MR imaging of absorbed dose distributions for radiotherapy using ferrous sulphate gels

Determination of paramagnetic ferrous gel sensitivity in low energy x-ray beam produced by a miniature accelerator

The INTRABEAM Carl Zeiss Surgical system (Oberkochen, Germany) is a miniature accelerator producing low energy photons (50 keV maximum). The published dosimetric characterization of the INTRABEAM was based on detectors (radiochromic films or ionization chambers) not allowing measuring the absorbed dose in the first millimeters of the irradiated medium, where the dose is actually prescribed. This study aims at determining with Magnetic Resonance Imaging (MRI) the sensitivity of a paramagnetic gel in order to measure the dose deposit produced with the INTRABEAM from 0 to 20 mm. Although spherical applicators are mostly used with the INTRABEAM system for breast applications, this study focuses on surface applicators that are of interest for cutaneous carcinomas. The irradiations at 12 different dose levels (between 2 Gy and 50 Gy at the gel surface) were performed with the INTRABEAM and a 4 cm surface applicator. The gel used in this study is a new « sensitive » material. In order to compare gel sensitivity at low energy with high energy, gels were irradiated by an 18 MV photon beam produced by a Varian Clinac 2100 CD. T₂ weighted multi echo MRI sequences were performed with 16 echo times. The T₂ signal versus echo times was fitted with a mono-exponential function with 95% confidence interval. The calibration curve determined at low energy is a linear function (r² = 0.9893) with a sensitivity of 0.0381 s^-1.Gy^-1, a similar linear function was obtained at high energy (0.0372 s^-1.Gy^-1 with r² = 0.9662). The calibration curve at low energy was used to draw isodose maps from the MR images. The PDD (Percent Depth Dose) determined in the gel is within 5%-1mm of the ionization chamber PDD except for one point. The dosimetric sensitivity of this new paramagnetic ferrous gel was determined with MRI measurements. It allowed measuring the dose distribution specifically in the first millimeters for an irradiation with the INTRABEAM miniature accelerator equipped with a surface applicator.

A very low diffusion Fricke gel dosimeter with functionalised xylenol orange-PVA (XOPVA)

A gel dosimeter has been developed utilising a recently reported system for reducing Fe³⁺ diffusion in a Fricke gel dosimeter which chelates xylenol orange to the gelling agent poly(vinyl alcohol) (PVA). Formulations were investigated using both gelatin and PVA as the gelling agent, along with the inclusion of glyoxal. The resulting gel had an optical density dose response of 0.0031 Gy^-1, an auto-oxidation rate of 0.000 23 h^-1, and a diffusion rate of 0.132 mm² h^-1 which is a significant improvement over previously reported gelatin based Fricke gel dosimeters. The gel was also shown to be energy and dose-rate independent and could be reused after irradiation. Thus, this gel dosimeter has the potential to provide a safe and practical solution to three dimensional radiation dosimetry in the medical environment.

The correction of time and temperature effects in MR-based 3D Fricke xylenol orange dosimetry

Previously developed MR-based three-dimensional (3D) Fricke-xylenol orange (FXG) dosimeters can provide end-to-end quality assurance and validation protocols for pre-clinical radiation platforms. FXG dosimeters quantify ionizing irradiation induced oxidation of Fe²⁺ ions using pre- and post-irradiation MR imaging methods that detect changes in spin-lattice relaxation rates (R ₁  =  [Formula: see text]) caused by irradiation induced oxidation of Fe²⁺. Chemical changes in MR-based FXG dosimeters that occur over time and with changes in temperature can decrease dosimetric accuracy if they are not properly characterized and corrected. This paper describes the characterization, development and utilization of an empirical model-based correction algorithm for time and temperature effects in the context of a pre-clinical irradiator and a 7 T pre-clinical MR imaging system. Time and temperature dependent changes of R ₁ values were characterized using variable TR spin-echo imaging. R ₁-time and R ₁-temperature dependencies were fit using non-linear least squares fitting methods. Models were validated using leave-one-out cross-validation and resampling. Subsequently, a correction algorithm was developed that employed the previously fit empirical models to predict and reduce baseline R ₁ shifts that occurred in the presence of time and temperature changes. The correction algorithm was tested on R ₁-dose response curves and 3D dose distributions delivered using a small animal irradiator at 225 kVp. The correction algorithm reduced baseline R ₁ shifts from  -2.8  ×  10^-2 s^-1 to 1.5  ×  10^-3 s^-1. In terms of absolute dosimetric performance as assessed with traceable standards, the correction algorithm reduced dose discrepancies from approximately 3% to approximately 0.5% (2.90  ±  2.08% to 0.20  ±  0.07%, and 2.68  ±  1.84% to 0.46  ±  0.37% for the 10  ×  10 and 8  ×  12 mm² fields, respectively). Chemical changes in MR-based FXG dosimeters produce time and temperature dependent R ₁ values for the time intervals and temperature changes found in a typical small animal imaging and irradiation laboratory setting. These changes cause baseline R ₁ shifts that negatively affect dosimeter accuracy. Characterization, modeling and correction of these effects improved in-field reported dose accuracy to less than 1% when compared to standardized ion chamber measurements.

