Radiochromic film based dosimetry of image-guidance procedures on different radiotherapy modalities.

In this work we compare doses from imaging procedures performed on today's state-of-the-art integrated imaging systems using a reference radiochromic film dosimetry system. Skin dose and dose profile measurements from different imaging systems were performed using radiochromic films at different anatomical sites on a humanoid RANDO phantom. EBT3 film was used to measure imaging doses from a TomoTherapy MVCT system, while XRQA2 film was used for dose measurements from kilovoltage imaging systems (CBCT on 21eX and TrueBeam Varian linear accelerators and CyberKnife stereoscopic orthogonal imagers). Maximum measured imaging doses in cGy at head, thorax, and pelvis regions were respectively 0.50, 1.01, and 4.91 for CBCT on 21eX, 0.38, 0.84, and 3.15 for CBCT on TrueBeam, 4.33, 3.86, and 6.50 for CyberKnife imagers, and 3.84, 1.90, and 2.09 for TomoTherapy MVCT. In addition, we have shown how an improved calibration system of XRQA2 film can achieve dose uncertainty level of better than 2% for doses above 0.25 cGy. In addition to simulation-based studies in literature, this study provides the radiation oncology team with data necessary to aid in their decision about imaging frequency for image-guided radiation therapy protocols.

Linearization of dose-response curve of the radiochromic film dosimetry system.

PURPOSE: Despite numerous advantages of radiochromic film dosimeter (high spatial resolution, near tissue equivalence, low energy dependence) to measure a relative dose distribution with film, one needs to first measure an absolute dose (following previously established reference dosimetry protocol) and then convert measured absolute dose values into relative doses. In this work, we present result of our efforts to obtain a functional form that would linearize the inherently nonlinear dose-response curve of the radiochromic film dosimetry system. METHODS: Functional form [ζ = (-1)[middle dot]netOD((2∕3))∕ln(netOD)] was derived from calibration curves of various previously established radiochromic film dosimetry systems. In order to test the invariance of the proposed functional form with respect to the film model used we tested it with three different GAFCHROMIC™ film models (EBT, EBT2, and EBT3) irradiated to various doses and scanned on a same scanner. For one of the film models (EBT2), we tested the invariance of the functional form to the scanner model used by scanning irradiated film pieces with three different flatbed scanner models (Epson V700, 1680, and 10000XL). To test our hypothesis that the proposed functional argument linearizes the response of the radiochromic film dosimetry system, verification tests have been performed in clinical applications: percent depth dose measurements, IMRT quality assurance (QA), and brachytherapy QA. RESULTS: Obtained R(2) values indicate that the choice of the functional form of the new argument appropriately linearizes the dose response of the radiochromic film dosimetry system we used. The linear behavior was insensitive to both film model and flatbed scanner model used. Measured PDD values using the green channel response of the GAFCHROMIC™ EBT3 film model are well within ±2% window of the local relative dose value when compared to the tabulated Cobalt-60 data. It was also found that criteria of 3%∕3 mm for an IMRT QA plan and 3%∕2 mm for a brachytherapy QA plan are passing 95% gamma function points. CONCLUSIONS: In this paper, we demonstrate the use of functional argument to linearize the inherently nonlinear response of a radiochromic film based reference dosimetry system. In this way, relative dosimetry can be conveniently performed using radiochromic film dosimetry system without the need of establishing calibration curve.

Radiochromic film dosimetry of HDR (192)Ir source radiation fields.

PURPOSE: A radiochromic film based dosimetry system for high dose rate (HDR) Iridium-192 brachytherapy source was described. A comparison between calibration curves established in water and Solid Water™ was provided. METHODS: Pieces of EBT-2 model GAFCHROMIC™ film were irradiated in both water and Solid Water™ with HDR (192)Ir brachytherapy source in a dose range from 0 to 50 Gy. Responses of EBT-2 GAFCHROMIC™ film were compared for irradiations in water and Solid Water™ by scaling the dose between media through Monte Carlo calculated conversion factor for both setups. To decrease uncertainty in dose delivery due to positioning of the film piece with respect to the radiation source, traceable calibration irradiations were performed in a parallel-opposed beam setup. RESULTS: The EBT-2 GAFCHROMIC™ film based dosimetry system described in this work can provide an overall one-sigma dose uncertainty of 4.12% for doses above 1 Gy. The ratio of dose delivered to the sensitive layer of the film in water to the dose delivered to the sensitive layer of the film in Solid Water™ was calculated using Monte Carlo simulations to be 0.9941 ± 0.0007. CONCLUSIONS: A radiochromic film based dosimetry system using only the green color channel of a flatbed document scanner showed superior precision if used alone in a dose range that extends up to 50 Gy, which greatly decreases the complexity of work. In addition, Solid Water™ material was shown to be a viable alternative to water in performing radiochromic film based dosimetry with HDR (192)Ir brachytherapy sources.

