Net Optical Density Research Articles

Poster - Thur Eve - 01: Development of simple and fast EBT2 film calibration procedure using PDD table.

Standard calibration procedure for EBT films is laborious and time-consuming. The objective of this work was to develop a simple and fast approach of EBT2 film calibration using PDD tables. EBT2 sheet is cut into 3 stripes of 5×25.5cm2 . The strips were exposed to dose of 600, 200 and 70cGy at dmax each while placed horizontally in the middle of a 30×30×30cm3 solid water phantom. Varian 21EX 6MV 10×10cm2 beam was used with the gantry rotated to 90° and SSD of 100cm to the phantom surface. After at least 24 hours, the films were digitized with flatbed scanner (Epson10000XL), according to a modified ISP scanning protocol. All images were analysed using an in-house Matlab code and ImageJ software. The net-optical densities against depths in the solid phantom were calibrated using PDD tables measured with ionization chamber for same machine. For verification, another calibration curve was generated for the same film batch following the same calibration protocol. Seven pieces of films were exposed to known doses and these doses were reconstructed using two derived calibration curves. The proposed approach was 3.6 times faster than the standard considering the number of films used in each methods, 3 stripes compared to11 pieces. The mean relative dose difference calculated for these films using the PDD calibration and the standard methods was 1.0±1.2% and 0.5±2.2% with maximum relative differences of 3.0% and 4.7% respectively. Our results show that PDD calibration approach is much easier, faster and predicts dose more reproducibly and accurately than the standard approach.

SU-D-BRCD-04: Validation of Film Dosimetry in the Dose Build-Up Region.

To establish a film dosimetric method for high-resolution measurement in the dose build-up region. Percent depth dose (PDD) curves were measured in water for TomoTherapy using Gafchromic EBT2 films. Depth dose measurements were also performed using Standard Imaging A1SL and PTW PinPoint ionization chambers, as well as a PTW DiodeE detector. The film and detector measurements were then compared to Monte Carlo (MC) simulation data computed using PENELOPE. An in- house support apparatus was constructed to hold the films parallel to the beam central axis while being suspended in the water, simultaneously demarcating the water surface on the film. The films were converted to dose using a corrected net optical density method involving the red and blue color channels of an Epson Expression 10000XL scanner. Film analysis was performed using ImageJ software and MATLAB code developed in clinic. In our film method, a systematic shift of 1.2mm downstream is seen for the sample of five films, with good reproducibility within the sample (s=0.3mm). The film measurements showed a mean PDD difference of 0.3% (s=0.4%) with a maximum of 1.3% from that of MC between depths of 0.5mm to 20mm. The average film dose measured at the water surface (d=0mm) was 13.1% greater than that calculated by MC. EBT2 film shows much better agreement with MC in the dose build-up region than the ionization chamber and detector measurements. This study demonstrates the capability of EBT2 films for simple and accurate superficial dose measurements. A suspected reason for the systematic shift in film alignment is attributed to difficulty in determining water surface due to the meniscus that forms at the film. Funded by SSRMP Research Grant 2011.

Variations in dose distribution and optical properties of GafchromicTM EBT2 film according to scanning mode