Three-dimensional radiation dosimetry using polymer gel and solid radiochromic polymer: From basics to clinical applications

Accurate dose measurement tools are needed to evaluate the radiation dose delivered to patients by using modern and sophisticated radiation therapy techniques. However, the adequate tools which enable us to directly measure the dose distributions in three-dimensional (3D) space are not commonly available. One such 3D dose measurement device is the polymer-based dosimeter, which changes the material property in response to radiation. These are available in the gel form as polymer gel dosimeter (PGD) and ferrous gel dosimeter (FGD) and in the solid form as solid plastic dosimeter (SPD). Those are made of a continuous uniform medium which polymerizes upon irradiation. Hence, the intrinsic spatial resolution of those dosimeters is very high, and it is only limited by the method by which one converts the dose information recorded by the medium to the absorbed dose. The current standard methods of the dose quantification are magnetic resonance imaging, optical computed tomography, and X-ray computed tomography. In particular, magnetic resonance imaging is well established as a method for obtaining clinically relevant dosimetric data by PGD and FGD. Despite the likely possibility of doing 3D dosimetry by PGD, FGD or SPD, the tools are still lacking wider usages for clinical applications. In this review article, we summarize the current status of PGD, FGD, and SPD and discuss the issue faced by these for wider acceptance in radiation oncology clinic and propose some directions for future development.

Technical Note: Preliminary investigations into the use of a functionalised polymer to reduce diffusion in Fricke gel dosimeters

PURPOSE

A modification of the existing PVA-FX hydrogel has been made to investigate the use of a functionalised polymer in a Fricke gel dosimetry system to decrease Fe(3+) diffusion.

METHODS

The chelating agent, xylenol orange, was chemically bonded to the gelling agent, polyvinyl alcohol (PVA) to create xylenol orange functionalised PVA (XO-PVA). A gel was created from the XO-PVA (20% w/v) with ferrous sulfate (0.4 mM) and sulfuric acid (50 mM).

RESULTS

This resulted in an optical density dose sensitivity of 0.014 Gy(-1), an auto-oxidation rate of 0.0005 h(-1), and a diffusion rate of 0.129 mm(2) h(-1); an 8% reduction compared to the original PVA-FX gel, which in practical terms adds approximately 1 h to the time span between irradiation and accurate read-out.

CONCLUSIONS

Because this initial method of chemically bonding xylenol orange to polyvinyl alcohol has inherently low conversion, the improvement on existing gel systems is minimal when compared to the drawbacks. More efficient methods of functionalising polyvinyl alcohol with xylenol orange must be developed for this system to gain clinical relevance.

Evaluation of a ferrous benzoic xylenol orange transparent PVA cryogel radiochromic dosimeter

A stable cryogel dosimeter was prepared using ferrous benzoic xylenol orange (FBX) in a transparent poly-(vinyl alcohol) (PVA) cryogel matrix. Dose response was evaluated for different numbers of freeze-thaw cycles (FTCs), different concentrations of PVA, and ratios of water/dimethyl sulfoxide. Linear relationships between dose and absorbance were obtained in the range of 0-1000 cGy for all formulations. Increasing the concentration of PVA and number of FTCs resulted in increased absorbance and sensitivity. The effects of energy and dose rate were also evaluated. No significant dose rate dependence was observed over the range 1.05 to 6.33 Gy min(-1). No energy response was observed over photon energies of 6, 10, and 18 MV.

Isotropic three-dimensional MRI-Fricke-infused gel dosimetry

PURPOSE

Fricke-infused gel has been shown to be a simple and attainable method for the conformal measurement of absorbed radiation dose. Nevertheless, its accuracy is seriously hindered by the irreversible ferric ion diffusion during magnetic resonance imaging, particularly when three-dimensional (3D) dose measurement in radiosurgery is considered. In this study, the authors developed a fast three-dimensional spin-echo based Fricke gel dosimetry technique to reduce the adverse effects of ferric ion diffusion and to obtain an accurate isotropic 3D dose measurement.

METHODS

A skull shaped phantom containing Fricke-infused gel was irradiated using Leksell Gamma Knife. The rapid image-based dosimetry technique was applied with the use of a 3D fast spin-echo magnetic resonance imaging sequence. The authors mathematically derived and experimentally validated the correlations between dose-response characteristics and parameters of the 3D fast spin-echo MR imaging sequence. Absorbed dose profiles were assessed and compared to the calculated profiles given by the Gamma Knife treatment planning system. Coefficient of variance (CV%) and coefficient of determination (R(2)) were used to evaluate the precision of dose-response curve estimation. The agreement between the measured and the planned 3D dose distributions was quantified by gamma-index analysis of two acceptance criteria.

RESULTS

Proper magnetic resonance imaging parameters were explored to render an accurate three-dimensional absorbed dose mapping with a 1 mm(3) isotropic image resolution. The efficacy of the dose-response estimation was approved by an R(2) > 0.99 and an average CV% of 1.6%. Average gamma pass-rate between the experimentally measured and GammaPlan calculated dose distributions were 83.8% and 99.7% for 2%/2 and 3%/3 mm criteria, respectively.

CONCLUSIONS

With the designed MR imaging sequence and parameters, total 3D MR acquisition time was confined to within 20 min postirradiation, during which time ferric ion diffusion effects were negligible, thus enabling an accurate 3D radiation dose measurement.

