Table of contents

Gel dosimetry is not merely another dosimetry technique. Gel dosimeters are integrating dosimeters that enable dose verification in three dimensions. The application of a 3D dosimetry technique in the clinic would give a real push to the implementation of advanced high-precision radiotherapy technologies in many institutes. It can be expected that with the recent developments in the field towards more user-friendly gel systems and imaging modalities, gel dosimetry will become a vital link in the chain of high-precision radiation cancer therapy in the near future.

Many researchers all over the world have contributed to the emerging technology of gel dosimetry. The research field of gel dosimetry is recognized to be very broad from polymer and analytical chemistry and material research to imaging technologies.

The DOSGEL conferences in the past have proven to be an important forum at which material scientists, chemists, medical physicists, magnetic resonance imaging and radiation specialists brought together a critical mass of thoughts, findings and considerations.

DOSGEL 2004 has been endorsed by many international, supra-national and national medical physics organizations and publishers. These proceedings contain 51 papers that cover various aspects of gel dosimetry.

This paper will review the development of the field of gel dosimetry as an introduction to this years DOSGEL 2004 conference.

This paper looks into the dosimetric challenges posed by IMRT and lists the requirements for the ideal dosimeter. Although gel dosimetry was never off the author's mind, gel dosimetry is deliberately kept in the background.

The innovation of adding a gel matrix to the traditional Fricke dosimeter to stabilize geometric information established the field of gel dosimetry for radiation therapy. A discussion of Fricke gels provides an overview of the issues that determine the dose response of all gel dosimeters in general. In this paper we review some of the features of Fricke systems to illustrate these issues and, in addition, to motivate renewed clinical interest in Fricke gels.

Hydrogels are an interesting class of materials that can be prepared by a variety of methods. The properties of these materials depend on their building blocks and the preparation procedures and can be largely varied.

Among the various classes of hydrogels, are the gels based of physically structuring biopolymers, like gelatine. By chemically modifying these biopolymers, hydrogels with a dual crosslinkage (physical and chemical) can be prepared.

Hydrogels serve a broad range of applications, including biomaterials, matrices for drug delivery and scaffolds for tissue engineering.

Three factors that prohibit widespread clinical use of polyacrylamide gel (PAG) dosimeters are polymerization after irradiation ceases, formation of additional polymer near the edges of irradiated zones, and monomer toxicity. Polymerization can occur long after irradiation ceases because polymeric radicals cannot diffuse and terminate with other radicals. Small monomer molecules can diffuse toward trapped polymeric radicals and polymerize. Edge enhancement occurs because acrylamide and bisacrylamide diffuse from regions of high concentration (where the radical concentration is low) to adjacent regions where monomer concentration is low and the radical concentration is high. As monomers diffuse into the irradiated zone, they are polymerized by radicals near the edge. Acrylamide is a neurotoxin and suspected carcinogen that can be absorbed through the skin and inhaled, making it an undesirable monomer for polymer gel dosimetry. Less toxic monomers, such as n-vinyl formamide, should give similar dosimetry results, with less concern for safety. When selecting alternative monomers and cross-linkers for polymer gel dosimetry, it would be advantageous to choose larger molecules that diffuse more slowly, resulting in less edge enhancement. Larger molecules should also lead to improved safety, because they are less easily absorbed through the skin and are less easily vaporized and inhaled.

For a physical measurement instrument different requirements have to be fulfilled such as its insensitivity to uncontrollable environmental parameters and its stability. On the other hand, in order to meet an assigned accuracy, all measurement instruments should be operated in an approved manner. Polymer gel dosimeters are unique in their kind as they are able to integrate the dose in three dimensions and can be shaped in a humanoid form. In this paper, we focus on different characteristics that determine the accuracy of polymer gel dosimeters from the point-of-view of their use as 3D dosimeters in radiotherapy. It is shown that the accuracy is highly dependent on the composition of polymer gel. The comparison of the radiological characteristics may help in understanding the underlying mechanisms of the polymer gel dosimeters and in optimizing the chemical composition in terms of both dose and spatial accuracy.

Gel dosimetry has been examined as a clinical dosimeter since the 1950s. During the last two decades, however, a rapid increase in the number of investigators has been seen, and the body of knowledge regarding gel dosimetry has expanded considerably. Gel dosimetry is still considered a research project, and the introduction of this tool into clinical use is proceeding slowly. This paper will review the characteristics of gel dosimetry that make it desirable for clinical use, the postulated and demonstrated applications of gel dosimetry, and some complications, set-backs, and failures that have contributed to the slow introduction into routine clinical use.

The first publications describing gel dosimetry used magnetic resonance imaging to detect changes in the proton longitudinal relaxation rate of a gel infused with ferrous ions and irradiated with ionizing radiation. Later, different gel dosimeter systems were proposed that are based on the free-radical polymerization of monomers dispersed in a gel matrix. In these polymer gels, changes in transverse relaxation rates were shown to be dependent on the absorbed dose. More recently, contrast in MR images based on the exchange of magnetization between polymer and water protons, following saturation of the polymer protons, has been exploited in polymer gel dosimeters. In addition, variations in relaxation times in the rotating frame (T_1ρ) have been shown to produce contrast in MR images of irradiated polymer gel dosimeters. The signal and contrast in MR images may be manipulated to reflect a variety of these and other processes within an irradiated sample, An attempt is made here to provide an overview of the main different types of MRI contrast that may be used in gel dosimetry and, where possible, to relate this contrast to the nature of the chemical processes and structural changes that occur within the gels following the absorption of ionizing radiation.

In radiotherapy gel dosimetry a humanoid phantom is irradiated according to the planned treatment of a patient. This results in a three-dimensional dose distribution. In order to read-out the gel dosimeter phantom, magnetic resonance imaging (MRI) is often used. Due to specific disturbances both the spatial and the dose reliability can be compromised. It is essential that the measurement sequence is optimized and that possible imaging artifacts are compensated in such a way that the proposed spatial and dose accuracy are met. In this review, several sources of disturbances are treated and compensation strategies are proposed. A code of good practice for the read-out technique is proposed. Finally, a tool for quality control of the imaging sequence is presented.

Optical computed tomography (CT) is physically similar to x-ray CT but is more versatile since many powerful light sources exist and optical elements such as mirrors, lenses, polarizers and efficient detectors are available. There are many potential forms of optical CT. Attenuation, fluorescence or scatter, polarization and refractive index spatial changes are all examples of optical CT. To date, optical CT for gel dosimetry has been limited to attenuation measurements that are the sum of scatter and absorption along defined lines. Polymerization gels turn white with absorbed dose and attenuation is due to scatter. Radiochromic gels also form a dose image due to changes in visible absorption.

This short review concentrates on the papers published since the DOSGEL 2001 meeting and highlights experimental results and issues that are important for obtaining good quality input data for reconstruction. The format involves selected highlights from the papers and associated points from our experience with optical CT experimentation. The comments are intended to assist researchers unfamiliar with optical measurements to obtain high quality transmission data, a necessary step in quantitative gel dosimetry.

The application of optical-CT scanning to achieve accurate high-resolution 3D dosimetry is a subject of current interest. The purpose of this paper is to provide a brief overview of past research and achievements in optical-CT polymer gel dosimetry, and to review current issues and challenges. The origins of optical-CT imaging of light-scattering polymer gels are reviewed. Techniques to characterize and optimize optical-CT performance are presented. Particular attention is given to studies of artifacts in optical-CT imaging, an important area that has not been well studied to date. The technique of optical-CT simulation by Monte-Carlo modeling is introduced as a tool to explore such artifacts. New simulation studies are presented and compared with experimental data.