The authors aim was to investigate the effects of using transmission and reflection scanning modes, the film orientation during scanning, and ambient room light on a dosimetry system based on the Gafchromic(TM) EBT2 film model. For calibration, the films were cut to 3 × 3 cm(2) and irradiated from 20 to 700 cGy at the depth of maximum dose using 6 and 10 MV photon beams in a 10 × 10 cm(2) field size. Absolute dose calibration of the linear accelerator was done according to the TRS398 protocol. An FG65-G ionization chamber was used to monitor the dose while irradiating the films in solid water. The film pieces were scanned with an EPSON Expression 1680 Pro flatbed scanner in transmission and reflection modes. Authors investigated the effect of orientation on films and examined the optical properties of EBT2 film using an ellipsometer and an ultraviolet (UV)/visible spectrometer to explain the dosimetric dependence of the film on orientation during the scanning process. To investigate the effect of ambient room light, films were preirradiated in 6 and 10 MV photon beams with intensity-modulated radiotherapy (IMRT) quality assurance (QA) plans, and then exposed to room light, either directly for 2 days in a workroom or for 2 months in a film box. Gamma index pass criteria of (3%, 3 mm) were used. The dose response curves based on net optical density (NOD) indicated that the reflection scanning mode can provide a better dose sensitivity than the transmission scanning mode, whereas the standard deviation of the dose is greater in reflection mode than in transmission mode. When the film was rotated 90° from the portrait orientation, the average dose of the EBT2 film decreased by 11.5-19.6% in transmission mode and by 1.5-2.3% in reflection mode. Using an ellipsometer, variation of the refractive index of EBT2 film-the birefringence property-was found to be the largest between 45° (1.72 and 1.71) and 135° (1.8 and 1.77) for 300 and 800 cGy. Absorption spectra of EBT2 films measured with spectrometer were the function of film orientation. The readings in reflection scanning mode were more stable against room light than those in transmission scanning mode, although dose readings increased in both modes after the films were exposed to room light. The transmission scanning mode exhibited a strong dependence on film orientation during scanning and a change in optical density resulting from room light exposure, so a constant scanning orientation and minimal exposure to light can reduce uncertainty in the measured dose (23 ± 3%). The angular dependence was analyzed using Jones matrices and optical properties of EBT2 film were obtained using an ellipsometer and an UV/visible spectrometer. The reflection scanning mode has relatively good stability with respect to room light and film orientation on a scanner, although the large standard deviation of dose is a disadvantage in measurements of absolute dose. Reflection scanning mode can offer a potential advantage for film dosimetry in radiotherapy, although transmission scanning mode is still recommended for dosimetry as it provides better uncertainty results.

Characteristics of Gafchromic XRQA2 films for kV image dose measurement

In this study, the relevant characteristics of the new Gafchromic XRQA2 film for its application in measuring kV cone beam computed tomography (CBCT) image doses were thoroughly investigated. The film was calibrated free in air to air kerma levels between 0 and 9 cGy using 120 kVp photon beams produced by the x-ray volume imager. Films were scanned using transmission and reflection scanning modes with the Epson Expression 10000 XL flat-bed document scanner. The impact of film size, region of interest for the analysis, scan uniformity, scan resolution, scan orientation and alternate scanning sides on the analysis process were investigated. Energy dependence, postirradiation growth of reflectance with time and irradiation angular dependence of the film were tested at different air kerma levels. The net reflectance changed by ∼3% when the size of the film piece changed from 1 cm × 2 cm to 10 cm × 11 cm and changed by ∼1% when ROI changed from 0. 7 cm × 0. 7 cm to 8 cm × 8 cm, suggesting a good uniformity of the film. The film was successfully analyzed using the transmission scanning mode, calibration curves from both transmission and reflection scanning modes showed similar behavior. The calibration uncertainty was somewhat lower when the film was scanned using reflection mode (6% and 8% for reflection and transmission modes, respectively.) Higher scanning resolution came with increasing calibration uncertainty. The calibration uncertainty for reflection and transmission modes increased from ∼3.5% to 7% and from ∼3.5% to 9%, respectively when scanning resolution was changed from 50 to 400 dpi. Scanning the film on alternate sides using transmission mode led to variation of 16%-19% in the net optical density at doses commonly used for CBCT procedures. The film response changed by almost 10% when it was exposed to beams of two different energies (100 and 120 kVp.) Other features of the film such as film orientation, postexposure growth, and irradiation angular dependence were also investigated. The size of film piece and analysis ROI used for calibration slightly affected the film response. Both transmission and reflection scanning modes can be used to analyze the Gafchromic XRQA2, with the reflection mode having a somewhat lower calibration uncertainty. Scanning films on alternate sides using transmission mode significantly affects the optical density. The film response was shown to be energy dependent. The films reached stability in about 6 h after exposure. The film response was proven to be independent of irradiation angle except when the beam is parallel to the film surface.

