Dosimetric and geometric characteristics of a small animal image‐guided irradiator using 3D dosimetry/optical CT scanner

A Review of PRESAGE Radiochromic Polymer and the Compositions for Application in Radiotherapy Dosimetry

Recent advances in radiotherapy technology and techniques have allowed a highly conformal radiation to be delivered to the tumour target inside the body for cancer treatment. A three-dimensional (3D) dosimetry system is required to verify the accuracy of the complex treatment delivery. A 3D dosimeter based on the radiochromic response of a polymer towards ionising radiation has been introduced as the PRESAGE dosimeter. The polyurethane dosimeter matrix is combined with a leuco-dye and a free radical initiator, whose colour changes in proportion to the radiation dose. In the previous decade, PRESAGE gained improvement and enhancement as a 3D dosimeter. Notably, PRESAGE overcomes the limitations of its predecessors, the Fricke gel and the polymer gel dosimeters, which are challenging to fabricate and read out, sensitive to oxygen, and sensitive to diffusion. This article aims to review the characteristics of the radiochromic dosimeter and its clinical applications. The formulation of PRESAGE shows a delicate balance between the number of radical initiators, metal compounds, and catalysts to achieve stability, optimal sensitivity, and water equivalency. The applications of PRESAGE in advanced radiotherapy treatment verifications are also discussed.

Temperature dependence and temporal stability of stacked radiochromic sheets for three-dimensional dose verification

PURPOSE

Recently a novel radiochromic sheet dosimeter, termed as PRESAGE sheets, consisting of leuco crystal violet dye and radical initiator had been developed and characterized. This study examines the dosimeter's temporal stability and storage temperature dependence postirradiation, and its applicability for dose verification in three dimensions (3D) as a stack dosimeter.

METHODS

PRESAGE sheets were irradiated using 6 MV photons at a dose range of 0-20 Gy with the change in optical density measured using a flatbed scanner. Following their irradiation, PRESAGE sheets were stored in different temperature environments (-18 °C, 4 °C, and 22 °C) and scanned at different time points, ranging from 1 to 168 h postirradiation, to track changes in measured signal and linearity of dose response. Multiple PRESAGE sheets were bound together to create a 12 × 13 × 8.7 cm³ film stack, with EBT3 film inserted between the sheets in the central region of the stack, that was treated using a clinical VMAT plan. Based on the results from the time and storage temperature study, two-dimensional (2D) relative dose distribution measurements in PRESAGE were acquired promptly following irradiation at selected planes in the coronal, sagittal, and axial orientation of the film stack and compared to the treatment planning system calculations in their respective axes. Dose distribution measurements on the coronal axis of the stack dosimeter were also independently verified using EBT3 film.

RESULTS

The dose response was observed to be linear (R² > 0.995) with sheets stored in colder temperatures retaining their signal and dose response sensitivity for extended periods postirradiation. Sheets stored in 22 °C environment should be measured within an hour postirradiation. Sheets stored in a 4 °C and -18 °C environment can be scanned up to 20- and 72 h postirradiation, respectively, while preserving the integrity of their dose response sensitivity and linearity of dose response within a mean absolute percent error of 2.0%. For instance, at 20 h postirradiation the dose response sensitivity for sheets stored in a -18 °C, 4 °C, and 22 °C temperature environment was measured to be 97%, 91%, and 77% of their original values measured within an hour postirradiation, respectively. The 2D gamma pass rate for central slices exceed 95% for PRESAGE film stack compared with treatment planning system on selected planes in the axial, coronal, and sagittal orientation and EBT3 film in the coronal orientation using a 2D gamma index of 2%/2mm. The gamma pass rate in comparing the calculated dose distribution with the measured dose distribution from PRESAGE-LCV was observed to decrease in sheets scanned at later elapsed times postirradiation. In one example, the gamma pass rate for 2%/2mm criteria in the coronal plane was observed to decrease from 97.7% pass rate when scanned within an hour postirradiation to 92.1% pass rate when scanned at 20 h postirradiation under room temperature conditions.

CONCLUSIONS

This is the first study to demonstrate that the temporal stability of PRESAGE sheets can be enhanced through its storage in colder temperature environments postirradiation and that sheets as a film stack dosimeter hold promise for precise relative dose distribution measurements in 3D where advanced optical CT is unavailable.

