Kodak XV Film Research Articles

TH‐C‐M100E‐04: Cerenkov Light From Phantom Cassettes in Absolute Dose Measurements Using Radiographic Film

Purpose: To determine the impact of Cerenkov light on radiographic film response and to recommend a methodology for correcting absolute dose measurements when using bare film, but calibrated using prepackaged film. Method and Materials: The Gammex RMI Film Dosimetry Cassette Model 436‐AST (Solid Water) and two in‐house cassettes (white opaque, high‐impact polystyrene) were studied using Kodak XV or EDR2 film. Films were exposed perpendicular to 16‐MeV electron (15×15 cm2) or 6 MV x‐ray (20×20 cm2) beams. Films were oriented such that quadrant ♯1 had bare film; quadrant ♯2 had film covered by the prepackaged white paper; quadrant ♯3 had film in its prepackaged container; and quadrant ♯4 had film covered by the prepackaged carbon jacket. To account for beam asymmetry, dose response for each quadrant was normalized to that in the corresponding quadrant of a film irradiated in the carbon jacket, which blocked phantom‐produced Cerenkov light. A prepackaged film, irradiated using a multi‐exposure technique, provided the dose‐response calibration. Results: The “carbon jacket only” dose values averaged 96.1% of the “prepackaged” dose values, indicating that the prepackaged white paper produced Cerenkov light that increased film response by 4.0%. No significant difference due to radiation modality or film type was evident. The “white paper only” dose values ranged from 103.6–107.5% of the “prepackaged” dose values, indicating that Cerenkov light from the phantom material contributed to an increased film response. For white opaque, high‐impact polystyrene the “bare film” dose values ranged from 102.2–109.6% of the “prepackaged” dose values, depending on phantom and modality. For Solid Water the “bare film” dose value was 117.3% of the “prepackaged” dose values. Conclusion: When making absolute dose measurements using bare film and calibrating using prepackaged film, a correction for excess film response arising from Cerenkov light is required, and the reported quadrant method is recommended.

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%.

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.

SU‐FF‐T‐426: The Response of RadioChromic EBT Film in High‐Energy Electron Beams

Purpose: To describe the response of a new radiochromic film to highenergy clinical electron beams. GAFChromic® EBT film was designed primarily for verification of high energy photon IMRT treatments. It has been shown to be a very accurate and convenient photon detector. However, there is no literature on this film's response to high‐energy electron beams. We initiated a study to characterize the response and utility of this film for routine electron beam dosimetry. Method and Materials: The linac was first calibrated to ensure that the delivered dose was known accurately. We exposed a series of EBT films at dmax in polystyrene to 6, 9, 12, 16, and 20 MeV electrons to develop standard characteristic curves. All films were from the same batch. We also exposed the EBT films in a solid water phantom to produce central axis depth dose curves. These data were compared against percent depth dose curves measured with either Kodak XV2 film in a solid water cassette, or in a water phantom using an IC‐10 ion chamber or a PTW electron diode. Results: Results showed that the characteristic curve for this film was the same for electron energies from 6–20 MeV. Also, the response of the EBT film along the central axis of the beam matched the central axis percentage depth dose characteristics (R50, Rp) determined using Kodak XV2 film, electron diodes, and corrected ionization chamber measurements. Conclusion: GAFChromic® EBT film is shown to be an accurate detector for clinical electron beam applications. It is convenient to use since the solid water film cassette can be loaded in the linac room and requires no post‐irradiation processing. It gives the same results for electron dosimetry as silver halide films, diodes, and ionization chambers. Research sponsored by International Specialty Products corporation.

