O. Lopatiuk-Tirpak

Dosimetric evaluation of a novel polymer gel dosimeter for proton therapy

PURPOSE The aim of this study is to evaluate the dosimetric performance of a newly developed proton-sensitive polymer gel formulation for proton therapy dosimetry. METHODS Using passive scattered modulated and nonmodulated proton beams, the dose response of the gel was assessed. A next-generation optical CT scanner is used as the readout mechanism of the radiation-induced absorbance in the gel medium. Comparison of relative dose profiles in the gel to ion chamber profiles in water is performed. A simple and easily reproducible calibration protocol is established for routine gel batch calibrations. Relative stopping power ratio measurement of the gel medium was performed to ensure accurate water-equivalent depth dose scaling. Measured dose distributions in the gel were compared to treatment planning system for benchmark irradiations and quality of agreement is assessed using clinically relevant gamma index criteria. RESULTS The dosimetric response of the gel was mapped up to 600 cGy using an electron-based calibration technique. Excellent dosimetric agreement is observed between ion chamber data and gel. The most notable result of this work is the fact that this gel has no observed dose quenching in the Bragg peak region. Quantitative dose distribution comparisons to treatment planning system calculations show that most (> 97%) of the gel dose maps pass the 3%/3 mm gamma criterion. CONCLUSIONS This study shows that the new proton-sensitive gel dosimeter is capable of reproducing ion chamber dose data for modulated and nonmodulated Bragg peak beams with different clinical beam energies. The findings suggest that the gel dosimeter can be used as QA tool for millimeter range verification of proton beam deliveries in the dosimeter medium.

Performance evaluation of an improved optical computed tomography polymer gel dosimeter system for 3D dose verification of static and dynamic phantom deliveries

The performance of a next-generation optical computed tomography scanner (OCTOPUS-5X) is characterized in the context of three-dimensional gel dosimetry. Large-volume (2.2 L), muscle-equivalent, radiation-sensitive polymer gel dosimeters (BANG-3) were used. Improvements in scanner design leading to shorter acquisition times are discussed. The spatial resolution, detectable absorbance range, and reproducibility are assessed. An efficient method for calibrating gel dosimeters using the depth-dose relationship is applied, with photon- and electron-based deliveries yielding equivalent results. A procedure involving a preirradiation scan was used to reduce the edge artifacts in reconstructed images, thereby increasing the useful cross-sectional area of the dosimeter by nearly a factor of 2. Dose distributions derived from optical density measurements using the calibration coefficient show good agreement with the treatment planning system simulations and radiographic film measurements. The feasibility of use for motion (four-dimensional) dosimetry is demonstrated on an example comparing dose distributions from static and dynamic delivery of a single-field photon plan. The capability to visualize three-dimensional dose distributions is also illustrated.

Direct response to proton beam linear energy transfer (LET) in a novel polymer gel dosimeter formulation.

Linear energy transfer (LET) of clinical proton beams is an important parameter influencing the biological effects of radiation. This work demonstrates LET-induced response enhancement in novel formulations of polymer gel dosimeters, potentially useful for LET mapping of clinical proton beams. A series of four polymer gel dosimeters (labeled A through D), prepared based on the BANG3-Pro2 formulation, but with varying concentrations of polymerization modifiers, were irradiated by a clinical proton beam with a spread out Bragg peak modulation (SOBP) and read out using the OCTOPUS-IQ optical CT scanner. The evaluation of optical density profiles in the SOBP (constant physical dose) revealed response deviations at the distal end consistent with variations in gel composition. Maximum response deviations were as follows: −3% (under-response) for gel A, and over-response of 2%, 12%, and 17% for gels B, C, and D, respectively, relative to the mean dose in the center of the SOBP. This enhancement in optical response was correlated to LET by analytical calculations. Gels A and B showed no measurable dependence on LET. Gel C responded linearly in the limited range from 1.5 to 3.5 keV/μm. LET response of gel D was linear up to at least 5.5 keV/μm, with the threshold at about 1.3 keV/μm. These results suggest that it may be possible to develop a polymer gel system with direct optical response to LET for mapping of LET distributions for particle therapy beams.

An experimental investigation into the effect of periodic motion on proton dosimetry using polymer gel dosimeters and a programmable motion platform

Organ motion in proton therapy affects treatment dose distribution during both double-scattering (DS) and uniform-scanning (US) deliveries. We investigated the dosimetric impact of target motion using three-dimensional polymer gel dosimeters and a programmable motion platform. A simple one-beam treatment plan with 16 cm range and 6 cm modulation was generated from the treatment planning system (TPS) in both the DS and US modes. One gel dosimeter was irradiated with a stationary DS beam. Two other gel dosimeters were irradiated with the DS and US beams while they moved in the same sinusoidal motion profile using a programmable motion platform. The dose distribution of the stationary DS delivery agreed with the TPS plan. Dosimetric comparisons between DS motion delivery and the MATLAB-based motion model showed insignificant differences. Dose-volume histograms of a cylindrical target volume inside the gel dosimeters showed target coverage degradation caused by motion. A three-dimensional gamma index calculation (3% and 3 mm) confirmed different dosimetric impacts from DS and US with the same target motion. This polymer-gel-dosimeter-based study confirmed the dosimetric impact of intrafraction target motion and its interplay with temporal delivery of different energy layers in US proton treatments.

Spatial correlation of proton irradiation‐induced activity and dose in polymer gel phantoms for PET/CT delivery verification studies

PURPOSE This work demonstrates a novel application of BANG3-Pro2 polymer gel dosimeter as a dosimetric phantom able to accurately capture both dose and induced activity. METHODS BANG3-Pro2 dosimeters were irradiated with a clinical proton beam using an unmodulated beam and a spread-out Bragg peak (SOBP) modulation, the latter with a Lucite compensator to introduce a range offset in one quadrant of the circular field. The dosimeters were imaged in a nearby positron emission tomography/computed tomography (PET/CT) unit starting within 5 min of beam-off. Induced positron emission (PE) activity along the central axis of the beam was compared to analytical calculations. Dose distributions were read out using an optical CT scanner and were validated against ion chamber measurements and the treatment plan. The offset between the distal fall-off of dose and activity (50% level) was determined over the entire irradiated field. Lateral profiles of PE were correlated to measured dose for the unmodulated beam delivery. RESULTS Measured profiles of PE activity along the central beam axis were found to be within 10% of the predictions of analytical calculations. The depth-dose profiles agreed with the reference values (ion chamber or treatment plan) within 3%. The offset between the depth profiles of dose and activity for the unmodulated beam was 8.4 ± 1.4 mm. For the compensator-based SOBP delivery, the distribution of offsets throughout the field was found to be bimodal, with the mean of 8.9 ± 2.8 mm for the thinner region of the compensator and 4.3 ± 2.5 mm for the thicker region. For the pristine beam delivery, lateral profiles of dose and activity were found to exhibit fair spatial correlation throughout the beam range, with the mean 2D gamma index of 0.42 and 91% of the evaluated pixels passing the test. CONCLUSIONS This work presents the first demonstration of simultaneous and accurate experimental measurement of three-dimensional distributions of dose and induced activity and lays the groundwork for further investigations using BANG3-Pro2 as a dosimetric phantom in PET/CT delivery verification studies.