Dose rate determination for preclinical total body irradiation

The accuracy of delivered radiation dose and the reproducibility of employed radiotherapy methods are key factors for preclinical radiobiology applications and research studies. In this work, ionization chamber (IC) measurements and Monte Carlo (MC) simulations were used to accurately determine the dose rate for total body irradiation (TBI), a classic radiobiologic and immunologic experimental method. Several phantom configurations, including large solid water slab, small water box and rodentomorphic mouse and rat phantoms were simulated and measured for TBI setup utilizing a preclinical irradiator XRad320. The irradiator calibration and the phantom measurements were performed using an ADCL calibrated IC N31010 following the AAPM TG-61 protocol. The MC simulations were carried out using Geant4/GATE to compute absorbed dose distributions for all phantom configurations. All simulated and measured geometries had favorable agreement. On average, the relative dose rate difference was 2.3%. However, the study indicated large dose rate deviations, if calibration conditions are assumed for a given experimental setup as commonly done for a quick determination of irradiation times utilizing lookup tables and hand calculations. In a TBI setting, the reference calibration geometry at an extended source-to-surface distance and a large reference field size is likely to overestimate true photon scatter. Consequently, the measured and hand calculated dose rates, for TBI geometries in this study, had large discrepancies: 16% for a large solid water slab, 27% for a small water box, and 31%, 36%, and 30% for mouse phantom, rat phantom, and mouse phantom in a pie cage, respectively. Small changes in TBI experimental setup could result in large dose rate variations. MC simulations and the corresponding measurements specific to a designed experimental setup are vital for accurate preclinical dosimetry and reproducibility of radiobiological findings. This study supports the well-recognized need for physics consultation for all radiobiological investigations.

Dosimetric characterization of a body-conforming radiochromic sheet

Purpose

A novel radiochromic PRESAGE sheet (Heuris Inc.) with 3 mm thickness has been developed as a measurement tool for 2D dosimetry. Its inherent ability to conform to irregular surfaces makes this dosimeter advantageous for patient surface dosimetry. This study is a comprehensive investigation into the PRESAGE sheet’s dosimetric characteristic, accuracy and its potential use as a dosimeter for clinical applications.

Methods

The characterization of the dosimeter included evaluation of the temporal stability of the dose linearity, reproducibility, measurement uncertainties, dose rate, energy, temperature and angular dependence, lateral response artifacts, percent depth dose curve, and 2D dose measurement. Dose distribution measurements were acquired for regular square fields on a flat and irregular surface and an irregular modulated field on the smooth surface. All measurements were performed using an Epson 11000XL high‐resolution scanner.

Results

The examined dosimeters exhibit stable linear response, standard error of repeated measurements within 2%, negligible dose rate, energy, and angular dependence. The same linear dose response was measured while the dosimeter was in contact with a heated water surface. Gamma test and histogram analysis of the dose difference between PRESAGE and EBT3 film, PRESAGE and the treatment planning system (TPS) were used to evaluate the measured dose distributions. The PRESAGE sheet dose distributions showed good agreement with EBT3 film and TPS. A discrepancy smaller than the statistical error of the two dosimeters was reported.

Conclusions

This study established a full dosimetric characterization of the PRESAGE sheets with the purpose of laying the foundation for future clinical uses. The results presented here for the comparison of this novel dosimeter with those currently in use reinforce the possibility of using this dosimeter as an alternative for irregular surface dose measurements.

3D isocentricity analysis for clinical linear accelerators

PURPOSE

To perform a three-dimensional (3D) concurrent isocentricity measurement of a clinical linear accelerator's (linac) using a single 3D dosimeter, PRESAGE.

METHODS

A 3D dosimeter, PRESAGE, set up on the treatment couch of a Varian TrueBeam LINAC using the setup lasers, was irradiated under gantry angles of 0 ∘ , 50 ∘ , 160 ∘ , and 270 ∘ with the couch fixed at 0 ∘ and subsequently, under couch angles of 10 ∘ , 330 ∘ , 300 ∘ , and 265 ∘ with the gantry fixed at 270 ∘ . The 1 cm 2 (at 100 cm SAD) square fields were delivered at 6 MV with 800 MU/field. After irradiation, the dosimeter was scanned using a single-beam optical scanner and images were reconstructed with submillimeter resolution using filtered back-projection. Postprocessing was used to extract views parallel to the star-shot planes from which beam trajectories and the smallest circles enclosing these were drawn and extracted. These circles and information from the view orthogonal to both star-shots were used to represent the rotational centers as spheroids. The linac isocenter was defined by the distribution of midpoints between any, randomly selected, points lying inside the center spheroids defined by the table and gantry rotations; isocenter location and size were defined by the average midpoint and the distribution's semi-axes. Collimator rotations were not included in this study.