SU-FF-T-49: A QA Test to Check MLC Carriage Calibration

Purpose: The leaf span for a Varian MLC is 15 cm. Fields larger than this in the leaf motion direction must be split into multiple ports with a “carriage shift” between ports. Leaf position is a function of both carriage position and leaf position within the carriage. The carriage position is dictated by the outermost leaf position, and the carriages do not move during radiation delivery. We have developed a test to assess carriage positioning accuracy by comparing the same abutting MLC‐shaped fields both with and without carriage motion between the delivery of the two fields. Methods and Materials: A 14 cm wide × 40 cm long field was split into two 7 cm wide rectangular ports shaped by the MLC. Width is defined in the leaf motion direction. These two fields were delivered using two separate static MLC files, then using a single DMLC file. In the former case, the carriages move while the leaves remain stationary with respect to the carriages. In the latter case, the carriages remain stationary while the leaves move with respect to the carriages. Kodak XV film was taped to the collimator face for both cases and compared. Results: Both films appeared identical upon visual inspection. A quantitative analysis of the profiles was performed using the RIT software system. Comparison of the profiles revealed that the FWHM of the abutting region agreed to within 0.2 mm between the two films. Conclusion: If accuracy of leaf calibration has been demonstrated, differences between these profiles would imply carriage miscalibration. Carriage miscalibration would result in mispositioning of all leaves within the carriage, thereby causing significant dose delivery errors. This carriage calibration test could be performed routinely as part of the QA procedure for MLCs used for IMRT delivery.

SU‐FF‐J‐92: Measurement of Linear Accelerator Photon Beam Spot Size and Assessment of Its Long‐Term Stability

Purpose: To characterize the beam spot size of Siemens linear accelerators and assess its long‐term stability for megavoltage cone‐beam CT (MV‐CBCT) application. Method and Materials: A laminated beam spot camera of length 20 cm was constructed for the measurement of the beam spot size. With the linac gantry at 180°, the camera was positioned on the compensator tray and a Kodak XV film placed on it. An exposure was delivered using photon (6, 10, 18 MV) beams and the FWHM of the resulting source intensity profile used as a measure of the beam spot size. Measurement of the beam spot size was performed in both the gun‐target direction (in‐plane) and the cross‐plane direction for seven Siemens accelerators. To assess the long‐term stability of the beam spot size, measurements taken 1 year apart were compared to each other. Results: The measured beam spot diameters (FWHM) range from 1.6–2.8 mm. For all accelerators, the in‐plane spot size was equal to or larger than the corresponding cross‐plane spot size by up to 0.6 mm. Treatment units of the same design had spot sizes that were, in general, not identical but differed by up to 1 mm. Comparison of measurements on the Primus' and Mevatrons showed the introduction of the former (new generation accelerators) did not necessarily lead to a reduction in the spot size. Also, beam spot sizes measured 1 year apart were found to be similar. Conclusions: The new accelerator models did not in general provide an improvement in the spot size compared to the old models. Assessment of the long‐term stability of the beam spot showed the spot size remains fairly stable over time. However, the observed spot sizes are large in relation to focal spot size of diagnostic x‐ray imaging devices, and this might compromise MV‐CBCT image quality.

MO‐D‐224A‐09: Validation of Synchronized Dynamic Dose Reconstruction Using Film Dosimetry

Purpose: To validate a method of retrospective dose reconstruction that uses real‐time intra‐treatment patient motion data that is synchronized with MLC leaf positions during IMRT treatments. Method and Materials: IMRT fields from an IRB‐approved prostate protocol were delivered to a water‐equivalent phantom on a programmable translation stage. Kodak XV film was placed at 5 cm depth with an SSD of 95cm and marked to register it with the Monte Carlo (MC) calculation grid. Motion was synchronized with beam delivery by using the target signal to trigger motion. Film measurements were repeated for each beam while the phantom was stationary, and while moving with both idealized and clinically measured motion profiles. MLC leaf positions and fluence state for each beam were obtained from the Varian DynaLog files. MC dose accumulation was performed which incorporated the real‐time phantom motion, DynaLog files and beam state. Films were digitized and compared to the results of the MC calculations. Results: Film measurements in stationary phantoms were measured three times on two machines. The measured dose distributions were compared and showed an average difference of 0.38 +/− 1.53 cGy. The average difference between MC and film measurements of the moving phantom was 0.44 +/− 3.4 cGy and was independent of the motion profile. Measured dose patterns, for both stationary and moving phantoms, were generally well reproduced by MC dose accumulation, including tongue‐and‐groove and motion related features. Doses in moving and static phantoms were compared, for both films and simulations. The measured dose deviations due to motion were well‐characterized by the MC dose accumulation method and not significantly different when a static phantom was compared to MC calculation. Conclusion: Real‐time motion and machine data may be used to reconstruct the dose delivered to the target volume, and may serve as a basis for dynamic refinement of treatments.

