a maximum of 240 MU or 360 MU and using 6 MV or 18 MV energies, had optical density (OD) standard deviations that were 7%-15% of their average values. This indicates that daily film calibrations are essential when EDR2 film is used to obtain absolute dose results. An elemental analysis of EDR2 film revealed that it contains 60% as much silver and 20% as much bromine as Kodak XV2 film. EDR2 film also has an unusual 1.69:1 silver:halide molar ratio, compared with the XV2 film's 1.02:1 ratio, which may affect its chemical reactions. To test EDR2's intermittency effect, the OD generated by a single 300 MU exposure was compared to the ODs generated by exposing the film 1 MU, 2 MU, and 4 MU at a time to a total of 300 MU. An ion chamber recorded the relative dose of all intermittency measurements to account for machine output variations. Using small MU bursts to expose the film resulted in delivery times of 4 to 14 minutes and lowered the film's OD by approximately 2% for both 6 and 18 MV beams. This effect may result in EDR2 film underestimating absolute doses for patient verifications that require long delivery times. After using a calibration to convert EDR2 film's OD to dose values, film measurements agreed within 2% relative difference and 2 mm criteria to ion chamber measurements for both sliding window and step-and-shoot fluence map verifications. Calibrated film results agreed with ion chamber measurements to within 5 % /2 mm criteria for transverse-plane full-plan verifications, but were consistently low. When properly calibrated, EDR2 film can be an adequate two-dimensional dosimeter for IMRT verifications, although it may underestimate doses in regions with long exposure times.