The role of dose distribution gradient in the observed ferric ion diffusion time scale in MRI-Fricke-infused gel dosimetry

Ferric ion diffusion is a detrimental factor in MRI-Fricke-infused gel dosimetry. In this study, a novel approach involving MR image subtraction and a fast image-based dosimetry technique to study ferric ion diffusion effects is presented. The fast image-based approach allows studying dose profile degradation within minutes post-irradiation. The relationship between the rate of dose profile deterioration and dose distribution gradients can be elucidated with the improved imaging temporal resolution also. Our results showed that for a dose distribution with gradient 4 Gy/mm or higher, ferric ion diffusion causes apparent dose profile degradation in 0.5-1 h post-irradiation. For a gradual dose gradient change of 2.1 Gy/mm or smaller, dose profile degradation appears insignificant for a two-hour elapsed diffusion time. These observations agree well with the theoretical analysis of a square dependence between dose profile degradation and dose distribution gradient. Because all stereotactic radiosurgery procedures produce steep dose distributions and because the ideal "snapshot" of MR scanning cannot be achieved, knowledge of the ferric ion diffusion time scale is important in experimental designs in order to avoid potential measurement errors in MRI-Fricke-agarose gel dosimetry.     

Development of high-radiation-sensitive polymer gel for magnetic resonance imaging in three-dimensional dosimetry

We developed a high radiation sensitive polymer gel by modifying the amounts of the gel components and the temperature for the gel preparation. We evaluated its relaxation time linearity against dose and compared the measured dose distribution with the calculated one. For the relaxation time-dose linearity, irradiations were carried out with a linear accelerator using 6 MV photons and doses ranging from 0-5.0 Gy. The relationship between dose and R(2) value (reciprocal of T(2) relaxation time) was measured and it had good linearity over a wide range (0.3-5 Gy). The measured dose distributions were in good agreement with calculated ones. Since the present gel has higher sensitivity and it is synthesized more easily at lower cost than conventional polymer gels, we expect to see improved three-dimensional (3D) dosimetry using it.     

Determination of Radiation Dose Distribution by Magnetic Resonance Imaging in the New Tissue Equivalent Gel

Abstract

A new tissue-equivalent substance for the MR dosimetry has been developed. It is composed of water, bovine serum albumin, acrylamide with N,N′-methylene-bis-acrylamide, ammonium ferrous sulphate and sulphuric acid. The elemental composition, mass density, and electron density of the PIRA gel are closer to real tissue than those of dosimeter gels previously investigated. Irradiation causes the changes in the NMR properties of the gel. The dose dependence of NMR longitudinal relaxation rate, R1, is reproducible (less than 2% variation) and is linear up to about 30 Gy, with a slope of 0.023 s^?1Gy^?1 at 0.48 T. The gel, referred to as PIRA, can be used to obtain accurate radiation dose distribution with conventional magnetic resonance imaging devices.     

Dosimetric evaluation of alanine-in-glass dosimeters at clinical dose levels using high-energy X-rays from a linear accelerator

This study presents the first dosimetric evaluation of the alanine-in-glass dosimeter in radiation therapy. The dosimeter is composed of a Pyrex glass tube filled with pure polycrystalline alanine. 6 MV X-ray beams from a linear accelerator were used to irradiate the dosimeter in a solid water phantom to therapy-level doses ranging from 0 to 30 Gy. An X-band electron paramagnetic resonance (EPR) spectrometer was utilized to measure the absorbed dose of the dosimeter. The doses measured by the dosimeter were compared to those from ion chamber dosimetry. It was found that the dosimeter exhibited a linear response in the dose range from 0.1 to 30 Gy. The deviation between measured and delivered doses was 0.11% over the 0.5–30 Gy range, whereas the deviation increased to about 25% at 0.1 Gy. The lowest detectable dose with an acceptable deviation limit of 5% or less was found to be 0.3 Gy. The inaccuracy in measurements at low doses can be attributed to background signals and instrument noise. The accuracy can be improved by proper selection of measurement conditions and better optimization of equipment. The findings of this study show that the alanine-in-glass dosimeter is suitable for dose measurements with acceptable accuracy down to 0.3 Gy. The dosimeter is therefore has the potential to be employed in radiotherapy applications and quality control procedures.     