Energy Dependence of the New Gafchromic EBT3 Film:Dose Response Curves for 50 KV, 6 and 15 MV X-Ray Beams

Purpose: To study and compare the dose response curves of the new GafChromic EBT3 film for megavoltage and kilovoltage x-ray beams, with different spatial resolutions. Methods: EBT3 films (lot#A101711-02) were exposed to each x-ray beam (6 MV, 15 MV, and 50 kV) at 7 dose values (50-3200 cGy). Each film piece was scanned three consecutive times in the center of Epson 10000XL flatbed scanner in 48-bit color at two separate spatial resolutions of 75 and 300 dpi. The data were analyzed using ImageJ and, for each scanned image, a region of interest (ROI) of 2 × 2 cm2 at the field center was selected to obtain the mean pixel value with its standard deviation in the ROI. For each energy, dose value and spatial resolution, the average net optical density (netOD) and its associated uncertainty were determined. The Student’s t-test was performed to evaluate the statistical differences between the net OD/dose values of the three energy modalities, with different color channels and spatial resolutions. Results and Discussion: The dose response curves for the three energy modalities were compared in three color channels. Weak energy dependence was found. For doses above 100 cGy, no statistical differences were observed between 6 and 15 MV beams, regardless of spatial resolution and color channel. However, statistical differences were observed between 50 kV and the megavoltage beams. The degree of energy dependence (from MV to 50 kV) was found to be a function of color channel, dose level, and spatial resolution. Conclusions: The dose response curves for GafChromic EBT3 films were found to be weakly dependent on the energy of the photon beams from 6 MV to 15 MV. For very low energy photon (e.g. 50 kV), variation of more than 11% due to the energy-dependence is observed, depending on the absorbed dose, spatial resolution and color channel used.

Characterizing the marker-dye correction for Gafchromic® EBT2 film: A comparison of three analysis methods

Gafchromic(®) EBT2 film has a yellow marker dye incorporated into the active layer of the film that can be used to correct the film response for small variations in thickness. This work characterizes the effect of the marker-dye correction on the uniformity and uncertainty of dose measurements with EBT2 film. The effect of variations in time postexposure on the uniformity of EBT2 is also investigated. EBT2 films were used to measure the flatness of a (60)Co field to provide a high-spatial resolution evaluation of the film uniformity. As a reference, the flatness of the (60)Co field was also measured with Kodak EDR2 films. The EBT2 films were digitized with a flatbed document scanner 24, 48, and 72 h postexposure, and the images were analyzed using three methods: (1) the manufacturer-recommended marker-dye correction, (2) an in-house marker-dye correction, and (3) a net optical density (OD) measurement in the red color channel. The field flatness was calculated from orthogonal profiles through the center of the field using each analysis method, and the results were compared with the EDR2 measurements. Uncertainty was propagated through a dose calculation for each analysis method. The change in the measured field flatness for increasing times postexposure was also determined. Both marker-dye correction methods improved the field flatness measured with EBT2 film relative to the net OD method, with a maximum improvement of 1% using the manufacturer-recommended correction. However, the manufacturer-recommended correction also resulted in a dose uncertainty an order of magnitude greater than the other two methods. The in-house marker-dye correction lowered the dose uncertainty relative to the net OD method. The measured field flatness did not exhibit any unidirectional change with increasing time postexposure and showed a maximum change of 0.3%. The marker dye in EBT2 can be used to improve the response uniformity of the film. Depending on the film analysis method used, however, application of a marker-dye correction can improve or degrade the dose uncertainty relative to the net OD method. The uniformity of EBT2 was found to be independent of the time postexposure.

SU‐E‐T‐165: Systematic Evaluation of Uncertainties Associated with GAFCHROMIC EBT2 Film Dosimetry for 6MV Photon Beams

Method and Materials:Eight packets of films were exposed to 13.5cm ×13.5cm, 6MV radiation fields in a solid water phantom. Dose levels of 1.1, 3.2, 5.3, 7.4, and 9.5 Gy were delivered to five films in each packet. Films were scanned both before and after irradiation using an Epson flat‐bed scanner (24hr wait‐time for post‐irradiation coloration). Corresponding 2D dose distributions were measured with a detector‐array (MatriXX). Point dose comparisons were performed with an ion chamber. Digitized film images were registered to the 2D dose distribution to generate a correction map that compensated the scanner non‐uniform response as a function of dose. Optical density (OD) and net optical density (NetOD) values were calculated for all images. Dose response curves were established using mean values of a central 0.5cm × 0.5cm region‐of‐interest (ROI). Images were converted to dose, and error uncertainties (1SD) were measured in the central 8cm × 8cm ROI. Results: The overall dosimetric uncertainties (1SD) of the NetOD approach were 2.2%, 1.9%, and 3.5% for red, green, and blue channels, respectively. The corresponding uncertainties of OD were 2.7%, 3.1%, and 8.3%, respectively. For low dose range (<3 Gy), the green channel revealed higher uncertainty (SDgreen= 3.3%) than the red channel (SDred=2.6%). However, for high doses (3∼9 Gy), the green channel showed less variability (SDgreen=1.6%, SDred=2.9%). Minimum SDred and SDgreen were 1.6% at 5.3Gy and 1.3% at 7.4 Gy, respectively. Scanner non‐uniformity correction mitigated the irregular response of scanner detector elements observed initially. Conclusion: NetOD may be a more useful metric for benchmarking EBT2 than OD. We demonstrated that the lowest dose uncertainties were achieved using the red channel for low dose range, while the green channel was preferred for higher doses. Scanner non‐uniformity correction is necessary for higher precision dosimetry.

