Browsing by Subject "Presage"

Purpose: The purpose of this work is to characterize three novel formulations of a radiochromic material Presage and identify optimal imaging procedures for accurate 3D dosimetry. The dosimetric qualities of interest were studied for each formulation of Presage dosimeter in the context of accurate 3D dosimetry. The formulation of Presage showing the most promise is compared to a clinical 3D quality assurance device to investigate the accuracy of a complex state-of-the-art brain IMRT treatment.

Methods and Materials: Three novel formulations of Presage were studied for their temporal stability, sensitivity, linearity of dose response, and feasibility of absolute dose calibration in large volume dosimeters (1 kg) with small volume cuvettes (4g). Large cylindrical dosimeters with 11 cm diameter and 10 cm height were irradiated with 5 2x2 cm fields on the upper flat surface with 3 distinct dose levels (3, 6 and 9.5 Gy, representing low, medium and high). This irradiation pattern is used to determine the dosimetric characteristics mentioned above and was chosen because of its repeatability and it lends to simple measurements of linearity and sensitivity. Measurements were taken at various time points from 0 hours to 24 hours post-irradiation using the high resolution (6.45 m pixels) Duke Medium-Sized Optical-CT Scanner (DMOS) and reconstructed with a Matlab-based reconstruction GUI created in-house. Analysis of the pertinent dosimetric characteristics was performed in the GUI. A comprehensive end-to-end QA test was performed on the optimal formulation using optimal scan timing determined from the formulation studies described above. A 5-field IMRT plan was created for head treatment. The plan was delivered both to a head phantom containing a Presage insert, and to the Delta4 QA device. Comparison of both delivered distributions together with the Eclipse predicted dose distribution enabled investigation of the accuracy of the delivery, and the consistency of independent measurement devices.

Results: The DEA-1 formulation showed up to 10% variation from 0-2 hours post-irradiation, but showed excellent temporal stability (<2% variation) between 3-7 hours post irradiation, and maintained good stability until 24 hours post-irradiation (up to 3% variation). The DEA-2 also showed up to 10% variation from 0-2 hours post-irradiation. The DEA-2 formulation then showed good stability (up to 2.1% variation) from 3-7 hours, but optical density values dropped by up to 11% after 24 hours. The DX formulation did not maintain stability of optical density for any significant time with values decreasing by ~20% by the 24-hour time point and optical density decreasing at different rates for different dose levels. Linearity of dose response was good for all formulations with an R2 value > 0.99. Gamma analysis with criteria of 3%/2mm was performed on two irradiations of the 5-field pattern on DEA-1 formulation. Voxel passing rates were 96.68% and 97.96%. Comparison of the DEA-1 formulation large dosimeter was done with small volume cuvettes of the same formulation and batch. Sensitivity of the large dosimeter was less than half the sensitivity of the cuvettes. For clinical 3D QA comparison, the DEA-1 formulation was used because it had optimal performance showed the most promise for accurate 3D dosimetry. Line dose profiles showed that Presage compared very well with the Eclipse calculation and had a much better 3D gamma passing rate for 3%/3mm criteria than the Delta4 (>99% vs 75%).

Conclusions: The DEA-1 formulation shows the most promise because of its temporal stability and linearity of dose response. The optimal imaging window for this formulation was determined to be 3-24 hours post-irradiation. The DEA-2 and DX formulation also showed potential for accurate dosimetry. The optimal imaging window for the DEA-2 formulation was determined to be 2-6 hours post-irradiation. The optimal scan time for the DX formulation was determined to be immediately post-irradiation. The amount of accuracy loss depending on the scan time is dependent on the formulation and when the dosimeter is scanned. Line dose profiles and gamma analysis results from the comparison of Presage and Eclipse calculation provide strong validation of the accuracy of the IMRT treatment delivery. Comparison of Presage to the Delta4 show the Delta4 to be somewhat lacking in its ability to calculate 3D dose in the phantom/Presage geometry.

Purpose: Radiosurgery has become a widely used procedure in the treatment of both solid tumors and secondary metastases in the brain. In cases with multiple brain lesions, isocenters are typically set up for each target, a process which can take hours and become very uncomfortable for the patient. Recently, multifocal treatments with a single isocenter have emerged as a solution. With the high doses delivered to small regions during radiosurgery, the importance of treatment verification is paramount, especially when delivering high doses to regions off isocenter.

Methods: A 5-arc RapidArc radiosurgery plan with a single isocenter and 5 targets was used to treat a dosimeter placed within a RPC-type head and neck phantom. The treatment was delivered five times at varying prescription doses, depending on the sensitivity of the PRESAGE dosimeter used. The delivered dose distribution was measured using an in-house optical-CT system and compared to the Eclipse-planned dose distribution using dose volume histograms and Gamma analysis.

Results: Reasonable dose agreement was measured between the majority of the dosimeters and the Eclipse plan (80-85% pass rate at 5%/3 mm Gamma critera). The failing voxels were located on the periphery of the dosimeter at regions of extremely high or low dose, suggesting a dose dependent stability of the PRESAGE formulation. The formulation with the best temporal stability had a much higher Gamma pass rate of 98% at 3%/2mm criteria.

Conclusions: The potential of accurate delivery of the complex radiosurgery plan was demonstrated with one of the three formulations of PRESAGE. While agreement was worse in the other formulations, the problem seemed to be an easily-fixable stability issue, resulting in improper scaling of doses. Replication of the most stable formulation would provide an excellent tool for verification of radiosurgery treatment delivery and other complex procedures.

The need for clinically intuitive metrics for patient-specific quality assurance in radiation therapy has been well-documented (Zhen, Nelms et al. 2011). A novel transform method has shown to be effective at converting full-density 3D dose measurements made in a phantom to dose values in the patient geometry, enabling comparisons using clinically intuitive metrics such as dose-volume histograms (Oldham et al. 2011). This work investigates the transform method and compares its calculated dose-volume histograms (DVHs) to DVH values calculated by a Delta4 QA device (Scandidos), marking the first comparison of a true 3D system to a semi-3D device using clinical metrics. