Water equivalence of NIPAM based polymer gel dosimeters with enhanced sensitivity for x-ray CT

Clinical applicability of Linac-integrated CBCT based NIPAM 3D dosimetry: a dual-institutional investigation

Objective.To develop and benchmark a novel 3D dose verification technique consisting of polymer gel dosimetry (PGD) with cone-beam-CT (CBCT) readout through a two-institution study. The technique has potential for wide and robust applicability through reliance on CBCT readout.Approach. Three treatment plans (3-field, TG119-C-shape spine, 4-target SRS) were created by two independent institutions (Institutions A and B). A Varian Truebeam linear accelerator was used to deliver the plans to NIPAM polymer gel dosimeters produced at both institutions using an identical approach. For readout, a slow CBCT scan mode was used to acquire pre- and post-irradiation images of the gel (1 mm slice thickness). Independent gel analysis tools were used to process the PGD images (A: VistaAce software, B: in-house MATLAB code). Comparing planned and measured doses, the analysis involved a combination of 1D line profiles, 2D contour plots, and 3D global gamma maps (criteria ranging between 2%1 mm and 5%2 mm, with a 10% dose threshold).Main results. For all gamma criteria tested, the 3D gamma pass rates were all above 90% for 3-field and 88% for the SRS plan. For the C-shape spine plan, we benchmarked our 2% 2 mm result against previously published work using film analysis (93.4%). For 2%2 mm, 99.4% (Institution A data), and 89.7% (Institution B data) were obtained based on VistaAce software analysis, 83.7% (Institution A data), and 82.9% (Institution B data) based on MATLAB.Significance. The benchmark data demonstrate that when two institutions follow the same rigorous procedures gamma passing rates up to 99%, for 2%2 mm criteria can be achieved for substantively different treatment plans. The use of different software and calibration techniques may have contributed to the variation in the 3D gamma results. By sharing the data across institutions, we observe the gamma passing rate is more consistent within each pipeline, indicating the need for standardized analysis methods.

Fluorescent Nanogel Sensors for X-ray Dosimetry

X-ray dosimeters are of significance for detecting the levels of ionizing radiation exposure in cells and phantoms; thus, they can further optimize X-ray radiotherapy in the clinic. In this paper, we designed a polyacrylamide-based nanogel sensor that is capable of measuring X-ray doses. The dosimeters were prepared by anchoring an X-ray-responsive probe (aminophenyl fluorescein, APF) to poly(acrylamide-co-N-(3-aminopropyl) methyl acrylamide) nanogels. The premise behind the dose measurement is the transition of APF to fluorescence in the presence of hydroxyl radicals that are caused by the radiolysis of water molecules under X-rays. Therefore, the dose of X-rays can be readily detected by measuring the fluorescence intensity of the resultant nanogel immediately after irradiation using fluorescence spectroscopy principles. Using an RS2000 X-ray biological irradiator, our dosimeters showed good linearity responsivity at X-ray doses ranging from 0 to 15 Gy, with a limit of detection (LOD) of 0.5 Gy. Additionally, the signals showed temperature stability (25-65 °C), durability (5 weeks), and dose-rate (1.177 and 6 Gy/min) and energy independence (160 kVp and 6 MV). As a proof-of-concept, we used our sensors to fluorescently detect X-ray doses in A549 tumor cells and 3D-printed eye phantoms. The results showed that our dosimeters were able to accurately predict doses similar to those used by treatment plan systems.

Evaluation of accuracy and precision in polymer gel dosimetry

PURPOSE

To assess the overall reproducibility and accuracy of an X-ray computed tomography (CT) polymer gel dosimetry (PGD) system and investigate what effects the use of generic, interbatch, and intrabatch gel calibration have on dosimetric and spatial accuracy.

METHODS

A N-isopropylacrylamide (NIPAM)-based gel formulation optimized for X-ray CT gel dosimetry was used, and the results over four different batches of gels were analyzed. All gels were irradiated with three 6 MV beams in a calibration pattern at both the bottom and top of the dosimeter. Postirradiation CT images of the gels were processed using background subtraction, image averaging, adaptive mean filtering, and remnant artifact removal. The gel dose distributions were calibrated using a Monte Carlo (Vancouver Island Monte Carlo system) calculated dose distribution of the calibration pattern. Using the calibration results from all gels, an average or "generic" calibration curve was calculated and this generic calibration curve was used to calibrate each of the gels within the sample. For each of the gels, the irradiation pattern at the bottom of the dosimeter was also calibrated using the irradiation pattern at the top of the dosimeter to evaluate intragel calibration.

RESULTS

Comparison of gel measurements with Monte Carlo dose calculations found excellent dosimetric accuracy when using an average (or generic) calibration with a mean dose discrepancy of 1.8% in the low-dose gradient region which compared to a "best-case scenario" self-calibration method with a mean dose discrepancy of 1.6%. The intragel calibration method investigated produced large dose discrepancies due to differences in dose response at the top and bottom of the dosimeter, but the use of a dose-dependent correction reduced these dose errors. Spatial accuracy was found to be excellent for the average calibration method with a mean distance-to-agreement (DTA) of 0.63 mm and 99.6% of points with a DTA < 2 mm in high-dose gradient regions. This compares favorably to the self-calibration method which produced a mean DTA of 0.61 mm and 99.8% of points with a DTA < 2 mm. Gamma analysis using a 3%/3 mm criterion also found good agreement between the gel measurement and Monte Carlo dose calculation when using either the average calibration or self-calibration methods (96.8% and 98.2%, respectively).

CONCLUSIONS

An X-ray CT PGD system was evaluated and found to have excellent dosimeteric and spatial accuracy when compared to Monte Carlo dose calculations and the use of generic and interbatch calibration methods were found to be effective. The establishment of the accuracy and reproducibility of this system provides important information for clinical implementation.

Effect of inorganic salts and glucose additives on dose-response, melting point and mass density of genipin gel dosimeters

Genipin gel dosimeters are hydrogels infused with a radiation-sensitive material which yield dosimetric information in three dimensions (3D). The effect of inorganic salts and glucose on the visible absorption dose-response, melting points and mass density of genipin gel dosimeters has been experimentally evaluated using 6-MV LINAC photons. As a result, the addition of glucose with optimum concentration of 10% (w/w) was found to improve the thermal stability of the genipin gel and increase its melting point (Tm) by 6 °C accompanied by a slight decrease of dose-response. Furthermore, glucose helps to adjust the gel mass density to obtain the desired tissue-equivalent properties. A drop of Tm was observed when salts were used as additives. As the salt concentration increased, gel Tm decreased. The mass density and melting point of the genipin gel could be adjusted using different amounts of glucose that improved the genipin gel suitability for 3D dose measurements without introducing additional toxicity to the final gel.