Small field dose delivery evaluations using cone beam optical computed tomography-based polymer gel dosimetry

This paper explores the combination of cone beam optical computed tomography with an N-isopropylacrylamide (NIPAM)-based polymer gel dosimeter for three-dimensional dose imaging of small field deliveries. Initial investigations indicate that cone beam optical imaging of polymer gels is complicated by scattered stray light perturbation. This can lead to significant dosimetry failures in comparison to dose readout by magnetic resonance imaging (MRI). For example, only 60% of the voxels from an optical CT dose readout of a 1 l dosimeter passed a two-dimensional Low's gamma test (at a 3%, 3 mm criteria, relative to a treatment plan for a well-characterized pencil beam delivery). When the same dosimeter was probed by MRI, a 93% pass rate was observed. The optical dose measurement was improved after modifications to the dosimeter preparation, matching its performance with the imaging capabilities of the scanner. With the new dosimeter preparation, 99.7% of the optical CT voxels passed a Low's gamma test at the 3%, 3 mm criteria and 92.7% at a 2%, 2 mm criteria. The fitted interjar dose responses of a small sample set of modified dosimeters prepared (a) from the same gel batch and (b) from different gel batches prepared on the same day were found to be in agreement to within 3.6% and 3.8%, respectively, over the full dose range. Without drawing any statistical conclusions, this experiment gives a preliminary indication that intrabatch or interbatch NIPAM dosimeters prepared on the same day should be suitable for dose sensitivity calibration.

Development and performance evaluation of a dynamic phantom for biological dosimetry of moving targets

A dynamic phantom has been developed to allow for measurement of 3D cell survival distributions and the corresponding distributions of the RBE-weighted dose (RBED) in the presence of motion. The phantom consists of two 96-microwell plates holding Chinese hamster ovary cells inside a container filled with culture medium and is placed on a movable stage. Basic biological properties of the phantom were investigated without irradiation and after irradiation with a carbon ion beam, using both a stationary (reference) exposure and exposure during motion of the phantom perpendicular to the beam with beam tracking. There was no statistically significant difference between plating efficiency measured in the microwells with and without motion (0.75) and values reported in the literature. Mean differences between measured and calculated cell survival for these two irradiation modes were within +/-5% of the target dose of 6 Gy (RBE).

Polymer gel dosimetry

Polymer gel dosimeters are fabricated from radiation sensitive chemicals which, upon irradiation, polymerize as a function of the absorbed radiation dose. These gel dosimeters, with the capacity to uniquely record the radiation dose distribution in three-dimensions (3D), have specific advantages when compared to one-dimensional dosimeters, such as ion chambers, and two-dimensional dosimeters, such as film. These advantages are particularly significant in dosimetry situations where steep dose gradients exist such as in intensity-modulated radiation therapy (IMRT) and stereotactic radiosurgery. Polymer gel dosimeters also have specific advantages for brachytherapy dosimetry. Potential dosimetry applications include those for low-energy x-rays, high-linear energy transfer (LET) and proton therapy, radionuclide and boron capture neutron therapy dosimetries. These 3D dosimeters are radiologically soft-tissue equivalent with properties that may be modified depending on the application. The 3D radiation dose distribution in polymer gel dosimeters may be imaged using magnetic resonance imaging (MRI), optical-computerized tomography (optical-CT), x-ray CT or ultrasound. The fundamental science underpinning polymer gel dosimetry is reviewed along with the various evaluation techniques. Clinical dosimetry applications of polymer gel dosimetry are also presented.

Polymer gel dosimetry

Polymer gel dosimeters are fabricated from radiation sensitive chemicals which, upon irradiation, polymerize as a function of the absorbed radiation dose. These gel dosimeters, with the capacity to uniquely record the radiation dose distribution in three-dimensions (3D), have specific advantages when compared to one-dimensional dosimeters, such as ion chambers, and two-dimensional dosimeters, such as film. These advantages are particularly significant in dosimetry situations where steep dose gradients exist such as in intensity-modulated radiation therapy (IMRT) and stereotactic radiosurgery. Polymer gel dosimeters also have specific advantages for brachytherapy dosimetry. Potential dosimetry applications include those for low-energy x-rays, high-linear energy transfer (LET) and proton therapy, radionuclide and boron capture neutron therapy dosimetries. These 3D dosimeters are radiologically soft-tissue equivalent with properties that may be modified depending on the application. The 3D radiation dose distribution in polymer gel dosimeters may be imaged using magnetic resonance imaging (MRI), optical-computerized tomography (optical-CT), x-ray CT or ultrasound. The fundamental science underpinning polymer gel dosimetry is reviewed along with the various evaluation techniques. Clinical dosimetry applications of polymer gel dosimetry are also presented.