Evaluation of EBT-2 model GAFCHROMIC film performance in water.

PURPOSE: The authors present results of the measurements on the impact of radiochromic film immersion in water. The impact of film piece size, initial optical density, postimmersion waiting time prior to scanning, and the time film was kept in water has been investigated. The authors also investigated the pathways of water penetration into the film during the film immersion in water. METHODS: To study the impact of water immersion on change in optical density, the authors used various sizes of the latest EBT-2 model GAFCHROMICTM film: 2 x 2, 4 x 4, and 8 x 8 in.2. In addition, to test any existing dependence of the film's optical density on water diffusion, the authors used two sets of films: Unexposed (0 Gy) and film pieces exposed to a dose of 3 Gy. Times that film pieces were left in water ranged from 30 min to 24 h, and once the film was permanently removed from water, the authors also studied the impact of the scanning time (deltat) that ranged from 0 (films scanned right after removal from water) to 72 h postimmersion. RESULTS: While the penetration depth can reach as much as 9 mm around the edges of the EBT-2 GAFCHROMIC film, the anticipated dose error due to the change in optical density due to the water immersion appears to be negligible for the short immersions of the order of 30 min. However, as the immersion time increases, the anticipated dose error may reach 22 cGy on a 2 x 2 in.2 piece of film, which corresponds to 7% dose error at 3 Gy of measured dose. CONCLUSIONS: In this work, the authors report on an undoubted impact of radiochromic film immersion in water on the measured change in optical density, which may lead to systematic errors in dose measurements if the film is kept in water for longer periods of time. The magnitude of the impact depends on many parameters: Size of the film piece, initial optical density, postimmersion waiting time prior to scanning (defined by the current radiochromic film dosimetry protocol in. place), and the time film was kept in water. The authors also suggested various approaches in correcting for the change in netOD due to water penetration into the film, but the authors believe that the use of the control film piece would be the most appropriate.

Absorption spectra time evolution of EBT-2 model GAFCHROMIC film.

PURPOSE: One of the major drawbacks of the current radiochromic film dosimetry protocols is the postirradiation waiting time. In this article, the authors study the postirradiation time evolution of the absorption spectrum of radiochromic EBT-2 GAFCHROMIC film model. METHODS: Postirradiation scanning times range from 3 min to 5 days and a dose range extends from 0 to 6 Gy. The authors compare the results of absorption spectra measurements for the latest GAFCHROMIC EBT-2 film model to the absorption spectra of the previous EBT GAFCHROMIC film model. The authors also describe a method that can establish the time error constraints on the postirradiation scanning time that will still provide an acceptable dose error for clinical applications if the protocol employing the shorter postirradiation scanning time is implemented in the clinic. RESULTS: The two film models experience the very same dose change in net absorbance with sensitivity of the latest EBT-2 model GAFCHROMIC film being slightly lower than its predecessor. The authors show that for two postirradiation scanning times of 30 min and 24 h, the 1% dose error can be achieved if the scanning time window is less than +/- 5 min and +/- 2 h, respectively. CONCLUSIONS: By comparing the resultant change in net absorbance between the latest EBT-2 and previous EBT GAFCHROMIC film models, the authors conclude that the addition of the yellow marker dye to the sensitive layer does not affect dosimetric properties of the latest film model. The authors also describe a procedure by which one can establish an acceptable time window around chosen postirradiation scanning time protocol that would provide an acceptable dose error for practical purposes.

Towards customizable thin-panel low-Z detector arrays: electrode design for increased spatial resolution ion chamber arrays.