SU-E-T-128: Characterizing the Uniformity of Gafchromic EBT2 Film Response in a Cobalt-60 Field

Purpose: The goal of this work is to evaluate the uniformity of International Specialty Products Gafchromic EBT2 film with a high-resolution metric. The effect on uniformity of the marker dye (blue-channel) correction and time post-exposure are investigated. Methods: Film uniformity was quantified by using EBT2 and Kodak EDR2 film to measure the flatness of a cobalt-60 field. EBT2 films were digitized 24, 48, and 72 hours post-exposure and analyzed using three Methods: the manufacturer-recommended blue-channel correction, an in-house blue-channel correction, and a net optical density (OD) measurement in the red color channel. The field flatness was calculated from the horizontal and vertical dose profiles taken through the center of the cobalt-60 field for each analysis method and compared to the EDR2 results. Results: The blue-channel correction methods improved the flatness measured with EBT2 film by up to 1% over the net OD method. Additionally, the percent standard deviations of the flatness measurements with the blue-channel correction methods were up to 0.7% (1-sigma) lower than the net OD method. The flatness measured with EDR2 film was 1.4% to 1.8% better than the flatness measured with EBT2 film using the net OD method. The flatness did not exhibit any unidirectional change with time and showed a maximum change of 0.4%. Conclusions: Using field flatness as a metric for response uniformity, EBT2 film analyzed with the net OD method demonstrates substantial non-uniformities relative to EDR2 film. The blue-channel correction considerably improves the uniformity of EBT2 response and reduces the film-to-film response variation relative to the net OD method. The uniformity of EBT2 is independent of post-exposure time. The blue-channel correction can be used to reduce uncertainty in relative dosimetric measurements with EBT2 film.

SU-E-T-120: Characterization of EBT2 Film for Absolute Dosimetry in Proton Beams

Purpose: The objective of this work is to fully characterize Gafchromic EBT2 film for absolute dose measurement (in‐vitro and in‐vivo) in the spread out Bragg peak (SOBP) of both scanned and scattered proton beams. Methods and Materials: The OD‐vs‐dose calibration curve is determined by irradiating film at different depths (0, 5, 10, 15, 20, and 25 cm) to different dose in a calibration field (range 28 g/cm2, modulation 15 g/cm2). Solid water slabs are used as phantom; 4x5 cm films are irradiated to nine dose levels spanning from 10 cGy to 1000 cGy. Beam output is measured using parallel plate ionization chamber both to calculate the specific doses delivered to the film and to compare the measured dose with the calculated dose from the film. The films are digitized with EPSON scanner and the net optical density computed. Measurement done at different depths (different average energy) are combined to create a single calibration curve. Verification measurements are performed for different range and modulation combinations, at various depths, and using both double‐scattering and uniform‐scanning delivery, thus validating the use of single sensitometric plot for different proton energy and delivery technique. In addition, reproducibility, angular response, and film uniformity are investigated. Results: Using the single calibration curve, the accuracy of absolute dose measurements using EBT2 film is found to be within 5% for any point within the SOBP excluding the distal fall off region and at very low dose level (<15cGy). No difference in response to scattering or scanning beams is observed. The day‐to‐day reproducibility is better than 1%. The uncertainty introduced by angular response, film nonuniformity and different delivery techniques are found to be insignificant. Conclusions: EBT2 film is a viable option for absolute dose measurements both for in‐vitro and in‐vivo.

