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PurposeIn advanced radiotherapy techniques such as intensity-modulated radiation therapy (IMRT), the quality assurance (QA) process is essential. The aim of the study was to assure the treatment planning dose delivered during delivery of complex treatment plans. The QA standard is to perform patient-specific comparisons between planned doses and doses measured in a phantom.Materials and methodThe Delta 4 phantom (Scandidos, Uppsala, Sweden) has been used in this study. This device consists of diode matrices in two orthogonal planes inserted in a cylindrical acrylic phantom. Each diode is sampled per beam pulse so that the dose distribution can be evaluated on segment-by-segment, beam-by-beam, or as a composite plan from a single set of measurements. Ninety-five simple and complex radiotherapy treatment plans for different pathologies, planned using a treatment planning system (TPS) were delivered to the QA device. The planned and measured dose distributions were then compared and analysed. The gamma index was determined for different pathologies.ResultsThe evaluation was performed in terms of dose deviation, distance to agreement and gamma index passing rate. The measurements were in excellent agreement between with the calculated dose of the TPS and the QA device. Overall, good agreement was observed between measured and calculated doses in most cases with gamma values above 1 in >95% of measured points. Plan results for each test met the recommended dose goals.ConclusionThe delivery of IMRT and volumetric-modulated arc therapy (VMAT) plans was verified to correspond well with calculated dose distributions for different pathologies. We found the Delta 4 device is accurate and reproducible. Although Delta4 appears to be a straightforward device for measuring dose and allows measure in real-time dosimetry QA, it is a complex device and careful quality control is required before its use.

Purpose To assess the dosimetric impact of a patient positioning device for prone breast radiotherapy and assess the accuracy of a treatment planning system (TPS) in predicting this impact. Methods Beam attenuation and build-up dose perturbations, quantified by ionization chamber and radiochromic film dosimetry, were evaluated for 3 components of the patient positioning device: the carbon fiber baseplate, the support cushions and the support wedge for the contralateral breast. Dose calculations were performed using the XVMC dose engine implemented in the Monaco TPS. All components were included during planning CT acquisition. Results Beam attenuation amounted to 7.57% (6 MV) and 5.33% (15 MV) for beams obliquely intersecting the couchtop–baseplate combination. Beams traversing large sections of the support wedge were attenuated by 12.28% (6 MV) and 9.37% (15 MV). For the support cushion foam, beam attenuation remained limited to 0.11% (6 MV) and 0.08% (15 MV) per centimeter thickness. A substantial loss of dose build-up was detected when irradiating through any of the investigated components. TPS dose calculations accurately predicted beam attenuation by the baseplate and support wedge. A manual density overwrite was needed to model attenuation by the support cushion foam. TPS dose calculations in build-up regions differed considerably from measurements for both open beams and beams traversing the device components. Conclusions Irradiating through the components of the positioning device resulted in a considerable degradation of skin sparing. Inclusion of the device components in the treatment planning CT allowed to accurately model the most important attenuation effect, but failed to accurately predict build-up doses.

The radiochromic film, which is used, in combination with a flatbed scanner has become a widely used tool for a quantitative evaluation of radiation dose in radiation therapy. One aspect of uncertainty using the radiochromic film is the magnitude of orientation effects when the orientation of the film is not kept constant during the digitization process. The aim of this note was to investigate the impact of using a combination of two crossed sheets of EBT2 film on various aspects of radiochromic film dosimetry. First the impact on the film sensitivity was studied. We also investigated the influence on orientation effects during scanning. The results show that the double crossed film combination increases the sensitivity with a factor 1.7-2.1 and practically eliminates the effects of film orientation on the optical density read-out and the lateral correction profiles.

Purpose: The dosimetric accuracy of EDR2 radiographic film has been rigorously assessed in regular and intensity modulated beams for various incidence angles, including the parallel and perpendicular orientation. There clearly exists confusion in literature regarding the effect of film orientation. The primary aim is to clarify potential sources of the confusion and to gain physical insight into the film orientation effect with a link to radiochromic film as well. Methods: An inverse pyramid IMRT field, consisting of six regular and elongated 3 Multiplication-Sign 20 cm{sup 2} field segments, was studied in perpendicular and parallel orientation. Assessment of film self-perturbation and intrinsic directional sensitivity were also included in the experiments. Finally, the authors investigated the orientational effect in composite beams in the two extreme orientations, i.e., perpendicular and parallel. Results: The study of an inverse pyramid dose profile revealed good agreement between the perpendicular film and the diamond detector within 0.5% in the low-scatter regions for both 6 and 18 MV. The parallel oriented film demonstrated a 3% under-response at 5-cm (6 MV) depth against the perpendicular orientation, but both orientations over responded equally in the central region, which received only scattered dose, at both 5- and 20-cm depths. In amore » regular 6-MV 5 Multiplication-Sign 5 cm{sup 2} field, a 4.1% lower film response was observed in the parallel orientation compared to perpendicular orientation. The under response gradually increased to 6% when reducing the field size to 0.5 Multiplication-Sign 5 cm{sup 2}. On the other hand, the film showed a 1.7% lower response in parallel orientation for the large field size of 20 Multiplication-Sign 20 cm{sup 2} at 5-cm depth but the difference disappeared at 10 cm. At 18 MV, similar but somewhat lower differences were found between the two orientations. The directional sensitivity of the film diminishes with increasing field size and depth. Surprisingly a composite IMRT beam consisting of 20 adjacent strip segments also produced a significant orientational dependence of film response, notwithstanding the large total field size of 20 Multiplication-Sign 20 cm{sup 2}. Conclusions: This analysis allowed the development of a hypothesis about the physics behind the orientational dependence of film response in general and to formulate precautions when using film dosimetry in the dosimetric verification of multibeam treatments.« less