Since Gore et al published their paper on Fricke gel dosimetry, the predominant method of evaluation of both Fricke and polymer gel dosimeters has been magnetic resonance imaging (MRI). More recently optical computer tomography (CT) has also been a favourable evaluation method. Other techniques have been explored and developed as potential evaluation techniques in gel dosimetry. This paper reviews these other developments.

Recent studies have used the ferrous-xylenol orange-gelatin (FXG) gel dosimeter for 3D radiation field mapping using the optical computed tomography technique. However the characterisation of the dosimetry performance of the FXG gel has not been detailed, such as the variation in dose-response of the FXG gel with changes in preparation techniques, constituent concentrations, pre-irradiation storage time, and concentrations of additives such as oxygen and saccharides. In this paper these issues are addressed with the aim of developing a standard FXG gel for future use in radiation dosimetry applications.

The knowledge of a radioactive beam energy or quality is important in radiotherapy once it is correlated with the type, size, and localization of the tumor. One indicative of the radiation quality is the half-value-layer (HVL), the material thickness which reduces the beam intensity to half. The analysis of a treatment beam spectrum can be inferred through its homogeneity coefficient (HC, ratio between the first and the second HVL) that for values ≥ 0.7 has the indication to be adequate for treatments. Another important indicator of radiation quality is the mass absorption coefficient (cm²/g), related to the photons energies absorbed in a particular exposed material. Once that several materials can be used as radiation detectors for X and γ dosimetry, this work has the purpose to verify the ferrous Xylenol gelatin (FXG) material performance, through its μ/ρ behavior and compare it with the μ/ρ behavior for soft tissue. The X and γ energies where selected, in the energies normally used in radiotherapy and their spectra were evaluated using the HC coefficient. The μ/ρ, for the FXG material, were obtained experimentally and from simulation with X-COM and a developed routine using the GEANT4 Library. From the results from all μ/ρ values obtained for the FXG material, when compared to those from water, one can see similar behaviors, when one considers measurements for energies greater than 78.0 keV. These results indicate that, once the human body is composed with ±80 % of water, the FXG for the energies used, could also be used as soft tissue simulator.

In this study we have investigated and evaluated some dosimetric properties of polymer gel dosimeter encountered when using gels in clinical radiation therapy environment.

A preliminary investigation is reported using both MRI and CT evaluation techniques of the hypoxic PAG formulation combined with the anti-oxidant THP in order to create a normoxic PAG polymer gel dosimeter. This formulation has been named PAGAT (polyacrylamide, gelatin and THP) polymer gel dosimeter.

Polyacrylamide gel (PAG) dosimetry provides a promising means for the calibration and verification of three-dimensional radiation dose delivery in modern conformal radiotherapy. PAG dosimetry is based on the radiation-induced copolymerization of acrylamide and bisacrylamide monomers in water and gelatin. While the basic mechanisms of the radiation-induced polymerization that gives rise to this technique have been studied experimentally, little work has been performed in modeling the polymerization processes. We have reported such models for homogeneous irradiations of PAG dosimeters previously. In this work we present a more complex model that can account for spatial variations in the dose distribution. The models are intended to provide a better understanding of fundamental dosimeter behaviour, so that the use of PAG systems may become more reliable and improved dosimeters may be developed in the future.

X-ray computed tomography (CT) has emerged as a promising method of extracting three-dimensional (3D) dose information from polymer gel dosimeters. CT gel dosimetry has many practical advantages: ease of use, relatively low cost and accessibility to cancer hospitals and has been shown to have clinical potential. However, a primary disadvantage remains poor dose resolution due in part to the low sensitivity of CT contrast to dose. This CT contrast is the result of a small density change that occurs on polymerization and advanced understanding of this density change is required in order to optimize gel sensitivity to CT. This work proposes a simple model that describes the radiation induced density change in polyacrylamide gel (PAG) as a function of polymer yield and an intrinsic density change that occurs, independent of polymer yield, on polymerization. Using this model in combination with experimental CT and Raman spectroscopy work, several fundamental properties of the PAG density response to dose are discovered. The result is a valuable framework on which to focus future developments of more sensitive CT polymer gel dosimetry systems.

