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With the increasing complexity of radiotherapy treatments typical 1D and 2D quality assurance (QA) detectors may fail to detect out-of-plane dose discrepancies, in particular in the presence of motion. In this work, small samples of the PRESAGE® 3D radiochromic dosimeter were used in combination with a motion phantom to measure real-time multileaf collimator (MLC)-tracked radiotherapy treatments. A different sample of PRESAGE® was irradiated for each of three different irradiation scenarios: (1) static: static sample, without tracking (2) motion: moving sample, without tracking and (3) tracking: moving sample, with tracking. Our in-house software DynaTrack dynamically moves the linac's MLC leafs based on the target position. The doses delivered to the samples were reconstructed based on the recorded positions of the MLC and phantom during the beam delivery. PRESAGE® samples were imaged with an in-house optical-CT scanner. Comparison between simulated and measured 3D dose showed good agreement for all three irradiation scenarios (static: 99.2%; motion: 99.7%; tracking: 99.3% with a 3%, 2 mm and a 10% threshold local gamma criterion), failing only at the edges of the PRESAGE® samples (~ 6 mm). Given that the dose distributions deposited using the DynaTrack system have been independently verified, this experiment demonstrates the ability of PRESAGE to measure 3D doses correctly in a tracking context. We conclude that this methodology could be used in the future to validate the delivery of dynamic MLC-tracked radiotherapy.

Measurements near the edges of PRESAGE® 3D dosimeters will be important for validating the electron return effect (ERE) that can occur at tissue-air interfaces during radiotherapy treatment delivered with the Elekta MR-linac. We investigate and characterize the spatially non-uniform response of small samples of PRESAGE® to radiation in a conventional linac. We develop a correction to compensate for these non-uniformities and obtain dose values near the dosimeter edges. Five samples from the same batch were uniformly irradiated in a water tank with a broad beam. The non-uniform response of the samples to radiation was investigated and a radial dose-correction function was generated from each sample to obtain a correction image. We then applied these correction images to another sample from the same batch, irradiated with four beams in an inhomogeneous medium, and compared this with the relevant simulated data. Additionally, we irradiated samples after physically removed their edges (axially and top and bottom edges). Higher sensitivity to radiation was observed at the edges (~6mm) of the samples in comparison with the central region. Applying the dose correction function improved agreement between simulations and measurements, but only partial correction was possible. A uniform response was observed on the samples with the edges removed, which we propose as the best option to measure dose at the edges of PRESAGE® samples.

The combination of MRI and radiotherapy on a single platform has the potential to revolutionise image-guided delivery of radiation doses. However, in order to realise these ambitions, good dosimetry must be available. The electron return effect gives rise to significant perturbations of dose at interfaces between tissue and air within the body, and this might lead to difficulties in dose compensation if air cavities move during treatment. In this article, I review briefly the ways in which the available methods of dosimetry are affected by the presence of magnetic fields and discuss the contribution that three-dimensional measurements can make to studies in this area. The methods of MRI and optical computed tomography have well known issues in imaging close to interfaces. These are described together with progress so far in providing solutions.

Medical image analysis remains a challenging application area for artificial intelligence. When applying machine learning, obtaining ground-truth labels for supervised learning is more difficult than in many more common applications of machine learning. This is especially so for datasets with abnormalities, as tissue types and the shapes of the organs in these datasets differ widely. However, organ detection in such an abnormal dataset may have many promising potential real-world applications, such as automatic diagnosis, automated radiotherapy planning, and medical image retrieval, where new multimodal medical images provide more information about the imaged tissues for diagnosis. Here, we test the application of deep learning methods to organ identification in magnetic resonance medical images, with visual and temporal hierarchical features learned to categorize object classes from an unlabeled multimodal DCE-MRI dataset so that only a weakly supervised training is required for a classifier. A probabilistic patch-based method was employed for multiple organ detection, with the features learned from the deep learning model. This shows the potential of the deep learning model for application to medical images, despite the difficulty of obtaining libraries of correctly labeled training datasets and despite the intrinsic abnormalities present in patient datasets.

MRI and laser-scanned optical-CT are widely reported as being three-dimensional readout methods for gel dosimetry. This preliminary study of proton irradiation of a PRESAGETM transparent solid plastic dosimeter is presented as a case study to illustrate the potential of a true-3D approach.

