M Oldham

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.

Dosimetry tools and techniques for IMRT

Intensity modulated radiation therapy (IMRT) poses a number of challenges for properly measuring commissioning data and quality assurance (QA) radiation dose distributions. This report provides a comprehensive overview of how dosimeters, phantoms, and dose distribution analysis techniques should be used to support the commissioning and quality assurance requirements of an IMRT program. The proper applications of each dosimeter are described along with the limitations of each system. Point detectors, arrays, film, and electronic portal imagers are discussed with respect to their proper use, along with potential applications of 3D dosimetry. Regardless of the IMRT technique utilized, some situations require the use of multiple detectors for the acquisition of accurate commissioning data. The overall goal of this task group report is to provide a document that aids the physicist in the proper selection and use of the dosimetry tools available for IMRT QA and to provide a resource for physicists that describes dosimetry measurement techniques for purposes of IMRT commissioning and measurement-based characterization or verification of IMRT treatment plans. This report is not intended to provide a comprehensive review of commissioning and QA procedures for IMRT. Instead, this report focuses on the aspects of metrology, particularly the practical aspects of measurements that are unique to IMRT. The metrology of IMRT concerns the application of measurement instruments and their suitability, calibration, and quality control of measurements. Each of the dosimetry measurement tools has limitations that need to be considered when incorporating them into a commissioning process or a comprehensive QA program. For example, routine quality assurance procedures require the use of robust field dosimetry systems. These often exhibit limitations with respect to spatial resolution or energy response and need to themselves be commissioned against more established dosimeters. A chain of dosimeters, from secondary standards to field instruments, is established to assure the quantitative nature of the tests. This report is intended to describe the characteristics of the components of these systems; dosimeters, phantoms, and dose evaluation algorithms. This work is the report of AAPM Task Group 120.

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.

Characterization of a new radiochromic three‐dimensional dosimeter

The development of intensity-modulated radiotherapy (IMRT) has created a clear need for a dosimeter that can accurately and conveniently measure dose distributions in three dimensions to assure treatment quality. PRESAGE is a new three dimensional (3D) dosimetry material consisting of an optically clear polyurethane matrix, containing a leuco dye that exhibits a radiochromic response when exposed to ionizing radiation. A number of potential advantages accrue over other gel dosimeters, including insensitivity to oxygen, radiation induced light absorption contrast rather than scattering contrast, and a solid texture amenable to machining to a variety of shapes and sizes without the requirement of an external container. In this paper, we introduce an efficient method to investigate the basic properties of a 3D dosimetry material that exhibits an optical dose response. The method is applied here to study the key aspects of the optical dose response of PRESAGE: linearity, dose rate dependency, reproducibility, stability, spectral changes in absorption, and temperature effects. PRESAGE was prepared in 1 x 1 x 4.5 cm3 optical cuvettes for convenience and was irradiated by both photon and electron beams to different doses, dose rates, and energies. Longer PRESAGE columns (2 x 2 x 13 cm3) were formed without an external container, for measurements of photon and high energy electron depth-dose curves. A linear optical scanning technique was used to detect the depth distribution of radiation induced optical density (OD) change along the PRESAGE columns and cuvettes. Measured depth-OD curves were compared with percent depth dose (PDD). Results indicate that PRESAGE has a linear optical response to radiation dose (with a root mean square error of -1%), little dependency on dose rate (-2%), high intrabatch reproducibility (< 2%), and can be stable (-2%) during 2 hours to 2 days post irradiation. Accurate PRESAGE dosimetry requires temperature control within 1 degrees C. Variations in the PRESAGE formulation yield corresponding variations in sensitivity, stability, and density. CT numbers in the range 100-470 were observed. In conclusion, the small volume studies presented here indicate PRESAGE to be a promising, versatile, and practical new dosimetry material with applicability for radiation therapy.