Adaptation of radiation field analyser (RFA) as optical CT scanner for gel dosimetry

Optical scanning is one of the emerging evaluation tools used for obtaining dose distributions in gel dosimetry. A radiation field analyzer adapted into an optical CT scanner to evaluate an irradiated Fricke gel has been already reported by others. This prototype optical CT scanner functions like a first generation x-ray CT scanner in the translate-rotate fashion. A similar scanner was constructed in our department for optical scanning of irradiated FX gel. At first, an aquarium was constructed and fitted into the water phantom of the RFA with provision to place the gel phantom to be scanned along with a light source and detector. The movements of the RFA were utilized to scan the gel phantom. A scan of a cuvette filled with colored solution was carried out and the resulting images were reconstructed and profiles obtained to evaluate the working of the optical scanner. A scan of the gel phantom was then obtained to evaluate the performance of the scanner. Thus a radiation field analyzer (DYNASCAN) was successfully adapted to an optical scanner to evaluate Fricke gels in our department.

Ferrous-Ferric Ion exchange dosemeter

In this work a three-dimensional ferrous-ferric ion exchange dosemeter is proposed and the dose response measured. The dosemeter consists of strong acid cation exchange resin beads in the H form in water. Amberlyst 15 Wet beads with a harmonic mean diameter of 0.600-0.850 mm were prepared by soaking them in an aqueous solution of ferrous ammonium sulphate to exchange ferrous ions for H(+) ions. The beads were rinsed with distilled water and packed in glass vials. Sets of samples with ferrous ion concentrations of 0.5 and 1.0 mM were dosed with 6 MV X rays from a Varian 2100C linac. The spin-lattice relaxation time constants (T1) for the samples were measured using an Apollo spectrometer (Tecmag, Houston, TX) interfaced to a 1.5 T magnet (Magnex, Abingdon, UK). Each sample had two T1 values; a long T1 at 1200 ms that did not significantly change with dose and a short T1 that ranged from 56 ms at 0 Gy to 36 ms at 100 Gy. The R1 vs. dose responses were linear with slopes of 0.066 and 0.079 s(-1) Gy(-1).

Experimental study of attenuation properties of normoxic polymer gel dosimeters

The change in linear attenuation coefficient with absorbed dose has been investigated for aqueous polyacrylamide, gelatine and tetrakis (PAGAT) and aqueous methacrylic acid, gelatine and tetrakis (MAGAT) normoxic polymer gel dosimeters using tetrakis (hydroxy methyl) phosphonium chloride as the antioxidant. The measured linear attenuation coefficient increased linearly with absorbed dose up to 15 Gy for PAGAT gels and 10 Gy for MAGAT gels. Computerized tomography (CT) numbers or Hounsfield units (H) were calculated from the linear attenuation coefficients and compared with values obtained using a CT scanner. Both calculated and measured CT numbers followed a similar pattern when fitted with a biexponential curve. The CT numbers obtained from linear attenuation measurements were found to be greater than that obtained with the CT scanner for both PAGAT and MAGAT polymer gels. The H-dose sensitivities of the MAGAT and PAGAT polymer gel dosimeters measured on a CT scanner were calculated to be (0.85 +/- 0.08) H Gy(-1) and (0.31 +/- 0.03) H Gy(-1), respectively. The H-dose sensitivities of the MAGAT and PAGAT polymer gel dosimeters from attenuation measurements were found to be (1.10 +/- 0.66) H Gy(-1) and (0.34 +/- 0.01) H Gy(-1), respectively.

Magnetic resonance imaging of radiation dose distributions using a polymer-gel dosimeter

A new formulation of a tissue-equivalent polymer-gel dosimeter for the measurement of three-dimensional dose distributions of ionizing radiation has been developed. It is composed of aqueous gelatin infused with acrylamide and N, N'-methylene-bisacrylamide monomers, and made hypoxic by nitrogen saturation. Irradiation of the gel, referred to as BANG, causes localized polymerization of the monomers, which, in turn, reduces the transverse NMR relaxation times of water protons. The dose dependence of the NMR transverse relaxation rate, R2, is reproducible (less than 2% variation) and is linear up to about 8 Gy, with a slope of 0.25 s(-1)Gy(-1) at 1.5 T. Magnetic resonance imaging may be used to obtain accurate three-dimensional dose distributions with high spatial resolution. Since the radiation-induced polymers do not diffuse through the gelatin matrix, the dose distributions recorded by BANG gels are stable for long periods of time, and may be used to measure low-activity radioactive sources. Since the light-scattering properties of the polymerized regions are different from those of the clear, non-irradiated regions, the dose distributions are visible, and their optical densities are dependent on dose.

Spatial resolution of magnetic resonance imaging Fricke-gel dosimetry is improved with a honeycomb phantom

The spatial accuracy of magnetic resonance imaging (MRI) Fricke-gel dosimetry is limited by diffusion of ferric ions. This paper describes a honeycomb structure to limit diffusion of Fe3+ ions in a three-dimensional phantom. Such a phantom containing the dosimeter gel was irradiated to a known dose distribution. Maps of dose distributions were produced from the MR images acquired at 2 and 24 hours after the dose was given. The dose distribution maps verified that the honeycomb structure precludes ion diffusion from one honeycomb cell to another, thus improving the usefulness of MRI Fricke-gel dosimetry.