Feasibility of quantitative PET/CT dosimetry for proton therapy using polymer gels

A feasibility study of proton beam PET/CT off-line quantitative dosimetry using polymer gels is presented. A newly developed proton-sensitive polymer gel dosimeter (BANG(®)3-Pro2) is used as a dosimeter and a tissue-equivalent phantom medium for this study. We explore a new approach to correlating measured proton 3-dimensional (3D) dose distributions directly to measured positron emission from in the gel medium using PET/CT imaging. A large cylindrical volume (2.2 Litres) of the gel was irradiated with a clinical modulated proton beam using irregular-shaped aperture geometry. The gel was imaged in a nearby PET/CT unit immediately (<3 min) after irradiation. Dose distribution in the gel was generated using an optical tomography scanning system. Direct 3D spatial comparison of dose and positron emission distributions was then performed. Profiles along the beam path show that the distal fall-off of the dose is nearly 2 cm deeper than the activity profile which is comparable to previous studies with plastic phantoms and Monte Carlo simulations of activity distributions. Planar PET and dose distributions at depth and perpendicular to beam axis show a strong one-to-one spatial correlation. This phantom study demonstrates that the gel medium could be potentially useful for quantifying various physical factors that can influence the PET activity range verification method in patients.

SU‐GG‐T‐270: Evaluation of a Novel 3‐D Polymer Gel Dosimetry System for Proton Radiotherapy

Purpose: Preliminary evaluation of a novel, large‐volume (2.2 liter), 3D polymergeldosimeter (BANG®‐3‐PRO) developed for protonradiotherapydosimetry is presented. The characteristics of dose response and its linear energy transfer (LET) dependence are addressed. The performance of the dosimeter is analyzed under the conditions of pristine and clinically relevant beam configurations. The ability to visualize and analyze protondose distributions in 3D is demonstrated. Method and Materials: The dosimeters were read out using a characterized optical computed tomographyscanner. The calibration was performed by correlating the change in optical density along the beam axis to the depth‐dose curve from a pristine proton beam delivery (28.4 cm range, 4.5 cm circular aperture) obtained using ion‐chamber measurements. To exclude the potential LET dependence, only the plateau region was used. Several pristine‐beam and spread‐out Bragg peak (SOBP) dose distributions were recorded and compared to ion‐chamber measurements, including the assessment of dose response in high‐LET regions. A volumetric dose distribution delivered using a pristine beam and an irregularly shaped aperture was recorded and reconstructed in three dimensions. Results: The dose response of the gel was linear in the studied range (25–150 cGy). Dose profiles from the gel showed good general agreement with ion chamber measurements, with excellent positional reproducibility of the distal and proximal edges along and perpendicular to the beam direction. In the case of SOBP delivery (140 cGy peak dose), no response quenching in the peak region (characteristic of other dosimeter types) was observed. For a pristine beam delivery (170 cGy peak dose), the geldosimeter demonstrated a ∼10% under‐response at the Bragg peak (BP). Conclusion: Preliminary investigations suggest that this 3D dosimeter is capable of reproducing proton beam dose distributions with high spatial accuracy, with little or no quenching at the BP or SOBP.

SU‐E‐T‐14: Modeling of 3D Positron Emission Activity Distributions Induced by Proton Irradiation: A Semi‐Empirical Method

PURPOSE to present and validate a method for modeling three-dimensional positron emission (PE) activity distributions induced by proton beam irradiation for PET/CT delivery verification studies in homogeneous media. METHODS the method relies on modeling the 3D proton flux distribution by combining the analytical expression for the depth reduction of proton flux with the empirically obtained lateral distribution. The latter is extracted from the corresponding dose distribution under the assumption that the projectile energy is nearly constant in the lateral plane. The same assumption allows calculating the 3D induced activity distributions from proton flux distributions by parameterizing the energy-dependent activation cross-sections in terms of depth via the energy-range relation. Results of this modeling approach were validated against experimental PET/CT data from three phantom deliveries: unmodulated (pristine) beam, spread-out Bragg peak (SOBP) delivery without a range compensator, and SOBP with a range compensator. BANG3-Pro2 polymer gel was used as a phantom material because of its elemental soft-tissue equivalence. RESULTS the agreement between modeled and measured activity distributions was evaluated using 3D gamma index analysis method, which, despite being traditionally reserved for dose distribution comparisons, is sufficiently general to be applied to other quantities. The evaluation criteria were dictated by limitations of PET imaging and were chosen to correspond to count rate uncertainty (6% value difference) and spatial resolution (4 mm distance to agreement). With these criteria and the threshold of 6%, the fraction of evaluated voxels passing the gamma evaluation was 97.9% for the pristine beam, 98.9% for the SOBP without compensator, and 98.5% for SOBP with compensator. CONCLUSIONS results of gamma evaluation indicate that the activity distributions produced by the model are consistent with experimental data within the uncertainties of PET imaging for clinical proton beams deliveries. This work was supported by the Bankhead-Coley Florida Biomedical Research Program under Grant No. 1BD10-34212.