A measure for quantifying the radiopacity of restorative resins

To evaluate the utility of the X-ray attenuation coefficient as a measure for quantifying the radiopacity of restorative resins at different exposure times and film speeds. Five restorative resins were made into disks, measuring 10 mm in diameter with thicknesses of 0.5, 1.0, 1.5, 2.0, and 2.5 mm. These resin disks, an aluminum step wedge, and a lead disk were placed on size 4 Ultraspeed® D and Insight® E/F films. X-ray parameters were 65 kVp, 10 mA, and 30-cm focus-to-film distance. Exposure times were 0.3, 0.15, and 0.10 s for the D-speed films and 0.15, 0.10, and 0.05 s for the E/F-speed films. At each exposure setting, three D- and E/F-speed films were exposed and processed immediately with an automatic processor. The optical density was measured with a transmission densitometer. The net optical density (D) was used to calculate the linear attenuation coefficients (μ) using lnD = −μ x + lnD 0, where D and D 0 denote the optical density of the specimen and background, respectively. The linear attenuation coefficients (mm−1) of the five restorative resins were 0.24–0.27 for Sorare®, 0.30–0.34 for Estelite®, 0.36–0.39 for Gradia®, 0.51–0.54 for Clearfil AP-X®, and 0.52–0.56 for Beautifil®. These were all higher than that of dentin (0.15–0.19). There was no significant difference in the attenuation coefficients at different exposure times or film speeds. Attenuation coefficients can be used instead of aluminum wedges of equivalent thickness to quantify the radiopacity of restorative resins.

Poster — Thur Eve — 06: Radiochromic Film Densitometry with Vista15 Optical Cone Beam CT Scanner

Purpose: To examine the performance of a scatter corrected commercial optical cone beam CT scanner, Vista15™, adapted for radiochromic film dosimetry. Methods: A slotted film mask adjacent to the EBT2 film was used to generate alternating open and dark areas. Three images were acquired with the mask centered along the optical axis and offset +/−0.7cm. The masked transmission images were processed by first forming composite images from the open and shadow areas of the three image sets. The composite shadow image was subtracted from the corresponding open image to create a “glare‐free image”. Net optical density (OD) images were generated by calculating −log10 (corrected post/corrected pre image). The calibration dose response in EBT2 film using multiple 4MV photon fields and a single 12MeV electron field was measured. Results: In the corrected data, net OD increased and the buildup region mirrored actual 12MeV dose deposition in water. This trend was also observed in the multiple 4MV photon field calibration data. From comparing the two calibration techniques, a discrepancy in net OD of up to 4% was observed. This suggests that EBT2 film users should have a consistent method for calibrating the dose response. Conclusions: The emission spectrum of Vista15™ designed for readout of radiochromic gels and plastics overlaps the absorption peak in EBT2 films, thus, making this scanner convenient and economical for both planar and 3D dosimetry. A method for veiling glare correction is promising but a more complex multiple grid approach will be needed in order to achieve quantitative densitometry.

Technical Note: Homogeneity of Gafchromic®EBT2 film

The self-developing Gafchromic EBT film is a radiochromic film, widely used for relative photon dosimetry. Recently, the manufacturer has replaced the well-investigated EBT film by the new Gafchromic EBT2 film. It has the same sensitive component and, in addition, it contains a yellow marker dye in order to protect the film against ambient light exposure and to serve as a base for corrections of small differences in film response. Furthermore, the configuration of the film layers as well as the binder material have been changed in comparison to the EBT film. When investigating the properties of EBT2 film, all characteristics were found to be similar to those of EBT film, except for the film response homogeneity. Thus, in this article special focus was put on examining the homogeneity of EBT2 film.A scan protocol established for EBT film and published previously was used. The uniformity of the film coloration was investigated for unirradiated and irradiated EBT2 film sheets. The dose response of EBT2 film was measured and the influence of film inhomogeneities on dose determination was evaluated.Inhomogeneities in pixel values of up to +/- 3.7% within one film were detected. The relative inhomogeneities were found to be approximately independent of the dose. Nonuniformities of the film response lead to uncertainties in dose determination of +/- 8.7% at 1 Gy. When using net optical densities for dose calibration, uncertainties in dose determination amount to more than +/- 6%.EBT2 films from the lot investigated in this study show response inhomogeneities, which lead to uncertainties in dose determination exceeding the commonly accepted tolerance levels. It is important to test further EBT2 lots regarding homogeneity before using the film in clinical routine.