A systematic review of the precision and accuracy of dose measurements in photon radiotherapy using polymer and Fricke MRI gel dosimetry

The purpose of this work is to undertake a critical appraisal of the evidence in the published literature concerning the basic parameters of accuracy and precision associated with the use of Fricke and polymer gels (in conjunction with MR imaging) as radiation dosimeters in photon radiotherapy, condensing and analysing the body of published information (to the end of April 2002). A systematic review was undertaken addressing specific issues of precision and accuracy asking defined questions of the published literature. Accuracy and precision in relation to gel dosimetry were defined. Information was obtained from published, peer-reviewed journals. A defined search strategy utilizing MeSH headings and keywords, with extensive use of cross-referencing, identified 115 references dealing with gel dosimetry. Exclusion criteria were used to select only data from publications which would give unequivocal evidence. For accuracy, results had to be compared with an ionization chamber as gold standard and all gel samples had to be manufactured in the same batch. For precision, in addition to gels being from the same batch, samples must all have been irradiated at the same time and scanned simultaneously (or within a short time frame). Many results were found demonstrating 'dose mapping' examples using gels. However, there were very few publications containing firm evidence of precision and accuracy. There was no evidence which fulfilled our criteria about accuracy or precision using Fricke gels. For polymer gels only one paper was found for accuracy (4% (Low et al 1999 Med. Phys. 26 1542-51)) and precision (1.7% (Baldock et al 1998 Phys. Med. Biol. 43695-702)); however, both were carried out at only one dose level. If the exclusion criteria were relaxed to include accuracy results comparing gel to a non gold standard dosimeter (e.g. TLD), results give a median accuracy of 10% (range 8-23.5%) for polymer gel (Cosgrove et al 2000 Phys. Med. Biol. 45 1195-210, De Deene et al 1998 Radiother: Oncol. 48 283-91, Farajollahi et al 2000 Br. J. Radiol. 72 1085-92, McJury et al 1999b Phys. Med. Biol. 44 2431-44, Murphy et al 2000b Phys. Med. Biol. 45 835-45, Oldham et al 2001 Med. Phys. 28 1436-45) and 5% for Fricke gel (Chan and Ayyangar 1995b Med. Phys. 22 1171-5). Evidence also points to accuracy worsening at lower dose levels for both gels. The precision data should be viewed with caution as repeated MR measurements were not performed with the same samples. The only precision data for Fricke gels was 1.5% (Johansson Back et al 1998 Phys. Med. Biol. 43 261-76), but for zero dose. In conclusion, despite the amount of published data, sparse research has been undertaken which provides clear evidence of the accuracy and precision for both gels. That which has been published has used higher doses than would be routine in radiotherapy. The basic radiation dosimeter qualities of accuracy and precision have yet to be fully quantified for polymer and Fricke gels at clinically relevant dose levels.

Modeling volume effects of experimental brachytherapy in the rat rectum: uncovering the limitations of a radiobiologic concept

PURPOSE

To analyze the significance of volume effects in experimental brachytherapy, based on modeling normal tissue complication probability.

METHODS AND MATERIALS

Experimental brachytherapy in the rat rectum was based on an eight-step 2.5-mm step size source configuration for 192Ir, afterloaded into an unshielded polystyrene applicator. Volume effects were studied using a half-circumferential lead-shielded applicator and a shorter (two-step) source configuration. The main end point was rectal stenosis.

RESULTS

Rectal stenosis was always caused by a radiation ulcer. With the shielded configuration, single-dose ED50 (50% incidence of rectal stenosis) increased from 23 Gy to 36.5 Gy. Single-dose ED50 for the short configuration was 77.9 Gy. The data showed a reasonable fit to a three-parameter version of the biophysical model described by Jackson et al. (1995). This model assumes that organs consist of a large number of radiobiologically independent subunits and that radiation causes a complication if the fraction of the organ damaged is greater than its functional reserve. The fraction of the organ damaged is calculated summing over fractions of the organ damaged at each dose level. The calculated mean functional reserve (nu50) of the rat rectum, assuming a cumulative functional reserve distribution in the group of experimental rats, was 0.53.

CONCLUSIONS

The volume effect observed within small brachytherapy volumes agreed well with clinical experience of large tolerance doses in contact X-ray therapy. However, the nu50 value was comparable to the high functional reserve value reported for liver. Experimental volume effects probably reflect repair processes originating in the areas adjacent to small radiation fields of brachytherapy more than the radiobiologic characteristics of the cells in the irradiated volume.

Energy and dose rate dependence of BANG-2 polymer-gel dosimeter

The purpose of this study was to evaluate dependence of BANG-2 polymer-gel dosimeter sensitivity on different photon and electron energies as well as on different mean dose rates expressed as repetition rates for a standard clinically used linear accelerator. The sensitivity of the dosimeter was represented by the slope of calibration curve in the linear region measured for each modality. A calibration curve (in the linear region) based on five dosimeters (four irradiated and one background) was obtained for each photon and electron energy and different repetition rates. Dosimeter sensitivity dependence on energy was studied for 4, 6, and 18 MV x-ray photons and for nominal electron energies 9, 12, 16, and 20 MeV. Dosimeter sensitivity dependence on mean dose rate was separately studied for electron and photon beams with the use of repetition rates 80, 160, 240, 320, and 400 MU min(-1). Evaluation of dosimeters was performed on Siemens MAGNETOM EXPERT 1T scanner in the head coil. A multiecho sequence with 16 equidistant echoes was used for the evaluation of irradiated polymer-gel dosimeters. The parameters of the sequence were as follows: TR 2000 ms, TE 22.5-360.0 ms, slice thickness 5 mm, FOV 255 mm, one acquisition. There was observed a trend in polymer-gel dosimeter sensitivity dependence on the quality index of high energy x-ray beams used and on mean electron energy absorbed in the center of the dosimeter. Polymer-gel dosimeter sensitivity was decreasing with increasing photon or electron energy. There was observed no trend in polymer-gel dosimeter sensitivity dependence on mean dose rate expressed as a repetition rate for both photon and electron beams.