The purpose of the present development is to employ 3D printing to prototype an ion chamber array with a scalable design potentially allowing increased spatial resolution and a larger active area. An additional goal is to design and fabricate a custom size thin-panel detector array with low-Z components. As a proof of principle demonstration, a medium size detector array with 30 × 30 air-vented ion chambers was 3D-printed using PLA as frame for the electrodes. The active-area is 122 mm × 120 mm with 4 × 4 mm2 spatial resolution. External electrodes are cylindrical and made from conductive PLA. Internal electrodes are made from microwire. The array is symmetric with respect to the central plane and its thickness is 10 mm including build-up/-down plates of 2.5 mm thickness. Data acquisition is realized by biasing only selected chamber rows and reading only 30 chambers at a time. To test the device for potential clinical applications, 1D dose profiles and 2D dose maps with various square and irregular fields were measured. The overall agreement with the reference doses (film and treatment planning system) was satisfactory, but the measured dose differs in the penumbra region and in the field size dependence. Both of these features are related to the thin walls between neighboring ion chambers and different lateral phantom scatter in the detector panel vs homogeneous material. We demonstrated feasibility of radiation detector arrays with minimal number of readout channels and low-cost electronics. The acquisition scheme based on selected row or column 'activation' by bias voltage is not practical for 2D dosimetry but it allows for rapid turn-around when testing of custom arrays with the aid of multiple 1D dose profiles. Future progress in this area includes overcoming the limitations due high chamber packing ratio, which leads to the lateral scattering effects.

Positional and angular tracking of HDR 192 Ir source for brachytherapy quality assurance using radiochromic film dosimetry.

PURPOSE: To quantify and verify the dosimetric impact of high-dose rate (HDR) source positional uncertainty in brachytherapy, and to introduce a model for three-dimensional (3D) position tracking of the HDR source based on a two-dimensional (2D) measurement. This model has been utilized for the development of a comprehensive source quality assurance (QA) method using radiochromic film (RCF) dosimetry including assessment of different digitization uncertainties. METHODS: An algorithm was developed and verified to generate 2D dose maps of the mHDR-V2 192 Ir source (Elekta, Veenendaal, Netherlands) based on the AAPM TG-43 formalism. The limits of the dosimetric error associated with source (0.9 mm diameter) positional uncertainty were evaluated and experimentally verified with EBT3 film measurements for 6F (2.0 mm diameter) and 4F (1.3 mm diameter) size catheters at the surface (4F, 6F) and 10 mm further (4F only). To quantify this uncertainty, a source tracking model was developed to incorporate the unique geometric features of all isodose lines (IDLs) within any given 2D dose map away from the source. The tracking model normalized the dose map to its maximum, then quantified the IDLs using blob analysis based on features such as area, perimeter, weighted centroid, elliptic orientation, and circularity. The Pearson correlation coefficients (PCCs) between these features and source coordinates (x, y, z, θy , θz ) were calculated. To experimentally verify the accuracy of the tracking model, EBT3 film pieces were positioned within a Solid Water® (SW) phantom above and below the source and they were exposed simultaneously. RESULTS: The maximum measured dosimetric variations on the 6F and 4F catheter surfaces were 39.8% and 36.1%, respectively. At 10 mm further, the variation reduced to 2.6% for the 4F catheter which is in agreement with the calculations. The source center (x, y) was strongly correlated with the low IDL-weighted centroid (PCC = 0.99), while the distance to source (z) was correlated with the IDL areas (PCC = 0.96) and perimeters (PCC = 0.99). The source orientation θy was correlated with the difference between high and low IDL-weighted centroids (PCC = 0.98), while θz was correlated with the elliptic orientation of the 60-90% IDLs (PCC = 0.97) for a maximum distance of z = 5 mm. Beyond 5 mm, IDL circularity was significant, therefore limiting the determination of θz (PCC ≤ 0.48). The measured positional errors from the film sets above and below the source indicated a source position at the bottom of the catheter (-0.24 ± 0.07 mm). CONCLUSIONS: Isodose line features of a 2D dose map away from the HDR source can reveal its spatial coordinates. RCF was shown to be a suitable dosimeter for source tracking and dosimetry. This technique offers a novel source QA method and has the potential to be used for QA of commercial and customized applicators.

Monte Carlo simulations of different CT X-ray energy spectra within CTDI phantom and the influence of its changes on radiochromic film measurements.