A comprehensive evaluation of the PRESAGE/optical-CT 3D dosimetry system

This work presents extensive investigations to evaluate the robustness (intradosimeter consistency and temporal stability of response), reproducibility, precision, and accuracy of a relatively new 3D dosimetry system comprising a leuco-dye doped plastic 3D dosimeter (PRESAGE) and a commercial optical-CT scanner (OCTOPUS 5x scanner from MGS Research, Inc). Four identical PRESAGE 3D dosimeters were created such that they were compatible with the Radiologic Physics Center (RPC) head-and-neck (H&N) IMRT credentialing phantom. Each dosimeter was irradiated with a rotationally symmetric arrangement of nine identical small fields (1 x 3 cm2) impinging on the flat circular face of the dosimeter. A repetitious sequence of three dose levels (4, 2.88, and 1.28 Gy) was delivered. The rotationally symmetric treatment resulted in a dose distribution with high spatial variation in axial planes but only gradual variation with depth along the long axis of the dosimeter. The significance of this treatment was that it facilitated accurate film dosimetry in the axial plane, for independent verification. Also, it enabled rigorous evaluation of robustness, reproducibility and accuracy of response, at the three dose levels. The OCTOPUS 5x commercial scanner was used for dose readout from the dosimeters at daily time intervals. The use of improved optics and acquisition technique yielded substantially improved noise characteristics (reduced to approximately 2%) than has been achieved previously. Intradosimeter uniformity of radiochromic response was evaluated by calculating a 3D gamma comparison between each dosimeter and axially rotated copies of the same dosimeter. This convenient technique exploits the rotational symmetry of the distribution. All points in the gamma comparison passed a 2% difference, 1 mm distance-to-agreement criteria indicating excellent intradosimeter uniformity even at low dose levels. Postirradiation, the dosimeters were all found to exhibit a slight increase in opaqueness with time. However, the relative dose distribution was found to be extremely stable up to 90 h postirradiation indicating excellent temporal stability. Excellent interdosimeter reproducibility was also observed between the four dosimeters. Gamma comparison maps between each dosimeter and the average distribution of all four dosimeters showed full agreement at the 2% difference, 2 mm distance-to-agreement level. Dose readout from the 3D dosimetry system was found to agree better with independent film measurement than with treatment planning system calculations in penumbral regions and was generally accurate to within 2% dose difference and 2 mm distance-to-agreement. In conclusion, these studies demonstrate excellent precision, accuracy, robustness, and reproducibility of the PRESAGE/optical-CT system for relative 3D dosimetry and support its potential integration with the RPC H&N credentialing phantom for IMRT verification.

Comparison of intensity-modulated tomotherapy with stereotactically guided conformal radiotherapy for brain tumors

PURPOSE Intensity-modulated radiotherapy (IMRT) offers the potential to more closely conform dose distributions to the target, and spare organs at risk (OAR). Its clinical value is still being defined. The present study aims to compare IMRT with stereotactically guided conformal radiotherapy (SCRT) for patients with medium size convex-shaped brain tumors. METHODS AND MATERIALS Five patients planned with SCRT were replanned with the IMRT-tomotherapy method using the Peacock system (Nomos Corporation). The planning target volume (PTV) and relevant OAR were assessed, and compared relative to SCRT plans using dose statistics, dose-volume histograms (DVH), and the Radiation Therapy Oncology Group (RTOG) stereotactic radiosurgery criteria. RESULTS The median and mean PTV were 78 cm3 and 85 cm3 respectively (range 62-119 cm3). The differences in PTV doses for the whole group (Peacock-SCRT +/-1 SD) were 2%+/-1.8 (minimum PTV), and 0.1%+/-1.9 (maximum PTV). The PTV homogeneity achieved by Peacock was 12.1%+/-1.7 compared to 13.9%+/-1.3 with SCRT. Using RTOG guidelines, Peacock plans provided acceptable PTV coverage for all 5/5 plans compared to minor coverage deviations in 4/5 SCRT plans; acceptable homogeneity index for both plans (Peacock = 1.1 vs. SCRT = 1.2); and comparable conformity index (1.4 each). As a consequence of the transaxial method of arc delivery, the optic nerves received mean and maximum doses that were 11.1 to 11.6%, and 10.3 to 15.2% higher respectively with Peacock plan. The maximum optic lens, and brainstem dose were 3.1 to 4.8% higher, and 0.6% lower respectively with Peacock plan. However, all doses remained below the tolerance threshold (5 Gy for lens, and 50 Gy for optic nerves) and were clinically acceptable. CONCLUSIONS The Peacock method provided improved PTV coverage, albeit small, in this group of convex tumors. Although the OAR doses were higher using the Peacock plans, all doses remained within the clinically defined threshold and were clinically acceptable. Further improvements may be expected using other methods of IMRT planning that do not limit the treatment delivery to transaxial arcs. Each IMRT system needs to be individually assessed as the paradigm utilized may provide different outcomes.