In this paper we describe the particular considerations relating to ultra-rapid, true-3D scanners based on charge-coupled device (CCD) or complementary metal-oxide semiconductor (CMOS) imaging detectors. Central to our ability to image dose distributions has been the development of novel materials whose optical properties change in response to radiation and a survey of these will be given. Finally, we will speculate briefly on the future of the technique. © 2006 IOP Publishing Ltd.

To investigate the feasibility of solid polymer dosimetry as an accurate 3-D dose verification applied to prostatic brachytherapy, the model 6711 Oncoseed TM was selected for a pilot study comparing experimental methods and Monte Carlo simulations. This study is presented as a typical case study of what can be achieved using solid polymer dosimetry. © 2006 IOP Publishing Ltd.

This paper describes the initial steps we have taken in establishing CCD based optical-CT as a viable alternative for 3-D radiation dosimetry. First, we compare the optical density (OD) measurements from a high quality test target and variable neutral density filter (VNDF). A modulation transfer function (MTF) of individual projections is derived for three positions of the sinusoidal test target within the scanning tank. Our CCD is then characterized in terms of its signal-to-noise ratio (SNR). Finally, a sample reconstruction of a scan of a PRESAGETM (registered trademark of Heuris Pharma, NJ, Skillman, USA.) dosimeter is given, demonstrating the capabilities of the apparatus.

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.

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).

An accurate value for the saturation dose of PRESAGE® is important in elucidating the mechanism of the so-called quenching phenomenon in the 3-D dosimetry of proton beams. This quantity is difficult to obtain, because it involves measuring the optical density of a set of very highly attenuating samples. We present a novel method that makes use of the ability to irradiate microscopically-thin "slices" of a cuvette using microplanar beams from a synchrotron. Using the new method, we were able to measure optical densities of up to 50 cm-1 using a standard spectrophotometer, and we derived a value 14000 Gy for the saturation parameter D 37.

International audience; Optical CT is a method that can potentially provide both accurate dosimetry at high spatial resolution and 3-D visualisation over a large field-of-view in a single dataset. The major factors limiting spatial resolution in previous studies are analysed here and it is shown that improvements in equipment specification can overcome many of these. The need for ultra-high spatial resolution in the verification of microbeam radiation therapy verification is demonstrated and example images of a PRESAGE (R) sample are presented

International audience; Optical Computed Tomography (CT) is a promising technique for dosimetry of Microbeam Radiation Therapy (MRT), providing high resolution 3D dose maps. Here different MRT irradiation geometries are visualised showing the potential of Optical CT as a tool for future MRT trials. The Peak-to-Valley dose ratio (PVDR) is calculated to be 7 at a depth of 3mm in the radiochromic dosimeter PRESAGE (R). This is significantly lower than predicted values and possible reasons for this are discussed

Chemical dosimeters, including PRESAGE® as used in optical CT, exhibit significant quenching effects in response to proton irradiation and this may limit their widespread uptake. This study performs careful measurements of the observed quenching of a recently developed variant of PRESAGE® in a 60 MeV proton beam and uses them to attempt an empirical correction of a simple superposition of two unmodulated beams.

High Intensity Focused ultrasound (HIFU) is a non-invasive cancer therapy that makes use of the mainly thermal effects of ultrasound to destroy tissue. In order to achieve reliable treatment planning, it is necessary to characterise the ultrasound source (transducer) and to understand how the wave propagates in tissue and the energy deposition in the focal region. This novel exploratory study investigated how HIFU affects PRESAGE®, an optical phantom used for radiotherapy dosimetry, which is potentially a rapid method of calibrating the transducer. Samples, of two different formulations, were exposed to focused ultrasound and imaged using Optical Computed Tomography. First results showed that, PRESAGE® changes colour on ultrasound exposure (darker green regions were observed) with the alterations being related to the acoustic power and sample composition. Future work will involve quantification of these alterations and understanding how to relate them to the mechanisms of action of HIFU.