PURPOSE: In this work we use Monte Carlo simulations to investigate change in Computed tomography (CT) X-ray energy spectra between exposures in air and within CT dose index (CTDI) phantom. While the results of these simulations will be relevant when measuring CTDI with any dosimeter, we apply the appropriate beam quality change correction for CTDI measurements using XR-QA2 model GafChromic™ film. METHODS: Dose profiles were measured with film strips, sandwiched between acrylic rods cut in half, placed within CTDI phantoms and scanned before and after irradiation with document scanner in reflective mode. Reference dosimetry system was calibrated in terms of air kerma in air, which was converted into absorbed dose using ratio of mass-energy absorption coefficients water-to-air for a given beam quality, following the AAPM TG-61 protocol. RESULTS: Beam qualities for all film positions within CTDI phantom show beam softening for HVLs above 6 mm Al and beam hardening for HVLs bellow 6 mm Al. Calculated CTDI values using HVL in air for all CTDI positions, and those calculated using the appropriate calibration curves based on beam quality correction show for Head CTDI phantom differences ranging from 0.3% to 2.1% and for Body CTDI phantom from 2.5% to 5.7%. CONCLUSIONS: We describe method for CTDI measurements using radiochromic film dosimetry protocol corrected by the beam quality change within the phantom. Our results show differences in CTDI measurements of up to 5.7% when compared to using film calibration curves for beam quality in air.

Modeling the primary source intensity distribution: reconstruction and inter-comparison of six Varian TrueBeam sources.

The primary source size is one of the most important beam model parameters in small photon fields. In this work we apply a recently suggested reconstruction technique to characterize the primary source of 6 Varian TrueBeam (TB) linacs. A series of photon fluence profile measurements were performed on 6 Varian TB linacs in the crossplane and inplane orientation using radiochromic film in air and a 2 mm Pb foil as a build-up layer. An image reconstruction algorithm was then applied, based on the maximum likelihood expectation-maximization (MLEM) algorithm, to estimate the source distribution. The method iteratively ray-traces photons from the source plane to the measurement plane to extract source profile corrections. The technique was first benchmarked using a Monte Carlo (MC) model of a Varian TrueBeam with known input Gaussian source sizes. The robustness of the suggested technique was also tested by randomly sampling different combinations of source and field size values and repeating the reconstruction. At the MC benchmarking stage the MLEM reconstruction algorithm was capable of reproducing the Gaussian shape with a RMSE less than 4.0%, while the reconstructed source size (FWHM) and field size were determined with an accuracy level of 0.14 mm and 0.10 mm respectively. Experimentally, the reconstructed TB sources presented FWHM values between 1.02-1.5 mm ([Formula: see text]-0.18 mm) and 1.08-1.42 mm ([Formula: see text]-0.13 mm) in the crossplane and inplane orientations respectively. All TB sources studied in this work can be considered symmetric within uncertainties with the exception of one. The source distribution presented systematic deviations from a Gaussian distribution mostly in the lower tail region. Multi-parameter functional forms, such as Pearson VII or double Gaussian presented improvements in modeling the source in this region, but increase the model complexity. The reconstructed sources measured in this work can serve as reference values for commissioning beam models in small fields and set upper and lower thresholds values of the expected source size for a TB linac.

Dose-response linearization in radiochromic film dosimetry based on multichannel normalized pixel value with an integrated spectral correction for scanner response variations.

PURPOSE: To introduce a model that reproducibly linearizes the response from radiochromic film (RCF) dosimetry systems at extended dose range. To introduce a correction method, generated from the same scanned images, which corrects for scanner temporal response variation and scanner bed inhomogeneity. METHODS: Six calibration curves were established for different lot numbers of EBT3 GAFCHROMIC™ film model based on four EPSON scanners [10000XL (2 units), 11000XL, 12000XL] at three different centers. These films were calibrated in terms of absorbed dose to water based on TG51 protocol or TRS398 with dose ranges up to 40 Gy. The film response was defined in terms of a proposed normalized pixel value ( n P V RGB ) as a summation of first-order equations based on information from red, green, and blue channels. The fitting parameters of these equations are chosen in a way that makes the film response equal to dose at the time of calibration. An integrated set of correction factors (one per color channel) was also introduced. These factors account for the spatial and temporal changes in scanning states during calibration and measurements. The combination of n P V RGB and this "fingerprint" correction formed the basis of this new protocol and it was tested against net optical density ( n e t O D X = R , G , B ) single-channel dosimetry in terms of accuracy, precision, scanner response variability, scanner bed inhomogeneity, noise, and long-term stability. RESULTS: Incorporating multichannel features (RGB) into the normalized pixel value produced linear response to absorbed dose (slope of 1) in all six RCF dosimetry systems considered in this study. The "fingerprint" correction factors of each of these six systems displayed unique patterns at the time of calibration. The application of n P V RGB to all of these six systems could achieve a level of accuracy of ± 2.0% in the dose range of interest within modeled uncertainty level of 2.0%-3.0% depending on the dose level. Consistent positioning of control and measurement film pieces and integrating the multichannel correction into the response function formalism mitigated possible scanner response variations of as much as ± 10% at lower doses and scanner bed inhomogeneity of ± 8% to the established level of uncertainty at the time of calibration. The system was also able to maintain the same level of accuracy after 3 and 6 months post calibration. CONCLUSIONS: Combining response linearity with the integrated correction for scanner response variation lead to a sustainable and practical RCF dosimetry system that mitigated systematic response shifts and it has the potential to reduce errors in reporting relative information from the film response.