High resolution gel-dosimetry by optical-CT and MR scanning

The increased intricacy of Intensity-Modulated-Radiation-Therapy (IMRT) delivery has created the need for a high-resolution 3D-dosimetry (three-dimensional) system capable of measuring and verifying the complex delivery. Present clinical methods are inadequate being restricted to single points (e.g., ion-chambers) or to 2D planes (e.g., film), and are labor intensive. In this paper we show that gel-dosimetry in conjunction with optical-CT scanning can yield maps of dose that are of sufficient accuracy, resolution and precision to allow verification of complex radiosurgery deliveries, and by extension IMRT deliveries. The radiosurgery dose-distribution represents the most challenging case encountered in external beam therapy by virtue of the steep dose-gradients and high resolution of delivery. We characterize the stringent radiosurgery requirements by the RTAP (Resolution-Time-Accuracy-Precision) criteria defined as < or = 1 mm3 spatial resolution, < or = 1 hour imaging time, accurate to within 3%, and within -1% precision. The RTAP criteria is applied to an in-house laser-based optical-CT scanning system presented here, and evaluated using gel-flasks containing BANG3 gel. The same gel flasks were subsequently imaged using the MR imaging protocol recommended by the gel manufacturer, but modified to match as closely as possible the RTAP. The resulting dose-maps demonstrate the high precision (< 1.3% noise at high dose) achievable with optical CT scanning while preserving high spatial resolution (<1 mm3). Using the sequence above, the MR gel-dose maps were found to have poorer precision by a factor of 5, under the strict conditions of the RTAP. The optical CT gel-dosimetry system was further evaluated for the verification of a complex 3-isocenter radiosurgery delivery. In conclusion, this work demonstrates that gel-dosimetry and optical-CT scanning approach an important long-term goal of radiation dosimetry, as specified by the RTAP criteria, and have potential to impact the clinic by improving and facilitating clinical dose verification for the most complex external beam radiation treatments.

Dosimétrie par gels radiosensibles en radiothérapie. Intérêt et méthodes

The goal of conformal radiotherapy is to concentrate the dose in a well-defined volume by avoiding the neighbouring healthy structures. This technique requires powerful treatment planning software and a rigorous control of estimated dosimetry. The usual dosimetric tools are not adapted to visualize and validate complex 3D treatment. Dosimetry by radiosensitive gel permits visualization and measurement of the three-dimensional dose distribution. The objective of this work is to report on current work in this field and, based on our results and our experience, to draw prospects for an optimal use of this technique. Further developments will relate to the realization of new radiosensitive gels satisfying, as well as possible, cost requirements, easy realization and use, magnetic resonance imagery (MRI) sensitivity, tissue equivalence, and stability. Other developments focus on scanning methods, especially in MRI to measure T1 and T2.Key words: gel dosimetry, radiation therapy, quality control.

Performance of a fluorescent screen and CCD camera as a two-dimensional dosimetry system for dynamic treatment techniques

A two-dimensionally position sensitive dosimetry system has been tested for different dosimetric applications in a radiation therapy facility with a scanning proton beam. The system consists of a scintillating (fluorescent) screen, mounted at the beam-exit side of a phantom and it is observed by a charge coupled device (CCD) camera. The observed light distribution at the screen is equivalent to the two-dimensional (2D)-dose distribution at the screen position. It has been found that the dosimetric properties of the system, measured in a scanning proton beam, are equal to those measured in a proton beam broadened by a scattering system. Measurements of the transversal dose distribution of a single pencil beam are consistent with dose measurements as well as with dose calculations in clinically relevant fields made with multiple pencil beams. Measurements of inhomogeneous dose distributions have shown to be of sufficient accuracy to be suitable for the verification of dose calculation algorithms. The good sensitivity and sub-mm spatial resolution of the system allows for the detection of deviations of a few percent in dose from the expected (intended or calculated) dose distribution. Its dosimetric properties and the immediate availability of the data make this device a useful tool in the quality control of scanning proton beams.

Bayesian estimation of relaxation times T(1) in MR images of irradiated Fricke-agarose gels

The authors present a novel method for processing T(1)-weighted images acquired with Inversion-Recovery (IR) sequence. The method, developed within the Bayesian framework, takes into account a priori knowledge about the spatial regularity of the parameters to be estimated. Inference is drawn by means of Markov Chains Monte Carlo algorithms. The method has been applied to the processing of IR images from irradiated Fricke-agarose gels, proposed in the past as relative dosimeter to verify radiotherapeutic treatment planning systems. Comparison with results obtained from a standard approach shows that signal-to noise ratio (SNR) is strongly enhanced when the estimation of the longitudinal relaxation rate (R1) is performed with the newly proposed statistical approach. Furthermore, the method allows the use of more complex models of the signal. Finally, an appreciable reduction of total acquisition time can be obtained due to the possibility of using a reduced number of images. The method can also be applied to T(1) mapping of other systems.