In this paper the influence of the use of alternative gelling agents on the dose response curve of normoxic gels is investigated. Part of the experiment is still in progress.

Over the past 45 years, the ideal physical composition sought from a three dimensional dosimeter was that it should be a transparent plastic that is firm in structure, and tissue equivalent. This paper describes such a dosimeter, which is composed primarily of the synthetic polymer polyurethane.

In this presentation we show results of investigations on gelatin-free dosimeters containing equal amounts of acrylamide and N,N'-methylene-bisacrylamide (named Aqueous Polyacrylamide, APA, dosimeters). The dosimeters were prepared with three different total monomer concentrations (2, 6, and 8% by weight). Nuclear magnetic resonance (NMR) spin-spin and spin-lattice proton relaxation measurements at 20 MHz, and gravimetric analyses performed on all three dosimeters, show a continuous degree of polymerization over the range of dose 0.5 – 25 Gy. The developed NMR model explains the relationship observed between the relaxation data and the amount of cross-linked polymer formed at each dose. This model may be extended with gelatin relaxation data to provide a fundamental understanding of radiation-induced polymerization in the conventional PAG dosimeters.

Static light scattering (SLS) could be a worthy technique to perform a structure analysis of the polymer structures inside radiation sensitive gels. The information obtained with SLS is a static characterization of the particle structures inside the gel. SLS will be combined with NMR relaxometry and NMR diffusion measurements, which deliver a hydrodynamic characterization of the microstructure of the gels.

The formulation for a new polymer gel, which is made under normal atmospheric conditions, has been published. Because manufacturing and processing can be performed under normal atmospheric conditions, the gel is also called normoxic. Only a few accessories for gel production are necessary, facilitating the use of this gel, which can be easily produced in a clinical environment.

The purpose of this study, which is work in progress, was to produce several gel batches with varying concentrations of their compounds, to measure dose response curves, to investigate ageing effects and to use this polymer gel in 3D absolute dose verifications in radiosurgery, IMRT and proton therapy.

In this study we report on our initial experience with MAGIC gels as a dosimetric tool. In particular, we address the issue of the reproducibility of the gel's response to radiation by measuring the spin-spin relaxation times of gels irradiated to known doses using magnetic resonance imaging (MRI) and a conventional multi-echo CPMG pulse sequence.

As a practical implementation of MAGIC gels into the clinic is required, the time to acquire images using MRI must be short. For this reason, the effect of the echo train length used in determining the spin-spin relaxation times was assessed as an initial investigation into whether alternative pulse sequences could be used to accurately measure the gels relaxation properties.

In the last few years there has been considerable interest in the use of polymer gels to measure complex dose distributions in radiotherapy. Despite considerable advantages they are still not widely used in clinical situations. This is due primarily to the difficulty in manufacture, particularly the need to exclude oxygen both from the gel and the manufacturing process, the limited number of suitable phantom materials and the need for easy access to an MRI facility.

The purpose of this paper is to report on an investigation of the basic properties of MAGIC gels namely: linearity of response, effects of temperature and stability.

Since the development of normoxic gels several others have been evaluated for dose response and spatial stability with MRI. Tetrakis (hydroxymethyl) phosphonium chloride (THP) has been used as an oxygen scavenger in MAGAT and PAGAT polymer gel dosimeters. X-ray CT has been used as an evaluation tool to measure dose response and dose distributions of irradiated polymer gel dosimeters. In this work the radiological attenuation properties of the PAGAT and MAGAT normoxic polymer gels are investigated as part of a feasibility study in using x-ray CT as an evaluation tool for normoxic polymer gel dosimeters.