Superficial dose from 6- and 18-MV photon beams has been studied by measuring surface dose and shallow build-up dose using radiographic film EDR2, radiochromic film EBT2 and plane-parallel chamber. Measurements have been made for intensity- and non-intensity-modulated beams. The results show that the surface dose was found to be 19.8% and 10% of maximum dose in unmodulated fields for 6 and 18 MV photon beams, respectively. The study further showed that intensity modulation decreased surface dose by 1.1% and 0.7% for the same field size at 6 and 18 MV, respectively, and surface dose was dropped by magnetically sweeping contaminating electrons. EDR2 and EBT2 films show in good agreement in shallow build-up region. This study demonstrated the capability of EDR2 film, in addition to radiochromic film, to measure surface and build-up dose in case of treatment planning system uncertainties with regard to skin toxicity or shallow target coverage.

Purpose: To evaluate the dose distribution of a 120-MeV laser-plasma accelerated electron beam which may be of potential interest for high-energy electron radiation therapy. Methods: In the interaction between an intense laser pulse and a helium gas jet, a well collimated electron beam with very high energy is produced. A secondary laser beam is used to optically control and to tune the electron beam energy and charge. The potential use of this beam for radiation treatment is evaluated experimentally by measurements of dose deposition in a polystyrene phantom. The results are compared to Monte Carlo simulations using the GEANT4 code. Results: It has been shown that the laser-plasma accelerated electron beam can deliver a peak dose of more than 1 Gy at the entrance of the phantom in a single laser shot by direct irradiation, without the use of intermediate magnetic transport or focusing. The dose distribution is peaked on axis, with narrow lateral penumbra. Monte Carlo simulations of electron beam propagation and dose deposition indicate that the propagation of the intense electron beam (with large self-fields) can be described by standard models that exclude collective effects in the response of the material. Conclusions: The measurements show that the high-energy electron beams produced by an optically injected laser-plasma accelerator can deliver high enough dose at penetration depths of interest for electron beam radiotherapy of deep-seated tumors. Many engineering issues must be resolved before laser-accelerated electrons can be used for cancer therapy, but they also represent exciting challenges for future research. (C) 2012 American Association of Physicists in Medicine. [http://dx.doi.org/10.1118/1.47199621 (Less)

The aim of this study is to determine the optimal backscatter thickness and lateral phantom dimension beyond the irradiated volume for the dosimetric verification with radiographic film when applying large field sizes. Polystyrene and Virtual Water™ phantoms were used to study the influence of the phantom backscatter thickness. EDR2 and XV films were used in 6 and 18 MV photon beams. The results show 11.4% and 6.4% over-response of the XV2 film when compared to the ion chamber for 6 MV 30×30 and 10×10 cm2 field sizes, respectively, when the phantom backscatter thickness is 5 cm. For the same setup, measurements with EDR2 films indicate 8.5% and 1.7% over-response. The XV2 film response in the polystyrene phantom is about 2.0% higher than in the Virtual Water™ phantom for the 6 MV beam and 20 cm backscatter thickness. Similar results were obtained for EDR2 film. In the lateral scatter study, film response was nearly constant within 5 cm of lateral thickness and it increases when lateral thickness increases due to more multiple scatter of low energy photons. The backscatter thickness of the phantom should be kept below 7 cm for the accuracy of the film dosimetry. The lateral extension of the phantom should not be more than 5 cm from the field boundary in case of large irradiated volumes.

The purpose of this investigation was the verification of both the measured data and quality of the implementation of the add-on ModuLeaf miniature multileaf collimator (ML mMLC) into the clinical treatment planning system for conformal stereotactic radiosurgery treatment. To this end the treatment head with ML mMLC was modeled in the BEAMnrc Monte Carlo (MC) code. The 6 MV photon beams used in the setup were first benchmarked with a set of measurements. A total ML mMLC transmission of 1.13% of the 10 x 10 cm2 open field dose was measured and reproduced with the BEAMnrc/DOSXYZnrc code. Correspondence between calculated and measured output factors (OFs) was within 2%. Correspondence between MC and measured profiles was within 2% dose and 2 mm distance, only for the smallest 0.5 x 0.5 cm2 field the results were within 3% dose. In the next step, the MC model was compared with Gafchromic film measurements and Pinnacle(3) 7.4 f (convolution superposition algorithm) calculated dose distributions, using a gamma evaluation comparison, for a multi-beam patient setup delivered to a Lucytrade mark phantom. The gamma evaluation of the MC versus Gafchromic film resulted in 3.4% of points not fulfilling gamma

Polymer gel dosimetry was used to verify the clinical use of IMAT. There was a satisfactory correlation between calculated and measured dose, thus validating the planning procedure, the calculation algorithm and the delivery. Although still elaborate and costly, polymer gel dosimetry has some unique features (3D absolute dosimetry) for verification of complex irradiation techniques.