SU‐FF‐T‐182: Dosimetric Comparison of An Elekta “Synergy S” Beam Modulator & Radionics MMLC Using Monte Carlo Simulations & Measurements

Purpose: To compare dosimetric parameters of a new Elekta “Synergy S” dedicated stereotactic radiosurgery MLC, namely the beam modulator (BM), with Radionics mini‐multileaf collimator (MMLC). Methods and Materials: The Beam Modulator maximum opening is 16cm×21cm and consists of 40 pairs of Tungsten leaves of 4mm thickness at the isocentre, with no back up jaws. Radionics MMLC has a maximum field size of 9.6cm×12cm and 3.75mm leaf thickness at the isocentre. Leakage and transmission, percentage depth doses (PDD) and dose profiles were measured and calculated for different field sizes and depths and for different source to surface distances (SSD). Kodak XV films, photon diode detector (diameter of active area 2mm), CC13 Wellhofer ion chamber (cavity volume 0.13 cm3) and Wellhofer water tank were used for measurements. BEAMnrc code was used for the Monte Carlo (MC) simulations. All the data are for a 6MV photon beam. Results: It is shown that the BM beams are slightly more energetic so that PDD at 10cm depth is 2% more for a 10.4cm × 9.6cm field, compared to Radionics MMLC. Dose profile results are generally comparable, except for the penumbra which is sharper for Radionics MMLC, especially in the leaf travel direction by up to 1.1 mm. Maximum and average leakage was 1.7 and 1.1 for BM and 1.2 and 0.9% for MMLC, respectively. MC calculation and measurement results for PDD and profiles agreed well to better than 1% and or 0.5mm. The uncertainty in simulation was less than 0.5%. Conclusion: Elekta “Synergy S” beam modulator and Radionics MMLC have successfully been modeled for the first time using the BEAMnrc MC simulations. The MC results showed an excellent agreement with the measurements. BM has a wider penumbra, mainly due to the larger isocentric distance and rounded leaf ends.

TH‐D‐224C‐06: An Optimal MLC Quality Assurance Procedure Using EPID

Purpose: IMRT QA is extremely time consuming and labor intensive, while IMRT delivery is complex. The purpose of this work is to develop an automated and quantitative IMRT QA process utilizing an EPID. One of the aspects of this is testing of MLC leaf position accuracy. The Memorial‐Sloan‐Kettering strip test was modified to provide an automated EPID‐based process.Method and Materials: To provide a baseline fluence map, Kodak XV film was irradiated to obtain an open field 2D beam profile, which was fitted with a Gaussian and used in the image correction routine. We used a strip test consisting of seven adjacent segments with an intentional gap of nominally 0.5 mm. The gap was intentionally varied for the positions located at regularly placed hexagonal regions in order to create a set of calibration curves. A fitting technique using a Lorentzian type function was developed to determine the dose‐peak‐height, dose‐peak‐position and FWHM. Results: The standard deviation of dose‐peak‐height at each abutment for traditional Memorial‐Sloan‐Kettering pattern was of the same order of magnitude as that of the average over the entire image. In contrast, in our pattern a difference of an order of magnitude was observed, which indicated the reducing of cross‐talk effect from adjacent leaves. The leaf positioning error can be identified by using standard calibration curves of dose‐peak‐height vs. gap. The coordinate of dose‐peak‐position is found to be an extremely sensitive indicator of leaf error with sub‐pixel standard deviation. Conclusions: This procedure is able to identify MLC positional errors less than 0.5 mm by using a fitting technique and by reducing the cross‐talk effect. Conflict of Interest: Research sponsored by Varian Corporation.