A Spatial Resolution Study of a New Optical Tomography-Based Polymer Gel Dosimetry System

A spatial resolution investigation of the OCTOPUS™-IQ scanner in combination with the new BANG3-Pro2® polymer gel was performed by scanning a high-contrast needle phantom. The phantom contained five thin needles (0.3 mm diameter) embedded in gel positioned in different patterns: needles were inserted (a) at 45° angle from the center of the gel container, and (b) vertically along the gel axis. The non-irradiated needle phantoms were scanned at various slice spacings (0.25–1.0 mm) and for two different laser beam orientations. Optical density profiles and their full width at half maximum (FWHM) were evaluated for resolution limit. The modulation transfer function (MTF) corresponding to measured point spread function (PSF) data was calculated. With high resolution scanning mode and 0.25 mm pixel resolution, the measured PSFs at the center of the gel dosimeter have a FWHM of 0.95 mm. The MTF for the 0.25 mm reconstruction pixel size suggests that the resolution of the system is 0.5 mm or less. We also observed a progressive degradation of the vertical needle images with off-axis distance, attributable to the defocusing of the laser beam. No significant degradation was observed up to the maximum useful reconstructed image radius of 50 mm from the gel dosimeter center axis.

TU‐G‐BRB‐07: Evaluation of Tissue‐Equivalent 3D Polymer Gel Dosimeters as Phantoms for PET/CT Verification of Proton Beam Deliveries

Purpose: to investigate the potential of polymergeldosimeters for concurrent measurements of three‐dimensional positron emission activity and dose distributions; to evaluate the ability of this technique to identify dosimetric errors due to delivery uncertainties, including those due to range modification and target motion.Methods: three BANG3‐Pro2 geldosimeters irradiated by protonbeams were imaged in a PET/CT scanner, starting within 3 minutes after irradiation. The radiation was delivered as a pristine beam under static conditions, as well as an SOBP, with and without phantom motion. The motion trace was defined by a sinusoidal curve with 2 cm peak‐to‐peak amplitude and the frequency of 0.25 Hz. The dose was read out using an established optical CT scanning procedure. PET/CT images of activated gels were validated against analytical calculations of activity and correlated to measured dose. The effects of target motion on activity and dose distributions were evaluated by volumetric gamma analysis against the treatment plan. Results: The profiles of positron emission activity along the central beam axis were found to be consistent with analytical calculations. Temporal dependence of activity decay suggests that the observed PET signal is due mainly to decay of 15O and 11C. Lateral profiles were found to exhibit good spatial correlation throughout the beam range. This allowed using a modified gamma analysis method to compare the signatures of target motion in PET and doseimages. Mean gamma for static PET and dose datasets was 0.07 and 0.11, respectively. For the motion delivery, the mean gamma value increased to 0.63 for both datasets. The spatial distributions of the gamma criterion for PET and dose datasets were qualitatively similar. Conclusions: Polymergels can accurately capture both dosimetric and activation information. Dosimetric errors due to target motion can be quantified by PET/CT using a novel method of analysis. This work was supported by the Bankhead‐Coley Florida Biomedical Research Program under Grant No. 1BD10‐34212.