Dose measurements nearby low energy electronic brachytherapy sources using radiochromic film.

PURPOSE: We investigate the effect of the GafChromic™ film EBT3 model absorbed dose energy response when used for dose measurements around low-energy photon sources. Monte Carlo based correction procedure in synergy with appropriate calibration curves was shown to provide more accurate absorbed dose (either relative or absolute). An assessment was made of possible dose errors that might be encountered if such energy dependent response is ignored. METHODS: We measured PDDs in water from a Xoft 50 kVp source using EBT3 film, and compared to PDD measurements acquired with a PTW-TN34013 parallel-plate ionization chamber. For the x-ray source, we simulated spectra using the EGSnrc (BEAMnrc) Monte Carlo code, and calculated Half Value Layer (HVL) at different distances from the source in water. Measurement strips of EBT3 film were positioned at distances of 2-6 cm from the Xoft source in a water phantom using a custom-made holder and irradiated simultaneously. RESULTS: Our results show that film calibration curves obtained at beam qualities near the effective energy of the Xoft 50 kVp source in water lead to variation in absorbed dose energy dependence of the response of around 5%. However, if the calibration curve was established in an MV beam quality, the error in absorbed dose could be as large as 20%. CONCLUSION: Accurate dose measurements using radiochromic films at low photon energies require that the radiochromic film dosimetry system be calibrated at appropriate corresponding low energies, as large absorbed dose errors are expected when film calibration is performed in MV beam qualities.

Comparison of dose response functions for EBT3 model GafChromic™ film dosimetry system.

OBJECTIVE: Different dose response functions of EBT3 model GafChromic™ film dosimetry system have been compared in terms of sensitivity as well as uncertainty vs. error analysis. We also made an assessment of the necessity of scanning film pieces before and after irradiation. METHODS: Pieces of EBT3 film model were irradiated to different dose values in Solid Water (SW) phantom. Based on images scanned in both reflection and transmission mode before and after irradiation, twelve different response functions were calculated. For every response function, a reference radiochromic film dosimetry system was established by generating calibration curve and by performing the error vs. uncertainty analysis. RESULTS: Response functions using pixel values from the green channel demonstrated the highest sensitivity in both transmission and reflection mode. All functions were successfully fitted with rational functional form, and provided an overall one-sigma uncertainty of better than 2% for doses above 2 Gy. Use of pre-scanned images to calculate response functions resulted in negligible improvement in dose measurement accuracy. CONCLUSION: Although reflection scanning mode provides higher sensitivity and could lead to a more widespread use of radiochromic film dosimetry, it has fairly limited dose range and slightly increased uncertainty when compared to transmission scan based response functions. Double-scanning technique, either in transmission or reflection mode, shows negligible improvement in dose accuracy as well as a negligible increase in dose uncertainty. Normalized pixel value of the images scanned in transmission mode shows linear response in a dose range of up to 11 Gy.

Image quality for radiotherapy CT simulators with different scanner bore size.