SU-FF-T-219: Evaluation of a Practical and Low Cost Gafchromic Film/flat-Bed Scanner Combination for Planar Dosimetry

Purpose: To assess the accuracy and practicality of using GAFCHROMIC® EBT Film in combination with an Epson flat‐bed scanner for the verification of advanced treatment techniques in radiation therapy. EBT is a new type of Gafchromic film with high sensitivity to ionizing radiation. The combination of EBT with flat‐bed scanner represents a very low cost and convenient dosimetry system. Methods and Materials: Experiments were conducted to evaluate two aspects of the EBT/flat‐bed‐scanner combination. First the stability of the scanner was evaluated by repeat scanning, over several weeks, of a conventional Kodak XV film, which is known to be highly stable post‐irradiation. Second the dose response and stability of EBT film was investigated by repeat scanning, over several weeks, of EBT film pieces irradiated to different doses (0 to 8Gy with 6MV photon beam). Results: The reproducibility of the Epson scanner was found to be highly stable, to within 1.03%, over all ranges of OD studied. A slight non‐uniformity in background was observed, but this background was consistent enabling efficient correction. The dose response was observed to be highest in the red channel of the scanner. The dose response curve of EBT film was found to be non‐linear but stable (within 1.32%) within one week after irradiation. Conclusions: The Epson® 4990 Scanner/ GAFCHROMIC® EBT Film dosimetry combination appears a promising, low‐cost, and highly convenient dosimetry system for radiation therapy.

MO‐E‐224C‐06: Improving Homogeneity of Abutment Dosimetry in Segmented‐Field Electron Conformal Therapy Using Variable Insert Positioning

Purpose: To improve dose homogeneity in segmented‐field electron conformal therapy by matching penumbra of abutted fields. Method and Materials: A Varian 21EX electron applicator was modified to allow for energy‐dependent positioning of Cerrobend inserts resulting in energy‐dependent air gaps. Air gaps were chosen based on theoretical calculations to approximately match penumbra for 6, 9, 12, 16, and 20 MeV beams at 1.5‐cm depth in a water phantom at 100‐cm SSD. Treatment plans developed for four simulated target volumes using the modified applicator were compared to identical plans using the standard applicator. Improvement in dose homogeneity was assessed by comparing maximum and minimum dose, mean dose, and sigma of the dose distribution in the target volume for the two plans. Subsequently, electron blocks were cut with diverging edges using the Compu•cutter® system, and dose plans using the modified applicator were delivered to Kodak XV film in a polystyrene phantom to demonstrate feasibility. Results: Treatment planning results using the modified applicator showed improved dose homogeneity in all four simulated target volumes as compared to plans using the standard applicator. Averaged for all four PTVs; dose spread (Dmax − Dmin) decreased by 35%, σ of the dose distribution decreased by 29%, and D90–10 decreased by 31%. Dose delivered to a film phantom using the modified applicator was found to agree well with predictions (within approximately 5%) of the Pinnacle treatment planning system in the abutment region of the PTV (±2 cm from the abutment edge at depths ⩾1.5 cm). Conclusion: The results of this study suggest segmented‐field electron conformal therapy can be delivered with significant improvement in dose homogeneity as compared to the current method by using energy‐dependent positioning of electron inserts to match beam penumbra.

Evaluation of the EDR-2 film for relative dosimetry of high-energy photon and electron beams

A sensitometric study of Kodak XV and EDR-2 radiographic films (Eastman Kodak Company, Rochester, NY) was performed using photons ranging from 75 kV to 18 MV and electrons ranging from 6 to 20 MeV. To investigate the applicability of the EDR-2 film for clinical radiation dosimetry, percentage depth-doses, profiles and distributions in open and dynamically wedged fields were measured using film and compared with data from a linear diode. Moreover, conventional quality assurance dose parameters were measured, including open-field dose profiles to determine flatness and symmetry of photon and electron beams. Finally, film was employed to validate dose distributions produced by complex computerised treatment planning techniques. Our conclusion is that the EDR-2 film is an effective tool for relative dosimetry of photon and electron beams.