RESULTS

Relative to the setup center defined by lasers, the table and gantry rotation center coordinates (lat., long., vert.) were measured in units of millimeters, to be (-0.24, 0.18, -0.52) and (0.10, 0.53, -0.52), respectively. Displacements from the setup center were 0.60 and 0.75 mm for the table and gantry centers, while the distance between them measured 0.49 mm. The linac's radiation isocenter was calculated to be at (-0.07, -0.17, 0.51) relative to the setup lasers and its size was found to be most easily described by a spheroid prolate in vertical direction with semi-axis lengths of 0.13 and 0.23 mm for the lateral-longitudinal and vertical directions, respectively.

CONCLUSIONS

This study demonstrates how to measure the location and sizes of rotational centers in 3D with one setup. The proposed method provides a more comprehensive view on the isocentricity of LINAC than the conventional two-dimensional film measurements. Additionally, a new definition of isocenter and its size was proposed.

A comprehensive geometric quality assurance framework for preclinical microirradiators

PURPOSE

The mechanical and geometric accuracy of small animal image-guided radiotherapy (SA-IGRT) systems is critical and is affected by a number of system-related factors. Because of the small dimensions involved in preclinical radiotherapy research, such factors can individually and/or cumulatively contribute to significant errors in the small animal radiation research. In this study, we developed and implemented a comprehensive quality assurance (QA) framework for characterizing the mechanical and geometric constancy and accuracy of the small animal radiation research platform (SARRP) system.

METHODS

We quantified the accuracy of gantry and stage rotation isocentricity and positional stage translations. We determined the accuracy and symmetry of field sizes formed by collimators. We evaluated collimator assembly system performance by characterization of collimator axis alignment along the beam axis during gantry rotation. Furthermore, we quantified the end-to-end precision and accuracy of image-guided delivery by examining the congruence of intended (e.g., imaging) and actual delivery (measured during experiment) isocenters.

RESULTS

The fine and broad beams showed different central axes. The center of the beam was offset toward the cathode (0.22 ± 0.05 mm) when switching the beam from a fine to a broad focus. Larger (custom-made) collimators were more symmetrically centered than smaller (standard) collimators. The field formed by a 1-mm circular collimator was found to deviate from the circular shape, measuring 1.55 mm and 1.25 mm in the X and Y directions, respectively. The 40-mm collimator showed a field that was 1.65 (4.13%) and 1.3 (3.25%) mm smaller than nominal values in the X and Y directions, respectively, and the 30-mm collimator field was smaller by 0.75 mm (2.5%) in the X direction. Results showed that fields formed by other collimators were accurate in both directions and had ≤2% error. The size of the gantry rotation isocenter was 1.45 ± 0.15 mm. While the gantry rotated, lateral and longitudinal isocenter displacements ranged from 0 to -0.34 and -0.44 to 0.33 mm, respectively. Maximum lateral and longitudinal displacements were found at obliques gantry angles of -135° and 45°, respectively. The stage translational accuracies were 0.015, 0.010, and 0 mm in the X, Y, and Z directions, respectively. The size of the stage rotation runout was 2.73 ± 0.3 mm. Maximum displacements of the stage rotational axis were -0.38 (X direction) and -0.26 (Y direction) mm at stage angles of -45° and -135°, respectively. We found that displacements of intended and actual delivery isocenters were 0.24 ± 0.10, 0.12 ± 0.62, and 0.12 ± 0.42 mm in the X, Y, and Z directions, respectively.

CONCLUSION

We used the SARRP built-in electronic portal imaging device (EPID) to perform most of the geometric QA tests, demonstrating the utility of the EPID for characterizing the geometric accuracy and precision of the SA-IGRT system. However, in principle, the methodology and tests developed here are applicable to any digital imaging detector available in SA-IGRT systems or film. The flexibility of film allows these tests to be adapted for QA of non-IGRT, cabinet irradiators, which make up many of preclinical small animal irradiators.