Tridimensional dosimetry for prostate IMRT treatments using MAGIC-f gel by MRI

Intensity-modulated radiation therapy is able to deliver complex dose distributions and a system able to determine tridimensional dose distribution during the quality assurance of the treatments would be of great interest. In this context polymeric gels can be a useful dosimeter. This work aims to apply this tridimensional dosimetry technique for two prostate IMRT treatments, using MAGIC-f as a gel dosimeter combined with the magnetic resonance imaging to determinate the dose distributions. Dose images obtained by the gel were compared with the treatment planning images and the gamma index was calculated as a quantitative comparison. In both plans a high pass rate was achieved in the gamma analyses (95.5% and 94.0% for plans 1 and 2 respectively) showing that they were approved in the tridimensional quality assurance.     

Investigation of the dose rate dependency of the PAGAT gel dosimeter at low dose rates

Medical physicists need dosimeters such as gel dosimeters capable of determining three-dimensional dose distributions with high spatial resolution. To date, in combination with magnetic resonance imaging (MRI), polyacrylamide gel (PAG) polymers are the most promising gel dosimetry systems. The purpose of this work was to investigate the dose rate dependency of the PAGAT gel dosimeter at low dose rates. The gel dosimeter was used for measurement of the dose distribution around a Cs-137 source from a brachytherapy LDR source to have a range of dose rates from 0.97 Gy h^?1 to 0.06 Gy h^?1. After irradiation of the PAGAT gel, it was observed that the dose measured by gel dosimetry was almost the same at different distances (different dose rates) from the source, although the points nearer the source had been expected to receive greater doses. Therefore, it was suspected that the PAGAT gel is dose rate dependent at low dose rates. To test this further, three other sets of measurements were performed by placing vials containing gel at different distances from a Cs-137 source. In the first two measurements, several plastic vials were exposed to equal doses at different dose rates. An ionization chamber was used to measure the dose rate at each distance. In addition, three TLD chips were simultaneously irradiated in order to verify the dose to each vial. In the third measurement, to test the oxygen diffusion through plastic vials, the experiment was repeated again using plastic vials in a nitrogen box and glass vials. The study indicates that oxygen diffusion through plastic vials for dose rates lower than 2 Gy h^?1 would affect the gel dosimeter response and it is suggested that the plastic vials or (phantoms) in an oxygen free environment or glass vials should be used for the dosimetry of low dose rate sources using PAGAT gel to avoid oxygen diffusion through the vials.     

Clinical tests of large area thermoluminescent detectors under radiotherapy beams

Two-dimensional (2D) thermoluminescence (TL) dosimetry systems based on LiF:Mg,Cu,P, together with the newly developed, based on CaSO4:Dy, were tested under radiotherapy beams. The detectors were irradiated in a water phantom with 6 MV X-ray beams from linac and read with a dedicated TLD reader. Dose distributions of differently shaped fields and of a full stereotactic plan were measured and compared with planned distributions.Maximum distance-to-agreement (DTA) in the penumbra region was 1 mm for both LiF:Mg,Cu,P and CaSO4:Dy TL sheets, for all the measured fields. Maximum percentage dose difference (DA%) between planned and measured dose value in low dose gradient regions was up to 11% for LiF:Mg,Cu,P TL sheets and 18% for CaSO4:Dy TL sheets. Concerning the full stereotactic plan, the percentage of points with γ-index below 1 is 54.9% for the LiF:Mg,Cu,P-based foil and 96.9% for the CaSO4:Dy TL sheets. Both 2D TL detector types can be considered to be a promising tool for bi-dimensional dose measurements in radiotherapy. Non-homogeneity, presumably due to the TL sheets manufacture, still affects dosimetric distribution and the agreement between planned and measured distributions may depend on the chosen sample.     

Fricke and polymer gel 2D dosimetry validation using Monte Carlo simulation

Complexity in modern radiotherapy treatments demands advanced dosimetry systems for quality control. These systems must have several characteristics, such as high spatial resolution, tissue equivalence, three-dimensional resolution, and dose-integrating capabilities. In this scenario, gel dosimetry has proved to be a very promising option for quality assurance. In this study, the feasibility of Fricke and polymer gel dosimeters suitably shaped in form of thin layers and optically analyzed by visible light transmission imaging has been investigated for quality assurance in external radiotherapy. Dosimeter irradiation was carried out with a 6-MV photon beam (CLINAC 600C). The analysis of the irradiated dosimeters was done using two-dimensional optical transmission images. These dosimeters were compared with a treatment plan system using Monte Carlo simulations as a reference by means of a gamma test with parameters of 1 mm and 2%. Results show very good agreement between the different dosimetric systems: in the worst-case scenario, 98% of the analyzed points meet the test quality requirements. Therefore, gel dosimetry may be considered as a potential tool for the validation of other dosimetric systems.     