Energy response of the new EBT2 radiochromic film to x-ray radiation

Gafchromic EBT2, Radiochromic film is assessed for its change in optical density response to x-ray radiation over a broad energy range, from low energy kilovoltage to megavoltage x-rays. A small energy dependence was found with variations in the change in optical density when scanned in the red component of a desktop scanner light source per unit dose of 6.5% from 50 kVp to 10 MV. This produces a slightly smaller and thus even more energy independent film than its predecessor, EBT film whose response varied by 7.7% over the same energy range. The energy response peaked at 100 kVp with a 5% over response compared to 6 MV x-rays and the minimum response found at both 50 kVp and 250 kVp being a 1.5% under response. It should be noted that the shape of the energy dependence response curve increases from 50 kVp up to 100 kVp followed by a decrease through to higher energies whilst the original EBT was found to increase in response from 50 kVp through to 10 MV. A reflected net optical density change of 0.215 ± 0.006 OD for the first Gray of radiation was found for EBT2 analysed in reflection mode at 6 MV x-ray energy. The minimal energy dependence of the EBT2 film provides further enhancement compared to EBT for its accuracy with respect to spectral changes in the beam to measure beams such as IMRT where complex field and multileaf collimator configurations exist causing small spectral changes to occur over the treatment field or at depth where spectral changes also occur.

Investigation of energy dependence of EBT and EBT‐2 Gafchromic film

The purpose of this study was to quantify the extent of energy dependence of Gafchromic film to x-ray energies ranging in quality from 105 kVp to 6 MV, and relate this dependency to the film's chemical composition and date of production. Lots of Gafchromic EBT film manufactured in 2004 and 2005 together with more recent batches produced in 2007 were evaluated for energy dependence. Multiple batches of EBT-2 film were also evaluated. Energy dependence was quantified as Rx-the ratio of net optical density (netOD) measured at a given energy x relative to the netOD measured at 6 MV, as measured on a linear accelerator. Rx was evaluated for beam qualities of 105 and 220 kVp on a clinical orthovoltage unit using two separate techniques-a flatbed scanner (Epson) and a real-time fiber-optic readout system. Neutron activation analysis for chlorine and bromine content was performed on all the films to determine whether the composition of the film had changed between batches of film exhibiting different energy dependence responses. For batches of EBT manufactured in 2007, R105 kVp was 0.75 and R220 kVp was 0.85, indicating an under-response at orthovoltage energies. These results were confirmed using both the Epson flatbed scanner as well as the real-time readout system. For batches of EBT film manufactured before 2006, Rl05 kVp ranged from 0.9 to 1.0. The results from the neutron activation analysis confirmed a direct relationship between the concentration of chlorine and the magnitude of under-response at orthovoltage energies. EBT-2 film exhibited R105 kVp values ranging from 0.79 (under-response) to 1.20 (over-response) among batches containing varying concentrations of bromine, chlorine, and potassium. The results of this study indicated that differences in energy response of EBT and EBT-2 films were due to differences in the chemical composition and therefore the effective atomic number of the film, which have changed over time. To achieve an energy independent dosimeter over a range of kilovoltage energies, the effective atomic number of the dosimeter must be closely matched to that of water. Small deviations in chemical composition can lead to large deviations in response as a function of energy.

Use of the Normalized Absorbance Ratio as an Internal Standardization Approach To Minimize Measurement Error in Enzyme-Linked Immunosorbent Assays for Diagnosis of Human Papillomavirus Infection

The serological detection of antibodies against human papillomavirus (HPV) antigens is a useful tool to determine exposure to genital HPV infection and in predicting the risk of infection persistence and associated lesions. Enzyme-linked immunosorbent assays (ELISAs) are commonly used for seroepidemiological studies of HPV infection but are not standardized. Intra- and interassay performance variation is difficult to control, especially in cohort studies that require the testing of specimens over extended periods. We propose the use of normalized absorbance ratios (NARs) as a standardization procedure to control for such variations and minimize measurement error. We compared NAR and ELISA optical density (OD) values for the strength of the correlation between serological results for paired visits 4 months apart and HPV-16 DNA positivity in cervical specimens from a cohort investigation of 2,048 women tested with an ELISA using HPV-16 virus-like particles. NARs were calculated by dividing the mean blank-subtracted (net) ODs by the equivalent values of a control serum pool included in the same plate in triplicate, using different dilutions. Stronger correlations were observed with NAR values than with net ODs at every dilution, with an overall reduction in nonexplained regression variability of 39%. Using logistic regression, the ranges of odds ratios of HPV-16 DNA positivity contrasting upper and lower quintiles at different dilutions and their averages were 4.73 to 5.47 for NARs and 2.78 to 3.28 for net ODs, with corresponding significant improvements in seroreactivity-risk trends across quintiles when NARs were used. The NAR standardization is a simple procedure to reduce measurement error in seroepidemiological studies of HPV infection.