PURPOSE: We compare image quality parameters derived from phantom images taken on three commercially available radiotherapy CT simulators. To make an unbiased evaluation, we assured images were obtained with the same surface dose measured using XR-QA2 model GafChromic™ film placed at the imaging phantom surface for all three CT-simulators. METHODS: Radiotherapy CT simulators GE LS 16, Philips Brilliance Big Bore, and Toshiba Aquilion LB were compared in terms of spatial resolution, low contrast detectability, image uniformity, and contrast to noise ratio using CATPHAN-504 phantom, scanned with Head and Pelvis protocols. Dose was measured at phantom surface, with CT scans repeated until doses on all scanners were within 2%. RESULTS: In terms of spatial resolution, the GE simulator appears slightly better, while Philips CT images are superior in terms of SNR for both scanning protocols. The CNR results show that Philips CT images appear to be better, except for high Z material, while Toshiba appears to fit in between the two simulators. CONCLUSIONS: While the image quality parameters for three RT CT simulators show comparable results, the scanner bore size is of vital importance in various radiotherapy applications. Since the image quality is a function of a large number of confounding parameters, any loss in image quality due to scanner bore size could be compensated by the appropriate choice of scanning parameters, including the exposure and by balancing between the additional imaging dose to the patient and high image quality required in highly conformal RT techniques.

Technical Note: Response time evolution of XR-QA2 GafChromic™ film models.

PURPOSE: To evaluate the response of the newest XR-QA2 GafChromic™ film model in terms of postexposure signal growth and energy response in comparison with the older XR-QA (Version 2) model. METHODS: Pieces of film were irradiated to air kerma in air values up to 12 cGy with several beam qualities (5.3-8.25 mm Al) commonly used for CT scanning. Film response was scored in terms of net reflectance from scanned film images at various points in time postirradiation ranging from 1 to 7 days and 5 months postexposure. To reconstruct the measurement signal changes with postirradiation delay, we irradiated one film piece and then scanned it at different point times starting from 2" min and up to 3 days postexposure. RESULTS: For all beam qualities and dose range investigated, it appears that the XR-QA2 film signal completely saturated after 15 h. Compared to 15 h postirradiation scanning time, the observed variation in net reflectance were 3%, 2%, and 1% for film scanned 2" min, 20 min, and 3 h after exposure, respectively, which is well within the measurement uncertainty of the XR-QA2 based reference radiochromic film dosimetry system. A comparison between the XR-QA (Version 2) and the XR-QA2 film response after several months (relative to their responses after 24 h) show differences in up to 8% and 1% for each film model respectively. CONCLUSIONS: The replacement of cesium bromide in the older XR-QA (Version 2) film model with bismuth oxide in the newer XR-QA2 film, while keeping the same single sensitive layer structure, lead to a significantly more stable postexposure response.

Physics aspects of the Papillon technique-Five decades later.

PURPOSE: The Papillon technique using 50-kVp soft X-rays to treat rectal adenocarcinomas was developed and clinically implemented in the 1960s. We describe differences between accurate dosimetry and clinical implementation of this technique that is extending from its very inception to date. METHODS AND MATERIALS: A renaissance of the Papillon technique occurred with two recently introduced 50-kVp systems: Papillon+ by Ariane and a custom-made rectal applicator (consisting of a surface applicator inserted into a proctoscope) by iCAD's Xoft Axxent Electronic Brachytherapy (eBT) System (iCad, Inc., Sunnyvale, CA). In contrast to the initial design, we investigated the impact of introducing a plastic lid, which would provide more reproducible and more accurate dose delivery across the rectal adenocarcinoma patient population. We use both parallel-plate chamber and radiochromic film dosimeters to determine differences in basic dosimetry characteristics (beam half-value layers, outputs, percent depth doses, and profiles) between the Xoft Electronic Brachytherapy rectal applicator system with and without the plastic lid in place. RESULTS: Compared to the open-cone applicator, the proposed applicator with the plastic lid produces a slightly harder (more penetrating) beam quality (half-value layer of 1.4 vs. 1.3-mm Al), but with reduced output (by 33%), and a slightly broader beam with flatness not worse than 3% and symmetry not worse than 2%. CONCLUSIONS: In addition to characterizing beam properties modified by the possible introduction of the plastic cap, we also pointed out and addressed misconceptions in the use of radiochromic films for dose measurements at low-energy photon beams.

Commissioning of applicator-guided stereotactic body radiation therapy boost with high-dose-rate brachytherapy for advanced cervical cancer using radiochromic film dosimetry.