In this study we investigate radiochromic film dosimetry around air cavities with particular focus on the perturbation of the dose distribution by the film when the film is parallel to the beam axis. We considered a layered polystyrene phantom containing an air cavity as a model for the air-soft tissue geometry that may occur after surgical resection of a paranasal sinus tumour. A radiochromic film type MD-55 was positioned within the phantom so that it intersected the cavity. Two phantom set-ups were examined. In the first case, the air cavity is at the centre of the phantom, thus the film is lying along the central beam axis. In the second case, the cavity and film are located 2 cm offset from the phantom centre and the central beam axis. In order to examine the influence of the film on the dose distribution and to interpret the film-measured results, Monte Carlo simulations were performed. The film was modelled rigorously to incorporate the composition and structure of the film. Two field configurations, a 1 x 10 cm2 field and a 10 x 10 cm2 field, were examined. The dose behind the air cavity is reduced by 6 to 7% for both field configurations when a film that intersects the cavity contains the central beam axis. This is due to the attenuation exerted by the film when photons cross the cavity. Offsetting the beam to the cavity and the film by 2 cm removes the dose reduction behind the air cavity completely. Another result was that the rebuild-up behind the cavity for the 10 x 10 cm2 field, albeit less significant than for the 1 x 10 cm2 field, could only be measured by the film that was placed offset with respect to the central beam axis. Although radiochromic film is approximately soft-tissue equivalent and energy independent as compared to radiographic films, care should be taken in the case of inhomogeneous phantoms when the film intersects air cavities and contains the beam central axis. Errors in dose measurement can be expected distal to the air cavity due to attenuation in the film itself. This attenuation would not occur in the absence of the film. Both experiments and Monte Carlo computations support this conclusion.

The physical and dosimetrical characteristics of a device, designed to visualize the isocenter position on electronic portal images, were examined. The device, to be mounted on the gantry head of the accelerator, containing five spheric lead markers, was designed in order to visualize the isocenter position on portal images. A quality control device was designed to check the reliability of this technique. The disturbance of the dose distribution by the markers was studied with gel dosimetry. The use of markers resulted in a precise and accurate method to visualize the isocenter on portal images. A maximum underdosage of 11%, due to attenuation by the markers, was observed. The use of markers to visualize the isocenter position on portal images, is a fast and reliable method when analyzing patient setup errors with online electronic portal imaging.

The aim of this work was to investigate the applicability of MR-based polymer gel dosimetry to measure the absorbed dose distribution at short distance from an iridium-192 brachytherapy point source. In this paper, different methodological problems that may result in significant errors in the measured dose distribution are discussed. First of all the extent to which physico-chemical mechanisms alter the dose response is discussed. The most important among these are the oxygen permeability of the catheter material and monomer-diffusion-related effects during irradiation. The effect of oxygen on the dose–R2 curve has been determined quantitatively and an oxygen map is performed using a well-defined external irradiation beam. The effect of diffusion of monomer during irradiation has been simulated. Another contribution of errors is related to magnetic susceptibility differences between the catheter and the gel during scanning the irradiated gel. The magnetic field distortion has been mapped by using both an experimental MRI technique and by simulation. Moreover, in constructing a dose-versus-distance curve by polar averaging, the sensitivity to the exact distance between source and point of measurement and to partial volume effects is illustrated. An optimization program is proposed to determine the location of the source on a sub-pixel scale.

In high-precision 3D gel dosimetry, long MR measurement times together with a high amount of RF energy being absorbed by the phantom are very common, and result in a spatially dependent temperature rise in the gel. As T2 of the dosimeter gel is temperature dependent, dose estimation will be affected. In this study we assess the temperature rise in the dosimeter gel by use of MR temperature mapping and computer modelling. It is shown that in conventional MR sequences, where linear k-space sampling is used, a temperature rise of 3 °C results in a dose underestimation of 10% over the whole dose map. To correct for these dose errors, a compensation method involving centric k-space ordering is suggested. Computer simulations have been performed to analyse the robustness of the proposed method. Applying the compensated sequence, a temperature rise of 3 °C leads to a narrow dose artefact of the order of 3% for a 'worst case' situation in which a single pixel dose gradient is assumed. Negligible deviations are found in the rest of the dose map.