Study on energy dependence of PAGAT polymer gel dosimeter evaluated using X-Ray CT

The normoxic polymer gel dosimeter evaluated with X-Ray computed tomography has emerged as a promising tool for measuring the dose delivered during radiotherapy in three dimensions. This study presents the dependence of PAGAT normoxic polymer gel sensitivity to different photon and electron energies. PAGAT polymer gel was prepared under normal atmospheric condition and irradiated with different photon energies of 1.25 MeV from Co-60 and 6 MV and 15 MV from linear accelerator and electron energies of 6, 9, 12, 15, 18 and 21 MeV from linear accelerator. Evaluation of dosimeter was performed with an X-Ray CT scanner. Images were acquired with optimum scanning protocols to reduce the signal-to-noise ratio. The averaged image was subtracted from the unirradiated polymer gel image for background. Central axis depth dose (PDD) curves obtained for each energy and polymer gel dosimeter measurements were in good agreement with diode and film measurements. Hounsfield (HU) – dose response curve for each photon and electron energy were derived from the PDD curve obtained from the gel dosimeter measurements. From the study it was clear that the HU-dose response curve was linear in the region 1–10 Gy. The dosimeter sensitivity was defined as a slope of these linear HU-dose response curves and found that the sensitivity of polymer gel decreases with increase in both photon and electron energies. This trend in dependence of PAGAT gel dosimeter sensitivity to different photon and electron energies was not dosimetrically significant. However, to evaluate the test phantom exposed with one energy using the calibration curve derived at another energy can produce clinically significant error.     

Linearity studies of HD-810 dosimeters by light ion beams

Radiochromic films (RCF), also called GafChromic? films, represent a performant material for accurate quantitative radiation dosimetry. Their compositions allow high dose sensitivity and fewer environmental dependence, giving a good feedback to the absorbed dose value and to the active media absorption, turning color upon being irradiated. The RCF take into account their reduced response near the Bragg peak due to a high linear energy transfer (LET). HD-810 GafChromic? films are tissue-equivalent, have easy optical readings and can be employed for ion dosimetry in radio diagnostic and therapy and for industrial applications. Such dosimeters were employed at Tandetron-Nuclear Physics Institute (?e?, Czech Republic) to study the responses of helium, proton and carbon beams, commonly employed in radiotherapy and microelectronics. The sensitivity of the detector is low enough to measure multiple-beam exposures. The induced effects by the ions in the energy range of 600?keV to 2.0?MeV were investigated in terms of optical absorbance, measured in the irradiated active region of the polymer. The employed ion dose range was between 40?Gy and 2.5?kGy. The experimental results show that the absorbance increases with the irradiation time (i.e. with the absorbed dose). The absorbance induced in the radio chromic film was measured at 673?nm, at which is observed the highest sensitivity of the films. Such data, together with the dose linearity and the dependence on the ion stopping power will be presented and discussed.     

Development of a protocol for small beam bi-dimensional dose distribution measurements with radiochromic films

Gafchromic™ films have become popular due to their ease of use and their near water equivalence. This last property is crucial for stereotactic small beam dosimetry as demonstrated in recent papers. An accurate bi-dimensional dose measurement with Gafchromic™ films is very challenging mainly because of the non-uniformity response of flatbed scanners (used for films digitalization) and their own non-uniformity. The first proposal of this work is to develop bi-dimensional protocol for small beams and evaluate the associated uncertainty. The second proposal is to validate this protocol for the bi-dimensional measurements of treatment plans performed with the CyberKnife^® system.First, the uniformity of an Epson V700 flatbed scanner and a batch of EBT3 Gafchromic™ films has been investigated. A “four films” dosimeter was designed to reduce the errors (statistic and systematic) due to their non-uniformity. Then, the “four films” dosimeter protocol in both a homogeneous (RW3 material) and heterogeneous (RW3, lung-like and bone-like materials) phantoms has been used to measure the bi-dimensional dose distributions of three simple CyberKnife^® treatment plans. Two tumor locations (middle of the lung and near lung/bone interface) were considered for the heterogeneous phantom. These plans were achieved with the 10 mm fixed collimator and modeled with the PENELOPE Monte Carlo code in order to calculate accurate dose distributions. Finally, the “four films” bi-dimensional dose distributions were compared to the PENELOPE Monte Carlo simulations.Regarding the uncertainty associated to the bi-dimensional dose measurement protocol, the relative standard deviation σ_D on the dose was 1.2% in the range from 0.5 to 4.0 Gy. Regarding the protocol validation on CyberKnife^® treatment plans, a very good agreement was found with all measurement points passing the {3% - 3 mm} Gamma Index criteria.     