There are different methods for obtaining access to quantitative information about the spatial resolution in dosimetric imaging. For polymer dosimetry these methods have mainly been based on comparisons between the dosimetric results of film or pin-point dose visualization to those of MR-based polymer dosimetry (MRPD) the resolution being typically 1 mm. With difference we present a more systematic approach as known from general imaging concepts based on the modulation transfer concept. We investigate periodic dose modulations in photon radiation fields down to spatial structures of a/2 = 280 μm (a: spatial period) using high resolution MR-microimaging at very small voxel sizes (Vs = 199 × 199 × 1000 μm³). The results are compared to those of a film scanner system as used for high resolution relative dosimetry in clinical routine.

Soon after the introduction of polymer gel dosimeters in 1993, edge enhancing effects were noticed in these gels near high dose gradient regions at higher dose levels (typically doses beyond the linear dose-R2 relationship). Our research group has also reported on this phenomenon in several studies. It is believed that due to a depletion of monomers in the high dose region after irradiation, a net diffusion occurs of unreacted monomer into this region where it reacts with long-living macroradicals. As a result, dose 'overshoots' can be observed in the gel-measured dose distribution. However, this hypothesis has never been substantiated.

When examining the dose-R2 relation post-irradiation, two types of instabilities may be observed: (a) a polymerization reaction and (b) the gelation process of gelatin. The polymerization reaction post-irradiation is also believed to be due to the creation of macroradical products.

A mathematical model is proposed that links the temporal instability to the overshoots near high dose gradients. This model is supported by experimental data. The work is still in progress.

In synchrotron stereotactic radiotherapy (SSR) the tumor is loaded with a high Z element, and exposed to monochromatic x-rays from a synchrotron source, in stereotactic conditions. The SSR treatment efficiency is related to the heavy element concentration achievable in the tumor, to the radiation energy, and the irradiation geometry.

The experimental dose verification of SSR in three dimensions with a good spatial resolution is highly desirable for radiotherapy. In this study, Fricke dosimeter and Monte Carlo calculations were employed for assessing 2D dose distribution.

The method used to perform dosimetry with Fricke-xylenol orange-infused gels in form of layers remains the most reliable method for in-phantom dose profiling and imaging in high fluxes of thermal and epithermal neutrons.

Gel-dosimeters in form of layers really give the possibility not only of obtaining spatial dose distributions but also of achieving measurements of each dose contribution in neutron fields. These advantages arise from the layer-geometry thanks to which neutron transport is not sensibly altered, even if the elemental gel composition is changed adding particular isotopes (for example ¹⁰B), as necessary to perform the separation of dose contributions.

The gel matrix composition and the experimental procedures, adopted for both dosimeter preparation and analysis, have been already described in previous works. In the present work, the improvements of the method employed for gel analysis, dose imaging and gel applications are illustrated.

The goal of this work is to calculate the effect of including the anterior and posterior ovoid shields on the dose distribution around a Fletcher Suit Delclos (FSD) ovoid (Nucletron Trading BV, Leersum, Netherlands) and verify these calculations with normoxic polymer gel dosimetry. To date, no Monte Carlo results verified with dosimetry have been published for this ovoid.

In this study, we investigated the feasibility of performing measurements of brachytherapy sources in close proximity to the source.

X-ray CT has been used to evaluate polymer gel dosimeters for dose response in the therapeutic dose range. This method of polymer gel dosimeter evaluation has been shown to be useful for instance in the comparison of complex sterotactic field distributions with treatment plans. Image averaging and subtraction techniques are used for noise reduction in polymer gel dosimeters resulting in the delivery of several CT slices across the polymer gel dosimeters.

It was a logical progression to evaluate normoxic polymer gel dosimeters with optimized CT scanning protocols. During these investigations it was found that unirradiated regions in irradiated normoxic polymer gel dosimetry phantoms polymerised possibly as a result of the evaluation using CT. This prompted an investigation of the CT diagnostic dose response of the normoxic polymer gel dosimeter in order to determine the dose contribution when evaluated using a CT scanner.