PURPOSE: To describe an EBT3 GAFCHROMIC film-based dosimetry method to be used in commissioning of a combined HDR brachytherapy (HDRB) and stereotactic body radiation therapy (SBRT) boost for treatment of advanced cervical cancer involving extensive residual disease after external beam treatment. METHODS AND MATERIALS: A cube phantom was designed to firmly fit an intrauterine tandem applicator and EBT3 radiochromic film pieces. A high-risk clinical target volume (CTVHR, Total) was contoured with an extended arm at one side. The HDRB treatment was planned to cover the proximal CTVHR, Total with 7 Gy and the distal volume, referred to as CTVHR, Distal, was planned by SBRT for dose augmentation. After HDRB treatment delivery, SBRT treatment was delivered within 1 hour by image guidance using the applicator geometry. Intentional 1D and 2D misalignments were introduced to evaluate the effect on target volumes. In addition, effect of film reirradiation at different time gaps and dose levels was evaluated. RESULTS: Film dosimetric accuracy, with up to 2 hours gap between irradiations, was shown to be unaffected. A 2%/2 mm gamma analysis between measured and planned doses showed agreement of >99%. Misalignments of more than 2 mm between applicator and SBRT isocenter resulted in suboptimal dose-volume histogram affecting mostly D98% and D90% of CTVHR, Distal. CONCLUSIONS: Visualizing how target dose-volume metrics are affected by minor misalignments between SBRT and HDRB dose gradients, in light of achievable phantom-based experimental quality assurance level, encourages the clinical applicability of this technique. Radiochromic film was shown to be a valuable tool to commission procedures combining two different treatment planning systems and modalities with varying dose rates and energy ranges.

Dose comparison between TG-43-based calculations and radiochromic film measurements of the Freiburg flap applicator used for high-dose-rate brachytherapy treatments of skin lesions.

PURPOSE: Current high-dose-rate brachytherapy skin treatments with the Freiburg flap (FF) applicator are planned with treatment planning systems based on the American Association of Physicists in Medicine TG-43 data sets, which assume full backscatter conditions in dose calculations. The aim of this work is to describe an experimental method based on radiochromic film dosimetry to evaluate dose calculation accuracy during surface treatments with the FF applicator at different depths and bolus thicknesses. METHODS AND MATERIALS: Absolute doses were measured using a reference EBT3 radiochromic film dosimetry system within a Solid Water phantom at different depths (0, 0.5, 1, 2, and 3 cm) with respect to the phantom surface. The impact of bolus (up to 3-cm thickness) placed on top of the applicator was investigated for two clinical loadings created using Oncentra MasterPlan: 5 cm × 5 cm and 11 cm × 11 cm. RESULTS: For smaller loading and depths beyond 2 cm and for larger loading and depths beyond 1 cm, the dose difference was less than 3% (±4%). At shallower depths, differences of up to 6% (±4%) at the surface were observed if no bolus was added. The addition of 2-cm bolus for the smaller loading and 1 cm for larger loading minimized the difference to less than 3% (±4%). CONCLUSIONS: For typical FF applicator loading sizes, the actual measured dose was 6% (±4%) lower at the skin level when compared with TG-43. Additional bolus above the FF was shown to decrease the dose difference. The consideration of change in clinical practice should be carefully investigated in light of clinical reference data.

FDG-PET-based differential uptake volume histograms: a possible approach towards definition of biological target volumes.