The aim of this work was to investigate MR-based polymer gel dosimetry as a three-dimensional (3D) dosimetry technique in conformal radiotherapy. A cylindrical container filled with polymer gel was placed in a water-filled torso phantom to verify a treatment plan for the conformal irradiation of a mediastinal tumor located near the esophagus. Magnetic resonance spin-spin relaxation rate images were acquired and, after calibration, converted to absorbed dose distributions. The dose maps were compared with dose distributions measured using radiographic film. The average root-mean-square structural deviation, for the complete dose distribution, amounted to less than 3% between gel and film dose maps. It may be expected that MR gel dosimetry will become a valuable tool in the verification of 3D dose distributions. The influence of imaging artifacts arising from eddy currents, temperature drift during scanning, and B1 field inhomogeneity on the dose maps was taken into account and minimized.

Background and purpose : It was our aim to investigate NMR-based BANG gel dosimetry as a three-dimensional dosimetry technique in conformal radiotherapy. Materials and methods : The BANG gel consisting of gelatin, water and co-monomers was first validated in a cylindrical glass flask for a single standard beam. Next, the gel contained in a human neck-shaped cast was used to verify a treatment plan for the conformal irradiation of a concave tumour in the lower neck. Magnetic resonance relaxation rate images were acquired and, based on an appropriate calibration of the gel, converted to absorbed dose distributions. The resulting maps were compared with dose distributions measured using radiographic film. Results : The gel-measured dose profiles of standard beams agreed within 3% (root mean square difference) with the profiles measured with high spatial resolution by a diamond detector. For the multi-beam conformal treatment, the difference map between gel-measured and film-measured dose distributions revealed a noise component and a more systematic deviation including structural or space-coherent patterns. The mean absolute value of the difference amounted to 8%. A number of possible causes for this deviation are designated. Conclusions : Polymer gel dosimetry in combination with magnetic resonance imaging is a promising method for dosimetric verification of conformal radiotherapy.

This study evaluates the performance of the Nikon Coolscan 9000 ED film scanner for high-gradient radiochromic film dosimetry. As a reference for comparison, analogue experiments were performed on the Epson Expression 10000XL flatbed scanner. Based on these results, a dosimetric protocol was established for the Nikon scanner and its overall performance for high-gradient dosimetry was evaluated. The Nikon scanner demonstrated a high sensitivity for radiochromic film dosimetry, resulting in more contrast in the digitized image. The scanner's optics also demonstrated excellent stability and did not necessitate warm-up scans prior to data acquisition. Moreover, negative effects of temperature changes of the film inside the scanner were shown to be limited. None of the digitized images showed significant disturbances by moire-patterns, by virtue of the absence of a glass plate for film positioning. However, scanner response was found to vary considerably across the reading area, requiring an optical density-dependent correction procedure to be incorporated into the scanning protocol. The main limitation of the Nikon Coolscan 9000 ED transmission scanner remains its film size restriction to 6.2 × 20 cm2. Nevertheless, its excellent characteristics render it the preferential tool for high-gradient radiochromic film dosimetry in applications limited to small film sizes, such as dosimetry in the build-up region.

The purpose of this study was the investigation of perturbation factors for microionization chambers in small field dosimetry and the influence of penumbra for different spot sizes. To this purpose, correlated sampling was implemented in the EGSnrc Monte Carlo (MC) user code cavity: CScavity. CScavity was first benchmarked against results in the literature for an NE2571 chamber. An efficiency increase of 17 was attained for the calculation of a realistic chamber perturbation factor in a water phantom. Calculations have been performed for microionization chambers of type PinPoint 31006 and PinPoint 31016 in full BEAMnrc linac simulations. Investigating the physical backgrounds of the differences for these small field settings, perturbation factors have been split up into (1) central electrode perturbation, (2) wall perturbation, (3) air-to-water perturbation (chamber volume air-to-water) and (4) water volume perturbation (water chamber volume to 1 mm(3) voxel). The influence of different spot sizes, position in penumbra, measuring depth and detector geometry on these perturbation factors has been investigated, in a 0.8 x 0.8 cm(2) field setting. p(cel) for the PP31006 steel electrode shows a variation of up to 1% in the lateral position, but only 0.4% for the PP31016 with an Al electrode. The air-to-water perturbation in the optimal scanning direction for both profiles and depth is most influenced by the radiation field, and only to a small extent the chamber geometry. The PP31016 geometry (shorter, larger radius) requires less total perturbation within the central axis of the field, but results in slightly larger variations off axis in the optimal scanning direction. Smaller spot sizes (0.6 mm FWHM) and sharper penumbras, compared to larger spot sizes (2 mm FWHM), result in larger perturbation starting in the penumbra. The longer geometries of the PP31006/14/15 exhibit in the non-optimal scanning direction large variations in total perturbation (p(tot) 1.201(4) (0.6 mm spot, 3 mm off axis, type A MC uncertainty) to 0.803(4) (5 mm off axis)) mainly due to volume perturbation. Therefore in IMRT settings, when the detector is not always in the optimal scanning direction, the PP31016 geometry requires less extreme perturbation (max p(tot) 1.130(3)) and shows less variation. However, these results suggest that small variations in positioning, spot size or MLC result in large differences in perturbation factors. Therefore even these 0.016 cm(3) ionization chambers are limited in their use for a field setting of 0.8 x 0.8 cm(2), as used in this investigation.