Having established that there was an effect on the normoxic polymer gel dosimeter when evaluating with a CT scanner the suitability of these gels in the determination of CT diagnostic dose measurement was further investigated.

Polymer gel dosimeters change their magnetic resonance (MR) and optical properties with the absorbed dose when irradiated and are suitable for narrow photon beam dosimetry in radiosurgery. Such dosimeters enable relative and absolute 3D dose verifications in order to check the entire treatment chain from imaging to dose application during commissioning and quality assurance.

For absolute 3D dose verifications in radiosurgery using Gamma Knife B, commercially available BANG™ Gels (BANG 25 Gy and BANG 3 Gy) together with dedicated phantoms were chosen in order to determine the potential of absolute gel dosimetry in radiosurgery.

Investigations of the dose dependent change in optical transmission, dose response, for radiochromic ferrous-xylenol orange-gelatin gels (FXG) 3D optical CT scanning has revealed that gelation time, temperature, and dose fractionation affect the dose response (Δμ/Δdose). Correction for these factors is important for developing a reproducible dosimeter that can be reliably calibrated and used clinically. The purpose of this report is to examine trends in dose response changes for the following parameters: gelation time-temperature, concentrations of ferrous ion and xylenol orange (XO), dose range and dose fractionation.

Environmental effects on the optical properties of a sensitive radiochromic gel dosemeter; in particular storage, irradiation and measurements temperature were studied. Knowledge of light temperature and other ambient effects help to optimise working conditions and minimize errors. A ferrous-sulphate dosemeter with xylenol orange ion indicator incorporated in a gelatin gel matrix (FXG) was prepared under normal working conditions, and the samples were then kept in closed storage area at different temperature ranging from 5°C up to the gel melting temperature about 35°C. The samples optical absorbance was then measured quantitatively using double beam spectrophotometry. There is a small and steady increase in the absorbance 0.3×10^-3 /°C with increasing temperature until about 30°C when we observe a big jump in the gel absorbance. Finally, additional important behaviour of FXG material was noticed, that is the changes occurred under the influence of rising temperature are reversible which is different from the permanent radiation caused changes.

This article demonstrates the resolution capabilities of the CCD scanner under ideal circumstances and describes the first CCD-based optical CT experiments on a new class of dosimeter, known as PRESAGE™ (Heuris Pharma, Skillman, NJ).

Preliminary optical density results on irradiated PRESAGE dosimeter are outlined in this article. PRESAGE is a solid dosimeter, based on a clear polyurethane combined with the leuco-dye leuco-malachite green. The purpose of these measurements was a) to obtain spectra for optimizing the wavelength of a new light source for the equipment and b) to obtain a dose-response relation. 10 PRESAGE cuvettes were given uniform doses from 0.1 to 40 Gy and later read out by spectrophotometer. The instrument used was CAMSPEC M350 Double Beam Spectrophotometer.

Our previous studies of ferrous xylenol orange gelatin gel have revealed a spatial dependence to the dose response of samples contained in 10 cm diameter cylinders. Dose response is defined as change in optical attenuation coefficient divided by the dose (units cm⁻¹ Gy⁻¹). This set of experiments was conducted to determine the reproducibility of our preparation, irradiation and full 3D optical cone beam CT scanning. The data provided an internal check of a larger storage time-dose response dependence study.

This work investigates factors which affect image noise in CT polymer gel dosimetry, discusses techniques that can be used to further improve image noise and provides overall recommendations for the CT imaging of polymer gels.

An X-ray micro-tomography system has been designed that is optimised for low dose imaging of radiation sensitive polymer gels. The scanner is based on a third-generation cone-beam X-ray CT scanner. A mini-focus X-ray source (Oxford XTF5011 or an Oxford series 1500) is used in conjunction with a 100mm diameter X-ray image intensifier (Hamamatsu C7336). The sample may be rotated and moved perpendicular to the beam axis.