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

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

Po-Poster - 04: A daily quality assurance measurement for dynamic MLC treatments

A daily test was developed to assure intensity modulated fields are delivered with a high degree of accuracy and precision using the dynamic multileaf collimator delivery mode. Quality assurance of the MLC, particularly for dynamic mode of delivery, requires positional accuracy of individual leafs, as well as gap width defined by opposing leaf pairs. The daily test was designed to be quick, simple to use, and easy to analyze. With the secondary collimating jaws at 25 cm wide and 39 cm long, a 5mm sliding gap over 5 cm at isocenter placed between two static fields 4 cm off axis was measured dosimetrically using Kodak XV film. The sliding gap allows us to test dosimetrically the positional accuracy defined by Varian in motor encoder counts, as a function of beam on time. The speed of the leaves is controlled by the MUs delivered. Relative film dosimetry with respect to a baseline film obtained after MLC alinment is performed daily and analyzed using RIT software. In order to facilitate the registration of the QA film to the baseline film, a mounting device utilizing the upper wedge block was used. The QA test was tested using beam shaper files with controlled errors introduced and dose differences of 5% are easily detected. In addition to the daily film QA, as part of morning photon output checks a sliding gap output is measured and these results will be presented as well.

SU‐FF‐T‐161: Practical Gap‐Width Threshold for MLC Quality Assurance for IMRT

Purpose: A standard IMRT quality assurance test for Multi Leaf Collimator (MLC) leaf position accuracy is the Memorial Sloan‐Kettering (MSK) leaf test. This test involves inspecting the widths of five 1 mm wide fields delivered 2 cm apart. A maximum error tolerance of 0.2 mm in the gap‐width between sets of opposing leaves is recommended. In a preliminary informal visual test performed with an independent observer, the observer detected all deliberately introduced errors greater than or equal to 0.5 mm, but not all 0.2 mm errors. Method and Materials: We investigated the use of the MSK leaf test on a Varian Clinac 2300C/D and Clinac 2300EX, with Mark I (52 leaf) and Millennium (120 leaf) MLCs, respectively. Kodak XV film was irradiated with 6 MV photons at an SSD of 100 cm, using 1000 monitor units for the delivery of the five segments. The film was scanned using a Vidar VXR‐16DP scanner, and subsequently analyzed with the RIT113 Version4 software. The RIT software recommends using a 3×3 median filter to remove background noise, which effectively reduces the image resolution. Since the smallest baseline resolution of the scanner is 0.089 mm, a 0.2mm error will be difficult to observe after a 3×3 filtration. Separate films were irradiated under the same conditions and scanned to determine if the scanner software combination was consistent in assessment of the magnitude of errors in the gap‐width. Results: The reproducibility of leaf positions on multiple films was found to be within one pixel. However, not all artificially introduced errors that were visually observable were detected by this software technique. Conclusion: It may not be feasible to reliably measure gap‐width errors of 0.2 mm, either by eye or by quantitative analysis using the technique presented here.

SU‐FF‐T‐102: Dosimetric Properties of Respiratory‐Gated Intensity Modulated Arc Treatments

Purpose: We investigated the dosimetric effects of gating on Intensity Modulated Arc Treatments. Method and Materials: One method to address respiratory motion during radiotherapy is 4DCT in conjunction with gated treatment. In the treatment of a solitary lung nodule or liver metastasis, intensity modulated arc therapy (IMAT) provides desired dose distributions. Here, we determined the dosimetric errors from delivering gated IMAT under a variety of conditions, including monitor units (MU), dose rate (MU/min), and direction of gantry rotation. We employed Varian Real Time Monitoring system for the gated delivery on a 2100C/D accelerator. To test the accuracy of the radiation delivery system, we used a static solid water phantom (IMRT phantom, MedTec). We chose a lateral 90° arc to stress test the gantry rotation delivery against gravity. Film dosimetry and point dose were performed. We delivered 30MU and 270MU treatments and used Kodak XV and EDR2 films respectively. RIT software was used to perform film dosimetry and analyses. Point doses at the isocentre were measured using a Scanditronix RK ion chamber (0.12cc). Dose distributions from gated deliveries are compared to non‐gated deliveries, normalized by the dose at isocenter. Amplitude gating was used with a 25% duty cycle. Results: With gating, we measured ≤1.5% change in dose at the isocentre. From film dosimetry, we observed beam transverse asymmetry in the gated delivery. Dose error < 3% was measured away from the isocentre using gated IMAT when the gantry is rotating at its maximum speed (295°/min). Errors less than 3% were measured when gantry rotates at or slower than 200°/min under clinical conditions. Conclusion: Gate IMAT treatments yield consistent output at the isocentre but accumulate beam symmetry errors. These errors are proportional to the gantry rotational speed which can be reduced to below 3% by lowering the delivery dose rate.