Small field dosimetry is challenging for radiotherapy dosimetry. We measured output factors for small fields with EBT film in polystyrene and sandwiched phantom. A comprising a combination of polystyrene and lung equivalent slabs is named Sandwiched phantom. EBT film response is in good agreement (within 1.7%) with the diamond detector for polystyrene phantom. Due to the density variation between the sandwiched phantom layers, the difference between EBT and diamond detector responses is significantly larger than for the homogeneous polystyrene phantom.

Values for the fluence perturbation correction factor were determined in a number of electron beams for the commonly applied PTW/Markus plane-parallel chamber and the cylindrical NE 2571 Farmer chamber. These data were determined relative to the NACP plane-parallel chamber as a function of the mean energy at depth, Ez, calculated according to the method recommended in modern dosimetry protocols. For the cylindrical chamber the results are in agreement with the data recommended in these protocols. For the PTW/Markus chamber a small but significant fluence perturbation correction is necessary: up to about 3% for a value of Ez=2 MeV. In order to check the value of these fluence perturbation correction values, dose measurements were performed ionometrically and compared with values determined with the Fricke dosimetry system.

The purpose of this study was both putting forward a statistically correct model for film calibration and the optimization of this process. A reliable calibration is needed in order to perform accurate reference dosimetry with radiographic (Gafchromic) film. Sometimes, an ordinary least squares simple linear (in the parameters) regression is applied to the dose-optical-density (OD) curve with the dose as a function of OD (inverse regression) or sometimes OD as a function of dose (inverse prediction). The application of a simple linear regression fit is an invalid method because heteroscedasticity of the data is not taken into account. This could lead to erroneous results originating from the calibration process itself and thus to a lower accuracy. In this work, we compare the ordinary least squares (OLS) inverse regression method with the correct weighted least squares (WLS) inverse prediction method to create calibration curves. We found that the OLS inverse regression method could lead to a prediction bias of up to 7.3 cGy at 300 cGy and total prediction errors of 3% or more for Gafchromic EBT film. Application of the WLS inverse prediction method resulted in a maximum prediction bias of 1.4 cGy and total prediction errors below 2% in a 0-400 cGy range. We developed a Monte-Carlo-based process to optimize calibrations, depending on the needs of the experiment. This type of thorough analysis can lead to a higher accuracy for film dosimetry.

Radiographic or silver halide film is a well-established 2D dosimeter with an unquestioned spatial resolution. But its higher sensitivity to low-energy photons has to be taken into consideration. Metal compensators or physical modulators to deliver intensity modulated radiation therapy (IMRT) are known to change the beam energy spectrum and to produce scattered photons and contaminating electrons. Therefore the reliability of film dosimetry in compensator-based IMRT might be questioned. Conflicting data have been reported in the literature. This uncertainty about the validity of film dosimetry in compensator-based IMRT triggered us to conduct this study. First, the effect of MCP-96 compensators of varying thickness on the depth dose characteristics was investigated using a diamond detector which has a uniform energy response. A beam hardening effect was observed at 6 MV that resulted in a depth dose increase that remained below 2% at 20 cm depth. At 25 MV, in contrast, beam softening produced a dose decrease of up to 5% at the same depth. Second, dose was measured at depth using EDR2 film in perpendicular orientation to both 6 MV and 25 MV beams for different compensator thicknesses. A film dose underresponse of 1.1% was found for a 30 mm thick block in a 25 MV beam, which realized a transmission factor of 0.243. The effect induced by the compensators is higher than the experimental error but still within the accepted overall uncertainty of film dosimetry in clinical IMRT QA. With radiographic film as an affordable QA tool, the physical compensator remains a low threshold technique to deliver IMRT.

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

Recently, Kodak introduced new light-tight packages (vacuum packaging, aluminium layer under black polyethylene and different paper) for their oncology films (EDR-2, X-Omat V and PPL-2). In order to avoid additional uncertainty and to ensure transferability of previously published results, we assessed in this study the effect of the old and new packages on the dosimetric response of EDR-2 radiographic film. Therefore, sensitometric measurements were performed for different film assemblies (new envelope + new paper, old envelope + old paper, new envelope without paper and old envelope without paper). In addition, to assess possible effects of the package on the film depth-dose response, packaged films were irradiated in parallel geometry, and central depth-dose curves were retrieved. For the perpendicular geometry, on the other hand, the effect of the package was assessed at large depth for a high intensity-modulated inverse-pyramid beam. The results of the sensitometric measurements reveal no difference between the packages. However, the white colour of the paper in both the packages induces a dose-dependent increase in optical density (0-0.12) of the film. The depth-dose curves show better reproducibility for the new package and the new paper improves the accuracy of film dosimetry, but despite the company's effort to evacuate the air out of the new envelope, it remains necessary to clamp the films in the phantom for the parallel irradiation geometry. At 5 cm depth, the films irradiated in parallel geometry show an under-response of 3-5% compared to films irradiated perpendicularly. Finally, even at locations of large photon scatter, no filtration effect from the aluminium layer incorporated in the new envelope has been observed for perpendicular irradiation geometry.