A method for rapid characterization of diffusion

This paper describes a method to determine molecular displacements as a function of time in just two scans: one reference scan using the Carr-Purcell-Meiboom-Gill (CPMG) sequence, a second scan using a modified CPMG sequence (KCPMG). Measurements on free diffusion in bulk fluids, and on restricted diffusion in porous rock samples are reported. This technique can also be used for rapid measurement of flow and chemical exchange.     

Small field size dose-profile measurements using gel dosimeters,gafchromic films and micro-thermoluminescent dosimeters

The introduction of mini-multi-leaf collimators (MMLC) into radiotherapy has seen the use of smaller field sizes become increasingly important. Small field sizes that tightly conform to precise target regions are sought in radiotherapy to deliver doses with a high therapeutic ratio. MMLCs have made it possible to shrink field sizes in radiotherapy to below half a centimetre. The dosimetry of such fields with conventional dosimeters such as gas-ionisation chambers is not feasible due to limitations caused by the chambers relatively large size compared to the size of the collimated beam. In this work, the dose distribution of radiotherapy beams collimated to such small sizes were examined using polyacrylamide gels dosimeters, Gafchromic films and micro-thermoluminescence dosimeters (micro-TLDs). Dose penumbra widths obtained with gel dosimeters, Gafchormic film and micro-TLDs were generally in agreement with each other, although a wider FWHM of the field was measured with gel in comparison to film. An asymmetric dose distribution between the two axis profiles of a 3 × 3 mm collimated field was observed and can be attributed to an inherent asymmetry of the MMLC.     

Experimental validation of a T2rho transverse relaxation model using LASER and CPMG acquisitions

The transverse relaxation rate (R2=1/T2) of many biological tissues are altered by endogenous magnetized particles (i.e., ferritin, deoxyhemoglobin), and may be sensitive to the pathological progression of neurodegenerative disorders associated with altered brain-iron stores. R2 measurements using Carr-Purcell-Meiboom-Gill (CPMG) acquisitions are sensitive to the refocusing pulse interval (2taucp), and have been modeled as a chemical exchange (CE) process, while R2 measurements using a localization by adiabatic selective refocusing (LASER) sequence have an additional relaxation rate contribution that has been modeled as a R2rho process. However, no direct comparison of the R2 measured using these two sequences has been described for a controlled phantom model of magnetized particles. The three main objectives of this study were: (1) to compare the accuracy of R2 relaxation rate predictions from the CE model with experimental data acquired using a conventional CPMG sequence, (2) to compare R2 estimates obtained using LASER and CPMG acquisitions, and (3) to determine whether the CE model, modified to account for R2rho relaxation, adequately describes the R2 measured by LASER for a full range of taucp values. In all cases, our analysis was confined to spherical magnetic particles that satisfied the weak field regime. Three phantoms were produced that contained spherical magnetic particles (10 microm diameter polyamide powders) suspended in Gd-DTPA (1.0, 1.5, and 2.0 mmol/L) doped gel. Mono-exponential R2 measurements were made at 4T as a function of refocusing pulse interval. CPMG measurements of R2 agreed with CE model predictions while significant differences in R2 estimates were observed between LASER and CPMG measurements for short taucp acquisitions. The discrepancy between R2 estimates is shown to be attributable to contrast enhancement in LASER due to T2rho relaxation.     

Protocol for emergency EPR dosimetry in fingernails

There is an increased need for after-the-fact dosimetry because of the high risk of radiation exposures due to terrorism or accidents. In case of such an event, a method is needed to make measurements of dose in a large number of individuals rapidly and with sufficient accuracy to facilitate effective medical triage. Dosimetry based on EPR measurements of fingernails potentially could be an effective tool for this purpose. This paper presents the first operational protocols for EPR fingernail dosimetry, including guidelines for collection and storage of samples, parameters for EPR measurements, and the method of dose assessment. In a blinded test of this protocol application was carried out on nails freshly sampled and irradiated to 4 and 20 Gy; this protocol gave dose estimates with an error of less than 30%.     

Correction of spatial distortion in magnetic resonance angiography for radiosurgical treatment planning of cerebral arteriovenous malformations.