A practical technique for verification of three-dimensional conformal dose distributions in stereotactic radiosurgery

The trend toward conformal techniques in stereotactic radiosurgery necessitates an accurate and practical method for verification of irregular three-dimensional dose distributions. This work presents the design and evaluation of a phantom system facilitating the measurement of conformal dose distributions using one or more arrays of up to 20 radiographic films separated by 3.2 mm-thick tissue-equivalent spacers. Using Electron Gamma Shower version 4 (EGS4) Monte Carlo simulation, we show that for 6 MV radiosurgical photon beams this arrangement preserves tissue-equivalence to within 1%. The phantom provides 0.25 mm in-plane spatial resolution and multiple sets of films may be used to resample the dose volume in orthogonal planes. Dedicated software has been developed to automate the process of ordering and orienting of scanned film images, conversion of scanned pixel value to dose, resampling of one or more sets of film images and subsequent export of images in DICOM format for coregistration of planned and measured dose volumes. Calculated and measured isodose surfaces for a simple, circular-beam treatment agree to within 1.5 mm throughout the dose volume. For conformal radiosurgical applications, the measured and planned dose distributions agree to within the uncertainty of the manufacture of irregularly shaped collimators. The sensitivity of this technique to minor spatial inaccuracies in beam shaping is also demonstrated.

Polyvinyl alcohol-Fricke hydrogel and cryogel: two new gel dosimetry systems with low Fe3+ diffusion

Two new Fricke dosimeter gel systems with low diffusion rates have been developed for 3D radiation dosimetry purposes. Both systems consist of a solution of 20% (by weight) polyvinyl alcohol (PVA) in a 50 mM H2SO4 solution with 0.4 mM ferrous ammonium sulphate and xylenol orange (FX). The difference in the two gels is the way that the gelation process was initiated: either by bringing the temperature to (a) +5 degrees C or (b) -20 degrees C before returning them to room temperature. These gels are termed 'hydrogel' and 'cryogel', respectively. The hydrogel is optically transparent, and can be used with either optical or MRI detection methods for dosimetric imaging. The cryogel is rubbery in texture but opaque, so its internal Fe3+ concentration can only be measured with MRI. The hydrogel's optical attenuation coefficient is linear (r2 = 0.99) with dose from 0 to 20 Gy with a sensitivity of 0.106 cm(-1) Gy(-1) (at 543 nm). In terms of MR relaxation rate, the dose response for both the hydrogel and cryogel was linear (r2 = 0.99) with a sensitivity of 0.020 s(-1) Gy(-1) (at 1.5 T). The Fe3+ diffusion coefficient (at 20 degrees C) was measured to be 0.14 mm2 h(-1), which is significantly lower than similar preparations reported for porcine gelatin or agarose. The PVA-FX gels can be stored for long periods of time before exposure to radiation, since the auto-oxidation rate was 10 times less than that of gelatin-Fricke recipes. The new gels developed in this work are a significant improvement on previous Fricke gel systems.

The use of a semi-customized phantom for verification of conformal plans

We have developed a technique for inverse treatment planning of prostate therapy designed to improve the degree of conformation between the dose distribution and the target volume. We compared the inverse plan with a "standard" four-field box technique as well as a four-field technique using oblique fields ("cross technique"). We validated the dosimetry of the inverse plan using Fricke gel solution in phantom specifically designed for this purpose. The phantom is a Plexiglas tank with a cross section, which approximates the dimensions of the pelvis. Anatomical data from computed tomography (CT) images of a patient were used to simulate organs in our phantom. This allows us to calculate dose distributions with the external geometry of the phantom and internal anatomy of the patient. Dose-volume histograms (DVHs) for the three different plans were calculated. The phantom containing the Fricke gel was irradiated according to the inverse plan. Magnetic resonance (MR) images was used to determine the dose distribution delivered to the phantom. We observe, on DVHs, that the inverse plan significantly reduces the dose to the rectum and the bladder but slightly increases the inhomogeneity inside the target volume. Correlation is good between isodoses on MR images and calculated isodoses. We conclude that inverse planning software can greatly improve the conformal degree of treatment to the prostate. This technique could be applied to other complex anatomic sites at which dose to organs at risk is a limiting factor and increased dose to the target volume is indicated. Our phantom and the Fricke gel solution are convenient to carry out validation of conformal treatments.

MRI image plane nonuniformity in evaluation of ferrous sulphate dosimeter gel (FeGel) by means of T1-relaxation time

MR image nonuniformity can vary significantly with the spin-echo pulse sequence repetition time. When MR images with different nonuniformity shapes are used in a T1-calculation the resulting T1-image becomes nonuniform. As shown in this work the uniformity TR-dependence of the spin-echo pulse sequence is a critical property for T1 measurements in general and for ferrous sulfate dosimeter gel (FeGel) applications in particular. The purpose was to study the characteristics of the MR image plane nonuniformity in FeGel evaluation. This included studies of the possibility of decreasing nonuniformities by selecting uniformity optimized repetition times, studies of the transmitted and received RF-fields and studies of the effectiveness of the correction methods background subtraction and quotient correction. A pronounced MR image nonuniformity variation with repetition and T1 relaxation time was observed, and was found to originate from nonuniform RF-transmission in combination with the inherent differences in T1 relaxation for different repetition times. The T1 calculation itself, the uniformity optimized repetition times, nor none of the correction methods studied could sufficiently correct the nonuniformities observed in the T1 images. The nonuniformities were found to vary considerably less with inversion time for the inversion-recovery pulse sequence, than with repetition time for the spin-echo pulse sequence, resulting in considerably lower T1 image nonuniformity levels.