Beam-orientation customization using an artificial neural network

A methodology for the constrained customization of coplanar beam orientations in radiotherapy treatment planning using an artificial neural network (ANN) has been developed. The geometry of the patients, with cancer of the prostate, was modelled by reducing the external contour, planning target volume (PTV) and organs at risk (OARs) to a set of cuboids. The coordinates and size of the cuboids were given to the ANN as inputs. A previously developed beam-orientation constrained-customization (BOCC) scheme employing a conventional computer algorithm was used to determine the customized beam orientations in a training set containing 45 patient datasets. Twelve patient datasets not involved in the training of the artificial neural network were used to test whether the ANN was able to map the inputs to customized beam orientations. Improvements from the customized beam orientations were compared with standard treatment plans with fixed gantry angles and plans produced from the BOCC scheme. The ANN produced customized beam orientations within 5 degrees of the BOCC scheme in 62.5% of cases. The average difference in the beam orientations produced by the ANN and the BOCC scheme was 7.7 degrees (+/-1.7, 1 SD). Compared with the standard treatment plans, the BOCC scheme produced plans with an increase in the average tumour control probability (TCP) of 5.7% (+/-1.4, 1 SD) whilst the ANN generated plans increased the average TCP by 3.9% (+/-1.3, 1 SD). Both figures refer to the TCP at a fixed rectal normal tissue complication probability (NTCP) of 1%. In conclusion, even using a very simple model for the geometry of the patient, an ANN was able to produce beam orientations that were similar to those produced by a conventional computer algorithm.

Experimental 3D dosimetry around a high-dose-rate clinical 192Ir source using a polyacrylamide gel (PAG) dosimeter

It is well known that the experimental dosimetry of brachytherapy sources presents a challenge. Depending on the particular-dosimeter used, measurements can suffer from poor spatial resolution (ion chambers), lack of 3D information (film) or errors due to the presence of the dosimeter itself distorting the radiation flux. To avoid these problems, we have investigated the dosimetry of a clinical 192Ir source using a polyacrylamide gel (PAG) dosimeter. Experimental measurements of dose versus radial distance from the centre of the source (cross-line plots) were compared with calculations produced with a Nucletron NPS planning system. Good agreement was found between the planning system and gel measurements in planes selected for analysis. Gel dosimeter measurements in a coronal plane through the phantom showed a mean difference between measured absorbed dose and calculated dose of 0.17 Gy with SD = 0.13 Gy. Spatially, the errors at the reference point remain within one image pixel (1.0 mm). The use of polymer gel dosimetry shows promise for brachytherapy applications, offering complete, three-dimensional dose information, good spatial resolution and small measurement errors. Measurements close to the source, however, are difficult, due to some of the limiting properties of the polyacrylamide gel.

Dynamics of polymerization in polyacrylamide gel (PAG) dosimeters: (I) ageing and long-term stability

Few quantitative data are available on the kinetics of polymerization reactions in polymer gel (PAG) dosimeters and their long-term stability. Post-irradiation polymerization reactions have been found to continue for several weeks, posing questions regarding dosimeter stability and its achievement. In this paper we report an investigation of polymerization kinetics in PAG dosimeters and the effect of diffusing oxygen into the dosimeter, post irradiation, as a potential method of inhibiting further polymerization and stabilizing the dose distribution. Results show continuous post-irradiation changes in transverse relaxation rate R2 with time over the five week study period and that a steady-state may not be reached for a period of months. An assessment is made of the appropriate time to image the dosimeter which shows that after three to four days the polymerization change is slow compared with imaging time. The implications of the time delay between irradiation and imaging are discussed in terms of the resultant sensitivity of the dosimeter and accuracy of the dose measured. In pairs of dosimeters, one sealed the other open to air, oxygen diffusing into the dosimeter arrests polymerization. However, the diffusion rate is too slow to make this method practicable. The slow diffusion means that while in regions near the air/gel interface polymerization is quickly arrested, in deeper regions it may continue for many hours, causing artefacts in the dose distribution. In the companion paper to this from a collaborating team, a study focusing on modelling oxygen diffusion in dosimeter gel will be presented.

Toward acquiring comprehensive radiosurgery field commissioning data using the PRESAGE®/optical-CT 3D dosimetry system

Achieving accurate small field dosimetry is challenging. This study investigates the utility of a radiochromic plastic PRESAGE read with optical-CT for the acquisition of radiosurgery field commissioning data from a Novalis Tx system with a high-definition multileaf collimator (HDMLC). Total scatter factors (Sc, p), beam profiles, and penumbrae were measured for five different radiosurgery fields (5, 10, 20, 30 and 40 mm) using a commercially available optical-CT scanner (OCTOPUS, MGS Research). The percent depth dose (PDD), beam profile and penumbra of the 10 mm field were also measured using a higher resolution in-house prototype CCD-based scanner. Gafchromic EBT film was used for independent verification. Measurements of Sc, p made with PRESAGE and film agreed with mini-ion chamber commissioning data to within 4% for every field (range 0.2-3.6% for PRESAGE, and 1.6-3.6% for EBT). PDD, beam profile and penumbra measurements made with the two PRESAGE/optical-CT systems and film showed good agreement with the high-resolution diode commissioning measurements with a competitive resolution (0.5 mm pixels). The in-house prototype optical-CT scanner allowed much finer resolution compared with previous applications of PRESAGE. The advantages of the PRESAGE system for small field dosimetry include 3D measurements, negligible volume averaging, directional insensitivity, an absence of beam perturbations, energy and dose rate independence.