A treatment planning system based on magnetic resonance (MR) angiographic imaging data for the radiosurgery of inoperable cerebral arteriovenous malformations is reported. MR angiography was performed using a three-dimensional (3D) velocity-compensated fast imaging with steady-state precession (FISP) sequence. Depending on the individual MR system, inhomogeneities and nonlinearities induced by eddy currents during the pulse sequence can distort the images and produce spurious displacements of the stereotactic coordinates in both the x-y plane and the z axis. If necessary, these errors in position can be assessed by means of two phantoms placed within the stereotactic guidance system--a "2D-phantom" displaying "pincushion" distortion in the image, and a "3D-phantom" displaying displacement, warp, and tilt of the image plane itself. The pincushion distortion can be "corrected" (reducing displacements from 2-3 mm to 1 mm) by calculations based on modeling the distortion as a fourth order 2D polynomial. Displacement, warp, and tilt of the image plane may be corrected by adjustment of the gradient shimming currents. After correction, the accuracy of the geometric information is limited only by the pixel resolution of the image (= 1 mm). Precise definition of the target volume could be performed by the therapist either directly in the MR images or in calculated projection MR angiograms obtained by a maximum intensity projection algorithm. MR angiography provides a sensitive, noninvasive 3D method for defining target volume and critical structures, and for calculating precise dose distributions for radiosurgery of cerebral arteriovenous malformations.     

Limits of retrospective accident dosimetry by EPR and TL with natural materials

The radiation response of natural materials and domestic articles was investigated by EPR and TL to select suitable materials for retrospective dose assessment in accident dosimetry. The thermal stability and the influence of UV-exposure to the radiation-induced EPR centres were investigated. Based on a required precision of ±20% for dosimetry the lower limits of applicability of the materials were determined. The lowest dosimetry limits of 0.5 Gy were found using sugar, boiler scale and egg shells by EPR and 0.3 Gy by using TL with boiler scale. A list of materials found not to be applicable for retrospective radiation accident EPR dosimetry is also given.     

Fiber-coupled radioluminescence dosimetry with saturated Al2O3:C crystals: Characterization in 6 and 18 MV photon beams

Radioluminescence (RL) and optically stimulated luminescence (OSL) from carbon-doped aluminum oxide crystals can be used for medical dosimetry in external beam radiotherapy and remotely afterloaded brachytherapy. The RL/OSL signals are guided from the treatment room to the readout instrumentation using optical fiber cables, and in vivo dosimetry can be carried out in real time while the dosimeter probes are in the patient. The present study proposes a new improved readout protocol based solely on the RL signal from Al₂O₃:C. The key elements in the protocol are that Al₂O₃:C is pre-dosed with ∼20 Gy before each measurement session, and that the crystals are not perturbed by optical stimulation. Using 6 and 18 MV linear accelerator photon beams, the new RL protocol was found to have a linear dose-response from 7 mGy to 14 Gy, and dosimetry in this range could therefore be performed using a single calibration factor (∼6 × 10⁶ counts per Gy for a 2 mg crystal). The reproducibility of the RL dosimetry was 0.3% (one relative standard deviation) for doses larger than 0.1 Gy. The apparent RL sensitivity was found to change with accumulated dose ((−0.45 ± 0.03)% per 100 Gy), crystal temperature ((−0.21 ± 0.01)%/ °C), and dose-delivery rate ((−0.22 ± 0.01)% per 100 MU/min). A temporal gating technique was used for separation of RL and stem signals (i.e. Cerenkov light and fluorescence induced in the optical fiber cable during irradiation). The new readout protocol was a substantial improvement compared with the combined RL/OSL protocol, that required relatively long readout times and where the optical stimulation greatly affected the RL sensitivity. The only significant caveat was the apparent change in RL-response with accelerator dose-delivery rate.     

Characterization of the gafchromic EBT3 films for dose distribution measurements in stereotactic radiotherapy

Radiochromic film dosimetry is a promising technique, but at this time there are some artefacts, including non-uniformity, energy dependence and scanner artefacts. Accurate dosimetry with radiochromic films requires characterizing the film as well as the scanning and analysis procedures. In this work, the performance of the EBT3 films in combination with the EPSON Dual Lens Perfection V700 scanner for dose distribution measurements in stereotactic radiotherapy has been evaluated. It has been shown that it was necessary to perform 20 blank scans to obtain the stability of the scanner. In order to reduce the uncertainties due to the non-uniformity of the scan field, it was then decided to use the 12 × 12 cm² central part of the scanner bed. Regarding EBT3 films, intra-sheet and inter-sheet uniformity of unexposed EBT3 films in terms of pixel value has been found to be respectively 0.27% (1 SD) and 0.15% (1 SD). No significant energy dependence has been observed above 30 keV and no angular dependence has been found.