OBJECTIVE: Integration of fluorine-18 fludeoxyglucose ((18)F-FDG)-positron emission tomography (PET) functional data into conventional anatomically based gross tumour volume delineation may lead to optimization of dose to biological target volumes (BTV) in radiotherapy. We describe a method for defining tumour subvolumes using (18)F-FDG-PET data, based on the decomposition of differential uptake volume histograms (dUVHs). METHODS: For 27 patients with histopathologically proven non-small-cell lung carcinoma (NSCLC), background uptake values were sampled within the healthy lung contralateral to a tumour in those image slices containing tumour and then scaled by the ratio of mass densities between the healthy lung and tumour. Signal-to-background (S/B) uptake values within volumes of interest encompassing the tumour were used to reconstruct the dUVHs. These were subsequently decomposed into the minimum number of analytical functions (in the form of differential uptake values as a function of S/B) that yielded acceptable net fits, as assessed by χ(2) values. RESULTS: Six subvolumes consistently emerged from the fitted dUVHs over the sampled volume of interest on PET images. Based on the assumption that each function used to decompose the dUVH may correspond to a single subvolume, the intersection between the two adjacent functions could be interpreted as a threshold value that differentiates them. Assuming that the first two subvolumes spread over the tumour boundary, we concentrated on four subvolumes with the highest uptake values, and their S/B thresholds [mean ± standard deviation (SD)] were 2.88 ± 0.98, 4.05 ± 1.55, 5.48 ± 2.06 and 7.34 ± 2.89 for adenocarcinoma, 3.01 ± 0.71, 4.40 ± 0.91, 5.99 ± 1.31 and 8.17 ± 2.42 for large-cell carcinoma and 4.54 ± 2.11, 6.46 ± 2.43, 8.87 ± 5.37 and 12.11 ± 7.28 for squamous cell carcinoma, respectively. CONCLUSION: (18)F-FDG-based PET data may potentially be used to identify BTV within the tumour in patients with NSCLC. Using the one-way analysis of variance statistical tests, we found a significant difference among all threshold levels among adenocarcinomas, large-cell carcinoma and squamous cell carcinomas. On the other hand, the observed significant variability in threshold values throughout the patient cohort (expressed as large SDs) can be explained as a consequence of differences in the physiological status of the tumour volume for each patient at the time of the PET/CT scan. This further suggests that patient-specific threshold values for the definition of BTVs could be determined by creation and curve fitting of dUVHs on a patient-by-patient basis. ADVANCES IN KNOWLEDGE: The method of (18)F-FDG-PET-based dUVH decomposition described in this work may lead to BTV segmentation in tumours.

Use of a control film piece in radiochromic film dosimetry.

PURPOSE: Radiochromic films change their color upon irradiation due to polymerization of the sensitive component embedded within the sensitive layer. However, agents, other than monitored radiation, can lead to a change in the color of the sensitive layer (temperature, humidity, UV light) that can be considered as a background signal and can be removed from the actual measurement by using a control film piece. In this work, we investigate the impact of the use of control film pieces on both accuracy and uncertainty of dose measured using radiochromic film based reference dosimetry protocol. METHODS: We irradiated "control" film pieces (EBT3 GafChromic(TM) film model) to known doses in a range of 0.05-1 Gy, and five film pieces of the same size to 2, 5, 10, 15 and 20 Gy, considered to be "unknown" doses. Depending on a dose range, two approaches to incorporating control film piece were investigated: signal and dose corrected method. RESULTS: For dose values greater than 10 Gy, the increase in accuracy of 3% led to uncertainty loss of 5% by using dose corrected approach. At lower doses and signals of the order of 5%, we observed an increase in accuracy of 10% with a loss of uncertainty lower than 1% by using the corrected signal approach. CONCLUSIONS: Incorporation of the signal registered by the control film piece into dose measurement analysis should be a judgment call of the user based on a tradeoff between deemed accuracy and acceptable uncertainty for a given dose measurement.

Radiochromic film-based quality assurance for CT-based high-dose-rate brachytherapy.

PURPOSE: In the past, film dosimetry was developed into a powerful tool for external beam radiotherapy treatment verification and quality assurance. The objective of this work was the development and clinical testing of the EBT3 model GafChromic film based brachytherapy quality assurance (QA) system. METHODS AND MATERIALS: Retrospective dosimetry study was performed to test a patient-specific QA system for preoperative endorectal brachytherapy that uses a radiochromic film dosimetry system. A dedicated phantom for brachytherapy applicator used for rectal cancer treatment was fabricated enabling us to compare calculated-to-measured dose distributions. Starting from the same criteria used for external beam intensity-modulated radiation therapy QA (3%, 3 mm), passing criteria for high- and low-dose gradient regions were subsequently determined. Finally, we investigated the QA system's sensitivity to controlled source positional errors on selected patient plans. RESULTS: In low-dose gradient regions, measured dose distributions with criteria of 3%, 3 mm barely passed the test, as they showed 95% passing pixels. However, in the high-dose gradient region, a more stringent condition could be established. Both criteria of 2%, 3 mm and 3%, 2 mm with gamma function calculated using normalization to the same absolute dose value in both measured and calculated dose distributions, and matrix sizes rescaled to match each other showed more than 95% of pixels passing, on average, for 15 patient plans analyzed. CONCLUSIONS: Although the necessity of the patient-specific brachytherapy QA needs yet to be justified, we described a radiochromic film dosimetry-based QA system that can be a part of the brachytherapy commissioning process, as well as yearly QA program.