Dose and absorption spectra response of EBT2 Gafchromic film to high energy x-rays

With new advancements in radiochromic film designs and sensitivity to suit different niche applications, EBT2 is the latest offering for the megavoltage radiotherapy market. New construction specifications including different physical construction and the use of a yellow coloured dye has provided the next generation radiochromic film for therapy applications. The film utilises the same active chemical for radiation measurement as its predecessor, EBT Gafchromic. Measurements have been performed using photo spectrometers to analyse the absorption spectra properties of this new EBT2 Gafchromic, radiochromic film. Results have shown that whilst the physical coloration or absorption spectra of the film, which turns yellow to green as compared to EBT film, (clear to blue) is significantly different due to the added yellow dye, the net change in absorption spectra properties for EBT2 are similar to the original EBT film. Absorption peaks are still located at 636nm and 585nm positions. A net optical density change of 0.590 +/- 0.020 (2SD) for a 1 Gy radiation absorbed dose using 6 MV x-rays when measured at the 636nm absorption peak was found. This is compared to 0.602 +/- 0.025 (2SD) for the original EBT film (2005 Batch) and 0.557 +/- 0.027 (2009 Batch) at the same absorption peak. The yellow dye and the new coating material produce a significantly different visible absorption spectra results for the EBT2 film compared to EBT at wavelengths especially below approximately 550nm. At wavelengths above 550nm differences in absolute OD are seen however, when dose analysis is performed at wavelengths above 550nm using net optical density changes, no significant variations are seen. If comparing results of the late production EBT to new production EBT2 film, net optical density variations of approximately 10 % to 15 % are seen. As all new film batches should be calibrated for sensitivity upon arrival this should not be of concern.

Energy dependence of EBT-1 radiochromic film response for photon (10kVp-15MVp) and electron beams (6-18MeV) readout by a flatbed scanner.

The aim of this article is to investigate the energy dependence of the radiochromic film type, Gafchromic™ EBT-1, when scanned with a flatbed scanner for film readout. Dose response curves were determined for 12 different beam qualities ranging from a10kVp x-ray beam to a 15MVp x-ray beam and include also two high energy electron beam qualities (6 and 18MeV). The dose responses measured as net optical density (netOD) for the different beam qualities were normalized to the response of a reference beam quality (6MVp). A strong systematic energy dependence of the film response was found. The lower the effective beam energy, the less sensitive the EBT-1 films get. The maximum decrease in dose for the same film response between the25kVp and 6MVp beam qualities was 44%. Additionally, a difference in energy dependence for different doses was discovered, meaning that higher doses show a smaller dependency on energy than lower doses. The maximum decrease in the normalized netOD was found to be 25% for a dose of 0.5Gy relative to the normalized netOD for 10Gy. Moreover, a scaling procedure is introduced, allowing the correction of the energy dependence for the investigated beam qualities and also for comparable x-ray beam qualities within the energy range studied. A strong energy dependence for EBT-1 radiochromic films was found. The films were readout with a flatbed scanner. If the effective beam energy is known, the energy dependence can be corrected with the introduced scaling procedure. Further investigation of the influence of the spectral band of the readout device on energy dependence is needed to understand the reason for the different energy dependences found in this and previous works.

MO‐FF‐A1‐03: Investigation of Discrepancy in Reports On GafChromic EBT Energy Dependence