From the proceeding of 7th International Conference on 3D Radiation Dosimetry (IC3DDose), Australia, Sydney, 4-8 December 2012. The aim of this study is to present the first experimental validation and quantification of the dose enhancement capability of bismuth oxide nanoparticles (Bi2O3-Nps). A recently introduced multi-compartment 3D radiochromic dosimeter for measuring radiation dose enhancement produced from the interaction of X-rays with metal nanoparticles was employed to investigate the 3D spatial distribution of ionizing radiation dose deposition. Doseenhancement factor for the dosimeters doped with Bi2O3-NPs was ~1.9 for both spectrophotometry and optical CT analyses. Our results suggest that bismuth-based nanomaterials are efficient dose enhancing agents and have great potential for application in clinical radiotherapy. Refereed/Peer-reviewed

In this article, which accompanies one of the morning "refresher" sessions at the IC3D Dose 2012 conference, a workflow for optical CT scanning is described and the following steps in that workflow are illustrated: sample positioning, refractive index matching, the importance of apparatus cleanliness to the imaging process, the pre-scan and image reconstruction and post-processing.

PRESAGE® has previously shown potential for 3D dosimetry of heavy particles. A new formulation has specifically developed for dosimetry of protons/heavy ions. This work provides a preliminary characterization the new formulation of PRESAGE® by measuring optical absorbance and dose response after irradiating by a 62 MeV proton beam for a dose range of 0.5 – 20 Gy. Results show linear dose response and the evolution over time of the optical density of the 3D dosimeter.

The aim of this study was to compare the conventional combination of three-dimensional dosimeter (nPAG gel) and readout method (MRI) with other combinations of three-dimensional dosimeters (nPAG gel/Presage (TM)) and readout methods (optical CT scanners). In the first experiment, the dose readout of a gel irradiated with a four field-box technique was performed with both an Octopus IQ scanner and MRI. It was seen that the MRI readout agreed slightly better to the TPS. In another experiment, a gel and a Presage (TM) sample were irradiated with a VMAT field and read out using MRI and a fast laser scanner, respectively. A comparison between the TPS and the volumes revealed that the MRI/gel readout had closer resemblance to the TPS than the optical CT/Presage (TM) readout. There are clearly potential in the evaluated optical CT scanners, but more time has to be invested in the particular scanning scenario than was possible in this study. (Less)

This study presents an investigation of the use of 3-D dosimetry using optical computed tomography to provide verification of synchrotron microbeam radiation therapy (MRT). MRT is based on the remarkable tolerance of normal tissues to high doses of radiation when this dose is constrained to very narrow beams. At beamline ID17 of the European Synchrotron Radiation Facility, pre-clinical radiation therapy is delivered using an array of parallel microbeams of x-rays generated by a synchrotron-wiggler source. Measurement of the dose distribution around these microbeams requires a dosimeter with high spatial resolution, and the radiochromic plastic dosimeter PRESAGETM, used in conjunction with optical CT, is highly appropriate for this task. Two solid cylinders of 9.7 mm diameter PRESAGETM were irradiated to create quality-assurance phantoms for the optical CT microscope using the dose-painting facilities at ID17. Images were analysed to ascertain the scanner linearity over the range 8 – 35 Gy and modulation-transfer function (MTF). With the initial scanner settings, MTF was found to be greater than 30% at 12 line pairs/mm and around 8% at 20.8 line pairs/mm, thus allowing individual lines of width 24 μm to be visualised. A further 9.7 mm PRESAGETM sample was irradiated with a typical array of microbeams of FWHM 50 μm and centre-to-centre distance 400 μm. Results demonstrate how optical CT dosimetry may be capable (after further analysis) of making quantitative measurements of the peak-to-valley ratio of the microbeams. Finally, two samples of diameters 9.7 and 22 mm were irradiated from four directions using a typical MRT cross-firing pattern, and then imaged at two different image resolutions. The results show how optical CT dosimetry is able both to visualise the planned dose distribution and identify an incorrect treatment delivery.

Optical computed tomography (CT) shows great potential for radiation therapy dose verification in 3D. However, an effective quality assurance regime for the various scanners currently available still remains to be developed. We show how the favourable properties of the PRESAGETM radiochromic polymer may be exploited to create highly sophisticated QA phantoms. Five 60 mm-diameter cylindrical PRESAGETM samples were irradiated using the x-ray microbeam radiation therapy facility on the ID17 biomedical beamline at the European Synchrotron Radiation Facility. Samples were then imaged on the University of Surrey parallel-beam optical CT scanner and were designed to allow a variety of tests to be performed, including linearity, MTF (three independent measurements) and an assessment of geometric distortion. A small sample of these results is presented. It is clear that, although the method produces extremely high quality test objects, it is not practical on a routine basis, because of its reliance of a highly specialised radiation source. Hence, we investigated a second possibility. Two PRESAGETM samples were illuminated with ultraviolet light of wavelength 365 nm, using cheap masks created by laser-printing patterns onto overhead projector acetate sheets. There was good correlation between optical density (OD) measured by the CT scanner and the expected UV "dose" delivered. The results are highly encouraging and a proposal is made for a scanner test regime based on calibrated and well characterised PRESAGETM samples.