Head-and-neck tumors are often situated at an air-tissue interface what may result in an underdosage of part of the tumor in radiotherapy treatments using megavoltage photons. especially for small fields. In addition to effects of transient electronic disequilibrium, for these small fields, an increased lateral electron range in air will result in an important extra reduction of the central axis (lose beyond the cavity. Therefore dose calculation algorithms need to model electron transport accurately. We simulated the trachea by a 2 cm diameter cylindrical air cavity with the rin) situated 2 cm beneath the phantom surface. A 6 MV photon beam from an Elekta SLi plus linear accelerator, equipped with the standard multileaf collimator (MLC), was assessed. A 10 x 2 cm(2) and a 10 K 1 cm(2) field, both widthwise collimated by the MLC, were applied with their long side parallel to the cylinder axis. Central axis dose rebuild-up was studied. Radiochromic film measurements were performed in an in-house manufactured polystyrene phantom with the films oriented either along or perpendicular to the beam axis. Monte Carlo simulations were performed with BEAM and EGSnrc. Calculations were also performed using the pencil beam (PB) algorithm and the collapsed cone convolution (CCC) algorithm of Helax-TMS (MDS Nordion, Kanata. Canada) version 6.0.2 and using the CCC algorithm of Pinnacle (ADAC Laboratories, Milpitas. CA, USA) version 4.2. A very good agreement between the film measurements and the Monte Carlo simulations was found. The CCC algorithms were not able to predict the interface dose accurately when lateral electronic disequilibrium occurs, but were shown to be a considerable improvement compared to the PB algorithm. The CCC algorithms overestimate the dose in the rebuild-up region. The interface dose was overestimated by a maximum of 31% or 54%, depending on the implementation of the CCC algorithm. At a depth of I rum, the maximum dose overestimation was 14% or 24%. (Less)

In this paper the performance of the LA48 linear ion chamber array (PTW, Freiburg, Germany) for characterization of intensity-modulated (IM) beams was investigated. First, some elementary properties were explored. A series of beam penumbras and output factors for small rectangular fields were measured at 6 and 18 MV, and the results were compared with data obtained using a diamond detector. The energy and dose rate dependence of the array response were examined, and the leakage current was assessed. In a second step, profiles were measured for two clinically delivered IM beams and for a dynamic wedge. The interplay between the sharpening of the penumbra by the upper metal electrode plate of the array and the volume averaging of the 4 x 4 mm ion chamber elements results in precise measurements, even in regions of high dose gradient. It is true, however, that the metal electrodes imply a small energy spectrum dependence in the array response. The dose rate dependence is found to be negligible. All of this makes, the LA48 linear array a suitable device for analysing dose distributions of clinical IM beams.

Volume averaging and lack of electronic equilibrium complicate accurate dosimetry of small photon fields. In this paper the performance of the PinPoint ion chamber for characterizing small fields used in intensity-modulated radiotherapy (IMRT) was investigated and the results were compared with those obtained using the Markus ion chamber and a diamond detector. Sharp beam penumbras were measured for a 5 x 5 cm field defined using a cerrobend block mounted on the accelerator head. In addition, output factors were measured for a 6 MV photon beam and a variety of small rectangular fields collimated widthwise using the multileaf collimator (MLC) in combination with the back-up jaws. From this study, a reference field of 5 x 5 cm and a measuring depth of 5 cm are recommended. This is related to the over-response of the PinPoint chamber to low-energy Compton scattered photons, an effect that was investigated rigorously and turned out to limit the scope of this ionization chamber. However, taking into account some limitations, the PinPoint chamber is an excellent detector for output measurements in small fields down to 2 cm. In profile measurements the chamber causes a broadening of the measured penumbras but its spatial resolution is superior to that of the Markus chamber.

In BANG gel dosimetry, the spin-spin relaxation rate, R2 = 1/T2, is related to the radiation dose that has been delivered to the gel phantom. R2 is calculated by fitting the pixel intensities of a set of differently T2-weighted base images. In gel dosimetry for radiotherapy, an accuracy of 5% in dose and 3 mm spatially, whichever is lower, is the objective. Therefore, possible sources of artefacts must be considered and dealt with. To obtain a set of base images a multiple spin-echo sequence is used. However, in a conventional MR scanner eddy currents will be provoked by switching the imaging gradients. As the eddy currents change in the course of the sequence, the net magnetization will be affected accordingly. Hence, eddy currents may have a significant influence on the quantitative R2 images themselves as well as on their slice position. In this study, we report an analysis of the eddy currents as they appear in the multiple spin-echo sequence. Eddy currents are measured using a frequency shift method resulting in eddy current field maps. The related geometrical displacements are obtained by use of a pyramidal phantom. The R2 versus dose relation is determined in the three main directions of the magnet, revealing a dependence of the measured R2 on slice orientation. The time course of eddy currents is then used in a computer simulation to estimate the effects they produce in the recorded R2 images. A compensation method for eddy current effects in multi-echo T2 mapping is described.