An application of GafChromic MD-55 film for 67.5 MeV clinical proton beam dosimetry

The purpose of this study is to explore the use of GafChromic MD-55 (RC) film for 67.5 MeV clinical proton beam dosimetry at the Crocker Nuclear Laboratory, University of California, Davis. Several strips of RC film 6 cm x 6 cm in dimension were irradiated at a depth of 18.2 mm corresponding to the middle of a 24 mm spread-out Bragg peak (SOBP). The films were irradiated to a proton dose in the range of 0.5 Gy to 100 Gy. The beam profiles were also measured at the middle of the 24 mm SOBP. The Bragg peak was measured by using a wedge shaped phantom made of Lucite. The Bragg peak measured with RC film was compared with diode and ionization chamber measurements. After background subtraction, the calibration of the dose response of RC film showed, to a maximum deviation of 10%, a linear increase of optical density (OD) with dose from 0.5 to 100 Gy. The uniformity of OD over a single sheet of film showed a variation of +/-6%. The distal-fall off between 90% and 20% measured with GafChromic film for the Bragg peak was 1.3 mm as compared to 1.1 mm for a diode measurement and 1.4 mm for an ionization chamber measurement. The FWHM of the Bragg peak was 7.5 mm when measured with GafChromic film, 5.3 mm when measured with a diode and 8.1 mm as measured by an ionization chamber. The peak/plateau ratio with GafChromic film was 3.3 as compared to 3.7 with a diode and 3.2 with an ionization chamber. In conclusion, GafChromic MD-55 film may be a useful and convenient detector for dose measurement and quality assurance programmes of proton beams.

A fast 3D look-locker method for volumetric T1 mapping

We introduce a fast technique, based on the principles of the 2D Look-Locker T1 measurement scheme, to rapidly acquire the data for accurate maps of T1 in three dimensions. The acquisition time has been shortened considerably by segmenting the acquisition of the k(y) phase encode lines. Using this technique, the data for a 256 x 128 x 32 volumetric T1 measurement can be acquired in 7.6 min. T1 measurements made in phantoms with T1s between 200 and 1200 ms had an accuracy of 4% and a reproducibility of 3.5%. Measurements of T1 made in normal brain using the fast 3D sequence corresponded well with inversion-recovery fast spin-echo measurements.

A system for three-dimensional dosimetric verification of treatment plans in intensity-modulated radiotherapy with heavy ions

The introduction of dynamic intensity modulation into radiotherapy using conventional photon beams or scanning particle beams requires additional and efficient methods of dose verification. Dose measurements in dynamically generated dose distributions with a single ionization chamber require a complete application of the treatment field for each single measurement. Therefore measurements are performed by simultaneous use of multiple ionization chambers. The measurement is performed by a computer controlled system and is comprised of the following steps: (a) automated positioning of the ionization chambers, (b) measurement at these points, (c) a comparison with the calculated dose from the treatment planning system, and (d) documentation of the measurement. The ionization chambers are read out by a multichannel electrometer and are densely packed into a mounting of polymethylmetacrylate, which is attached to the arm of a three-dimensional motor-driven water phantom. The measured and planned dose values are displayed numerically as well as graphically. The mean deviation between measured and planned doses as well as their standard deviation are calculated and displayed. Through printouts complete documentation of the measurement is obtained and a quick decision can be made whether the dose distribution is acceptable for the patient. The system is now routinely used for dose verification at the heavy ion therapy project at the Gesellschaft für Schwerionenforschung in Darmstadt. Up to now 242 measurements have been performed for heavy ion treatment of 30 patients. The system allows efficient verification and documentation of carbon ion fields and is in principle also applicable to intensity-modulated photon beams.

The radiation induced magnetic resonance image intensity change provides a more efficient three-dimensional dose measurement in MRI-Fricke-agarose gel dosimetry

A detailed methodology has been developed to map the spatial dose distribution in a Fricke-agarose gel based on the radiation induced image intensity change in the gel's magnetic resonance (MR) images. Besides the linear correlation between the change in the gel's spin-lattice relaxation rate and the absorbed dose, it is shown here that the radiation induced image intensity change for T1-weighted spin-echo images with TE << TR correlates exponentially to the absorbed dose. Furthermore, at the lower dose region (< 15 Gy), the correlation is fairly linear and its sensitivity is high. The minimum detectable dose is shown to be equivalent to the one obtained using the conventional R1-based approach. Since only one T1-weighted image is required for the dose evaluation, compared to the R1-based method, the total MR imaging time can be reduced from hours to a few minutes. This extensive time reduction avoids ferric ion diffusion effects and provides a practical way to simply and effectively measure the three-dimensional dose distribution using the Fricke-agarose dosimeter gel.