The conference refresher session associated with this abstract reviews the main principles of the two most important imaging readout modalities for 3-D dosimetry: MRI and optical CT. Best practices for both these techniques are already described in several different places in the literature, but, for the uninitiated, there are a number of pitfalls. Here, I list some of the important considerations required to obtain good results from these methods and point to relevant prior work.

Measurements of the 62 MeV proton beam at the Clatterbridge Centre for Oncology using the radiosensitive plastic PRESAGETM have previously shown a dependence of the dosimeter sensitivity (dose-response slope) on the linear energy transfer (LET) of the ionising particles. This work focuses on a possible explanation in terms of track structure theory (TST). Experimental measurements of highly irradiated PRESAGETM samples established the D37 parameter of the theory to be of the order of 1000 Gy. Initial attempts at applying the theory showed good agreement of the theoretical and experimental values of relative effectiveness, but more work is needed to verify the model and understand its different parameters.

Tissue inhomogeneities such as bones or air cavities give rise to significant perturbations of dose during electron radiotherapy. Whilst these can be calculated using a variety of computational methods, accurate experimental verification has hitherto been difficult. In this study, we used 3-D optical computed tomography (CT) dosimetry of PRESAGETM samples to obtain central-axis depth dose curves and to study the dose distribution around a simple air cavity. Some concerning anomalous results were obtained for the build-up region of the depth-dose curve, which are currently under investigation. However, despite this, the ability to measure rapidly varying dose distributions show great promise.

Laser scanning optical computed tomography (optical CT) is potentially a better technique for analyzing 3-D dosimeters than CCD based dosimetry due to the fact that narrow cone of light passing through the sample is not affected by the refractive index inhomogeneities of surrounding structures. We have previously demonstrated fast laser scanning of 3-D dosimeters using an optical arrangement borrowed from confocal microscopy. The main rationale for this design is that it guarantees good telecentricity at high scanning speeds. Here we demonstrate a number of advantages of this arrangement as compared to broadbeam optical CT based around CCD detectors. Additionally, we provide initial characterization of the apparatus in terms of signal-to-noise and dynamic range.

The introduction of optical computed tomography (optical-CT) offers economic and easy to use 3-D optical readout for gel dosimeters. However, previous authors have noted some challenges regarding the accuracy of such imaging techniques at high values of optical density. In this paper, we take a closer look at the 'cupping' artefact evident in both light-scattering polymer systems and highly light absorbing phantoms using our CCD-based optical scanner. In addition, a technique is implemented whereby the maximum measurable optical absorbance is extended to correct for any errors that may have occurred in the estimated value of the dark current or ambient light reaching the detector. The results indicate that for absorbance values up to 2.0, the optical scanner results have good accuracy, whereas this is not the case at high absorbance values for reasons yet to be explained.

This paper describes a new device, RadBall™ for mapping environmental radiation fields, as found in the area of nuclear decontamination. The system consists of a specially shaped PRESAGE™ dosimeter, which sits inside a custom-designed lead collimator. This is imaged using optical CT to yield data from which the position of either point sources or extended objects may be reconstructed. The principle of the technique is explained, simulations and preliminary data are given and the current design of the dosimeter and collimator are presented.

The current status of optical CT for 3-D radiation dosimetry is reviewed. The technique is first placed in its historical context, pointing out the relationship with other methods of optical imaging and showing how optical-CT has emerged independently in several different fields and under different names. The theoretical background of the method is described briefly and this provides the foundation for an explanation of the different types of scanner. The relative advantages and disadvantages of instruments based on scanned lasers and pixelated (area) detectors are presented. The latest generation of fast laser scanners is described and the review is concluded with a discussion of the different radiation-sensitive materials used as samples in optical CT.

This brief work-in-progress outlines two methods that we have attempted for determining the dose at which the linear relation between optical density of a PRESAGE™ dosimeter and the dose deposited breaks down. Both methods were equally successful in mapping the linear relation up to an optical density of approximately 6.25 cm−1 (absorbance 2..5), but no saturation was found in this region.