Background and purpose : Concave dose distributions generated by intensity modulated radiotherapy (IMRT) were applied to re-irradiate three patients with pharyngeal cancer. Patients, materials and methods : Conventional radiotherapy for oropharyngeal (patients 1 and 3) or nasopharyngeal (patient 2) cancers was followed by relapsing or new tumors in the nasopharynx (patients 1 and 2) and hypopharynx (patient 3). Six non-opposed coplanar intensity modulated beams were generated by combining non-modulated beamparts with intensities (weights) obtained by minimizing a biophysical objective function. Beamparts were delivered by a dynamic MLC (Elekta Oncology Systems, Crawley, UK) forced in step and shoot mode. Results and conclusions : Median PTV-doses (and ranges) for the three patients were 73 (65–78), 67 (59–72) and 63 (48–68) Gy. Maximum point doses to brain stem and spinal cord were, respectively, 67 Gy (60% of volume below 30 Gy) and 32 Gy (97% below 10 Gy) for patient 1; 60 Gy (69% below 30 Gy) and 34 Gy (92% below 10 Gy) for patient 2 and 21 Gy (96% below 10 Gy) at spinal cord for patient 3. Maximum point doses to the mandible were 69 Gy for patient 1 and 64 Gy for patient 2 with, respectively, 66 and 92% of the volume below 20 Gy. A treatment session, using the dynamic MLC, was finished within a 15-min time slot.

Dosimetric detectors used in high-energy photon radiation dosimetry mainly perform a zero- or one-dimensional measurement. These low-dimensional methods are not always adequate in the context of conformal radiotherapy. Therefore, two-dimensional film dosimetry has attracted attention. We studied a 12-bit CCD-based film digitizer (Vidar VXR-12) with regard to accurate film dosimetry. We investigated the stability, linearity, noise, effects of aberrant light scatter and built-in conversion tables. A digitizing resolution of 75 dpi and a digitizing speed of 20 ms/line result in an optimal signal-to-noise ratio. At optical densities above 2.0, the reading accuracy of the digitizer is limited by noise. The results of various experiments prove both the capabilities and limitations of the digitizer studied. We also propose a method to acquire and process film data using such a digitizer.

Purpose: Radiotherapy treatment modalities are becoming more complex in order to enable advanced patient treatments with a higher dose to irregularly shaped tumor volumes while sparing nearby organs at risk. VMAT as well as RapidArc incorporate capabilities such as variable doserate, variable gantry speed, and accurate and fast dynamic multileaf collimators (DMLC), to optimize dose conformity, delivery efficiency, accuracy and reliability. There is very little information on QA systems and techniques regarding the patient‐specific QA. Therefore, we compared the results obtained with radiographic film to those from the Delta4 phantom. Methods: RapidArc treatment plans were generated using the Varian Eclipse treatment planning software version 8.9 (Varian Medical Systems, Palo Alto, CA). A single but complete (358°) RapidArc was planned. RapidArc treatment plans were delivered to the Delta4 (Delta4) cylindrical (1069 p‐Si Silicon diodes) phantom from Scandidos Uppsala, Sweden, as well as to an equivalent home‐made polyethylene (HD 1000) cylindrical phantom (two longitudinal halves, density 0.94 g/cm3) for EDR2 film dosimetry. Results: The correspondence between D4 and cylindrical phantom measured were analyzed in radial and longitudinal (GT) profiles. The film in the 40° plane displayed a ±3.1% agreement in radial and a 2.2% agreement in GT direction with Delta4 phantom. A small difference was found between the planes because the cylindrical phantom joining the two halves parts has some gaps which might generate the discrepancy between radial and GT direction. The film‐measured dose in the isocenter of the 50° radial and GT plane showed an agreement within ±2.7% with the Delta4 phantom. Conclusion: Film dosimetry validated the Delta4 measurements and it clearly provided more useful information than single point dose measurement. GHENT UNIVERSITY BOF08/DOS/052

Purpose: The aim of this study is to determine the optimal backscatter thickness and later phantom dimension beyond the field size on the dosimetry verification with radiographic film when irradiating large volumes. Methods: The experiments were conducted for 6 and 18 MV 20 × 20 cm2 field sizes with XV2 and EDR2 films positioned at 10 cm depth inside (30 × 30 cm2) polystyrene and Virtual Water™ phantoms by varying the phantom backscattered thickness and changing the table height accordingly. A constant 100 cGy dose was delivered to EDR2 film for 6 and 18 MV by adapting the MU setting. A Farmer‐type ionization chamber was inserted centrally in a dedicated slab so that chamber position was kept at the same depth as the film. A laterally increased phantom studies were done by symmetrically increasing the lateral dimension in the gun‐target direction of the linear accelerator. Results: For 6 MV beam at 20 cm backscatter thickness, EDR2 film response in polystyrene is 20% higher than in Virtual Water™ phantom. Approximately the same difference exists between the XV2 results. The results show 11.4% and 6.4% over‐response of the XV2 film when compared to the ion chamber for 6 MV 30 × 30 cm2 and 10 × 10 cm2 field sizes respectively when the backscatter phantom thickness is 5 cm. For the same setup, measurements with EDR2 films indicate 8.5% and 1.7% over‐response. The film response on later scattered phantom study show nearly firm within 5 cm of lateral thickness and it increases when lateral thickness increases due to more multiple scatter of low energy photons. Conclusions: The backscattered phantom should not exceed more than 7cm for film accuracy. The lateral extension of the phantom should not be more than 5 cm from the field boundary in the case of large volumes.