Purpose: Radiochromic films, such as GafChromic EBT, provide a convenient and versatile dosimeter, and are particularly useful for small field dosimetry. Due to previous reports showing energy independence, GafChromic EBT films were implemented as dosimeters across a wide range of energies. However, in recent years, reports showing energy dependence have been presented, contradicting previously published data. This discrepancy was investigated by comparing two batches of film manufactured 4 years ago to two recently manufactured batches. Method and Materials: Small pieces of film were irradiated to 100 cGy using a 6 MV and a 105 kVp beams. A 30 cm × 30 cm × 4.5 cm phantom equipped with optical fibers was used, with appropriate amount of Solid Water as backscatter. Net optical density was measured by two completely independent methods: a 10 nm range at main absorbance peak was used for real‐time measurements, and a red channel of an Epson 1680 flatbed scanner was used for the measurements performed 24 hours after irradiation. Results: The ratios of optical density (105 kVp to 6 MV) were markedly different between the old and new batches of film, both for real‐time and scanner measurements. While the ratio of the older films demonstrated energy independent within 10%, as previously reported, the newer films show approximately a 25% drop in response when orthovoltage energies were used. Conclusion: A statistically significant difference in energy dependence exists between older and newer GafChromic EBT films, confirming both the publications demonstrating energy independence and those showing a decrease in sensitivity at low energies. Chemical analysis is underway to investigate the potential causes of the observed results.

SU‐FF‐T‐361: Dose Response Characteristics of the New EBT‐2 Film for Different Megavoltage Beams

Purpose: We studied the dose response characteristics of the newly introduced EBT‐2 radiochromic films for megavoltage photon and electron beams of different energies. Method and Materials: EBT‐2 films (ISP prototype lot♯020609), 2.5×3.2cm2, were individually irradiated by 6MV and 15MV photons, and 6MeV and 20MeV electrons (Varian Cl‐iX linac) at dmax and the center of 10×10cm2 field size in a Solid Water phantom (RMI457). The dose range was up to 50 Gy for 6MV photon and 20 Gy for the other three energy modalities. The dose was calibrated against an ion chamber with ADCL calibration. All the films were scanned using Epson 4870 and 10000XL flatbed scanners with 48 bit color (RGB) at least one day after irradiation. The pixel values in a central region of 0.5×0.5cm2 in each film were sampled and the mean value was assigned to each film. The optical density and net optical density (NOD) values for each film were determined from the mean pixel value. Results: The NOD values for the red, green and blue channels were plotted against the doses for each energy modality. The curves from the Epson 10000XL and 4870 scanners were also compared. The dose response curves of the new EBT‐2 films were found to be quite similar for the four megavoltage beams and similar to those of EBT film. For dose of 20 Gy, the NOD values from Epson 10000XL were about 1.6, 1.4 and 0.8 for red, green, and blue channels, respectively. Conclusion: The dose response characteristics of the newly introduced EBT‐2 films have been studied for four megavoltage photon and electron beams, and found to be similar to the predecessor, EBT films. The new EBT‐2 films are expected to be feasible for 2D dosimetry applications. Conflict of Interest: Prototype EBT‐2 films were provided by the International Specialty Products.

TU-D-BRB-01: Comparison of the Response of EBT Emulsion to 90Sr/Y Beta Particles and 60Co Gamma Rays

Purpose: To test the hypothesis that GAFCHROMIC EBT emulsion responds the same to beta particles from 90 Sr / 90 Y and gamma rays from 60 Co . Method and Materials: Single emulsion layer EBT film with minimal covering layer was obtained and 1 cm2 pieces were irradiated, 1) with the center of the film emulsion at a depth of 7 mg/cm2 in a 90 Sr / 90 Y reference radiation field of known absorbed dose rate (0.0911 mGy/s), or 2) with the films located at a depth of 5 cm in water in a 60 Co gamma ray beam of known absorbed dose rate (2.67 mGy/s). Six samples were irradiated in each radiation field at 15 logarithmically evenly spaced absorbed dose levels ranging from 30 mGy to 7 Gy. For the films irradiated in water, a commercially available food sealer was used to vacuum seal films in water‐proof packs which were held perpendicular to the beam axis with a spring‐loaded mounting jig. Irradiated films were read out between 6 and 9 days post irradiation in a 48‐bit color photo scanner and the red component of the TIFF image data was extracted for analysis of the average optical density. Results: The average net optical density change per unit of absorbed dose delivered was evaluated for each of the 15 dose levels used in the study. Using the uncertainty in the delivered absorbed dose for each radiation (1.5% for 90 Sr / 90 Y and 1% for 60 Co , both 1σ) and the statistics of the six film replicates, a two‐sided Student's t test was applied and no difference in the means in the net responses per unit absorbed dose was found at any of the applied absorbed dose levels. Conclusion: Within the measurement uncertainties (∼4% at 1σ), the response of the EBT emulsion to beta particles from 90 Sr / 90 Y and gamma rays from 60 Co is the same.