Modern radiotherapy methods require sophisticated treatment planning and radiation dose delivery routines, which in turn demand stringent quality assurance. This paper describes recent advances in three-dimensional optical computed tomography (OCT) readout for radiation dosimetry in which a tissue-equivalent radiochromic gels is used to achieve sub-millimetre spatial resolution for (512)³ voxel arrays in readout times of about half an hour.

Optical computed tomography (CT) scanners have been developed for quantitative reading of dose images written into radiation sensitive gels. Laser equipped, single ray CT scanners, similar to the first generation x-ray CT scanner geometry, provide high quality data because of efficient scatter rejection. These systems are effective for quantitative transmission measurements with both polymerization and radiochromic gels. Widespread research into optical CT gel dosimetry is limited by the lack of readily available commercial instruments. This project demonstrates how scanning water phantoms, which are common equipment in radiotherapy institutions, can easily be modified to perform optical laser CT scanning.

In this study we have attempted to setup a simple optical cone beam CT using the geometry used by Wolodzko et al and Jordan et al using an Intel webcam. This approach of recording transmission images of the gel is the inverse of x-ray cone beam CT if you consider only the rays, which contribute to image formation. This simple optical cone beam CT could be setup with minimum cost and could be used to demonstrate the principle of optical CT for teaching and if further investigated could be a potential optical readout device for gel dosimetry.

We present first results on self-manufactured normoxic gels and their applicability for measuring high dose gradients with high resolution in stereotactic radiation therapy.

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

Spatially fractionated radiotherapy through a grid is a concept which has a long history and was routinely used in orthovoltage radiation therapy in the middle of last century to minimize damage to the skin and subcutaneous tissue. With the advent of megavoltage radiotherapy and its skin sparing effects the use of grids in radiotherapy declined in the 1970s. However there has recently been a revival of the technique for use in palliative treatments with a single fraction of 10 to 20 Gy.

In this work the absolute 3D dose distribution in a grid irradiation is measured for photons using a combination of film and gel dosimetry.

Conventional dosimeters such as ionization chambers, thermoluminescent dosimeters and diodes are point dose measuring devices and therefore difficult to use for spatial evaluation of dose distributions. The film dosimeter is limited to 2D measurements, and the response is dependent of radiation direction. However, using gel dosimetry, dose information can be obtained in 3D with high spatial resolution. Intensity modulated radiation therapy (IMRT) usually involves steep dose gradients in all directions. For verification of this type of treatments gel dosimetry has shown to be suitable.

The aim of this study was to investigate the feasibility of using a new type of normoxic polymer gel dosimeter and a water filled homogenous pelvis phantom with a gel insert for verification of IMRT prostate dose distributions.

Stereotactic irradiation with the Leksell gamma knife (Elekta Instrument AB, Stockholm, Sweden) is one of the primary methods used for the stereotactic radiosurgery treatment of intracranial lesions. To assure the quality of the whole treatment procedure a proper dosimetric system is required. The polymer-gel dosimeter evaluated by nuclear magnetic resonance (NMR) is a promising tool to satisfy this requirement.

The purpose of this study was to investigate the use of polymer-gel dosimeter as a dosimetric tool for the quality control of stereotactic radiosurgery procedures performed by the Leksell gamma knife.

A laser-based optical-CT scanning system has been developed in our laboratory with the capability for high resolution 3D dosimetry. Basic characterization of the performance of the scanner, presented in previous work showed that relative 3D dose mapping with accuracy ≥96% at a spatial resolution of 1 mm³ was a feasible goal. Here we present initial clinical application of the system to verify a 5 field IMRT prostate patient treatment plan.

In this study BANG® polymer gels in conjunction with OCTOPUS™ optical CT scanner (MGS Research Inc., Madison, CT) was employed to measure the relative 3D dose distribution of an IMRT treatment. Measured relative dose distributions from the gel measurement were compared with those from treatment planning system calculations and EDR2 film measurements with regard to planar dose distributions in axial, coronal, and sagittal orientations.