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.

Development of a radiophotoluminescent glass plate detector for small field dosimetry

Development of a radiophotoluminescent glass plate detector for small field dosimetry

Matchline dosimetry in step and shoot IMRT fields: a film study

The Varian millennium 120 multileaf collimator has curved leaf ends. Transmission through the leaf ends generates a small asymmetric penumbral dose effect. This design can lead to hot spots between neighbouring beam segments during step and shoot IMRT dose delivery. We have observed some matchlines with film for clinical beams optimized using the pinnacle radiotherapy treatment planning system; hence we sought to verify the optimum leaf offset required to minimize the matchline effect. An in-house program was created to control the MLC leaf banks in 2 cm steps with a 2 cm gap. The gap was varied by the following offset values from 0.0 to 0.1 cm. Two types of radiographic films (Kodak EDR and XV films) and a radiochromic film (Gafchromic MD-55-2) were used to measure the optical density maps. The films were positioned in a solid water phantom perpendicular to the beam axis and irradiated at dmax using a 6 MV photon beam. An ion chamber (IC4) was used to measure point doses for normalization in a beam umbral minima position. The relative mean peak to valley dose ratios measured with no leaf offset were 1.31, 1.30 and 1.31 for the XV, EDR2 and Gafchromic films, respectively. For a 0.07 cm gap per leaf and a performance of end leaf repeatability of 0.01 cm, the central matchline was reduced to about 1.0 for all dosimeters, with two mini-peaks measured as 1.05, 1.05 and 1.08 each side of the matchline, for XV, EDR2 and Gafchromic, respectively. The average relative dose across the umbra for this offset was XO-mat V = 1.01, EDR = 1.01 and radiochromic film = 1.02, respectively.While we expected the beam penumbral tails from segment neighbours to cause overprediction of the dose in the central valley regions due to the energy response of radiographic films, by normalizing all dosimeters to an ion chamber reading in the minimum we could not observe any major shape distortion between the radiographic film and radiochromic film results. In conclusion, relative doses measured by radiographic and radiochromic films agree well with IC4 within ±2%.

The response of Kodak EDR2 film in high-energy electron beams.

Kodak XV2 film has been a key dosimeter in radiation therapy for many years. The advantages of the recently introduced Kodak EDR2 film for photon beam dosimetry have been the focus of several IMRT verification dosimetry publications. However, no description of this film's response to electron beams exists in the literature. We initiated a study to characterize the response and utility of this film for electron beam dosimetry. We exposed a series of EDR2 films to 6, 9, 12, 16, and 20 MeV electrons in addition to 6 and 18 MV x rays to develop standard characteristic curves. The linac was first calibrated to ensure that the delivered dose was known accurately. All irradiations were done at dmax in polystyrene for both photons and electrons, all films were from the same batch, and were developed at the same time. We also exposed the EDR2 films in a solid water phantom to produce central axis depth dose curves. These data were compared against percent depth dose curves measured in a water phantom using an IC-10 ion chamber, Kodak XV2 film, and a PTW electron diode. The response of this film was the same for both 6 and 18 MV x rays, but showed an apparent energy-dependent enhancement for electron beams. The response of the film also increased with increasing electron energy. This caused the percent depth dose curves using film to be shifted toward the surface compared to the ion chamber data.