Purpose: Small fields has been used in radiotherapy due to development of new technology in tomotherapy, radiosurgery(SRS),intensity modulated radiotherapy(IMRT), Gamma Knife, and Cyber Knife. We study the dosimetry of small fields like output factor, profiles in homogenous and non‐homogeneous phantom. Material and Methods:Field sizes of 0.8, 1.6, 2.4, 3.2, 4.8 and 9.6 cm square (defined at isocenter level) were used for this experiment from Elekta Synergy accelerator (Elekta, Crawley, UK). Two phantoms were used in this study: polystyrene (Polystyrol 495F, BASF, Germany) 30 × 30 × 20 cm3 phantom of 1‐cm thick slabs and a sandwiched (non‐homogeneous) lung equivalent phantom, which is further described. The sandwiched phantom is a combination of nine polystyrene slabs (1‐cm thick each) of density 1.02 ± 0.02 g/cm3 positioned above and below two slabs (1.2‐cm thick each) of lung equivalent density 0.3 ± 0.02g/cm3. EBT film (International Specialty Products, NJ, USA) has been applied in this study and results were compared with diamond detector (Type 60003‐9‐0002 — PTW‐Freiburg). Results: A 4.3% and 1.1%output difference was measured in homogeneous polystyrene phantom between EBT film and diamond measurement at 0.8cm and 1.6cm square field sizes respectively for 6 MV. This value is 2.4% and 1.4% in higher photon energy of 18 MV because lateral range of electrons is the critical parameter that influences the charge particle equilibrium rather than forward range of the electrons. Due to electronic disequilibrium, this value is high (33.6%) in non‐homogeneous phantom. The profile showed that 18MV has lower response for small field than 6MV photon beam in both phantom materials. Conclusion: EBT film can be used for output factor measurements and relative dosimetry for small fields. This uncertainty in non homogeneous medium between two detectors has activated us to continue this study.

Purpose:Radiographic EDR2 film is the most commonly used radiographic film for dose distribution measurement,quality assurance and treatment plan verification. The compensator is an alternative way to deliver intensity modulated treatment. The EDR2 film reliability was investigated for dosimetry and QA of compensator based IMRT.Method and Materials: MC‐96 was used to fabricate the compensator blocks (10, 20, 30 and 50 mm thickness). For this study 6 and 25 MV photon beam qualities from an Elekta SL‐25 were used. The transmission factors of the blocks were measured with a Farmer type ion chamber of 0.6cc for a 10×10cm field size. The film was placed at isocentric distance between the slabs of a polystyrene phantom in a perpendicular geometry. To determine the effect of block thickness on the dose response of EDR2 film for both beam energies, dosemeasurements were made for various field sizes and offsets and compared to Farmer chamber measurements. To obtain a similar exposure of the film, transmission factors were applied to adjust the MUs. An inverse pyramid intensity modulated beam, achieved with a compensator block, was dosimetrically investigated to assess the value of the EDR2 film in a more clinical setting. Results and Discussion: The film showed a systematic under‐response of maximally 1.3% for a 50mm thick block and 25 MV (10×10 cm) beam, which is within the 3% overall uncertainty of film dosimetry for perpendicular geometry. The field offset measurement showed also a film under‐response in the order of 1.3% in the direction towards the gun, which might be due to spectral changes related to the beam bending system. The cross‐sectional dose profiles by compensator blocks showed that the film is within 1.2% to the diamond detector. EDR2 film can be safely and easily used as a 2D dose detector in compensator based IMRT.

In polymer gel dosimetry, the spin–spin relaxation rate R 2=1/ T 2, is related to the absorbed dose that is delivered to a gel phantom by high-energy radiation beams. In a two-points method, R 2 is calculated from two differently T 2-weighted images. In the many-points method, R 2 is calculated by fitting the pixel intensities of a set of differently exponentially T 2-weighted images. An analysis of the influence of noise on the resulting R 2 image may contribute considerably to the enhancement of the accuracy of the dose map. The relation between the noise level in the differently T 2-weighted images and the noise level in the R 2 image is derived mathematically. This relation is dependent on the actual R 2, on the choice of the echo times in the sequences used, and the fitting algorithm. Both a least square fit to the semi-logarithmic T 2-relaxation plot and a maximum-likelihood estimation on the T 2-relaxation plot were investigated. It was found that dose images obtained from R 2 images through calibration will contain noise that originates from the noise in the R 2 image and from the probability distribution of the coefficients of the calibration curve. We present a method for optimizing the echo times in order to maximize the signal-to-noise ratio (SNR) in the resulting R 2 image for both methods and for both fitting algorithms. The mathematical considerations on the SNR as presented, can also be applied on other data sets which display a mono-exponential behavior (e.g. diffusion measurements, T 1 relaxation).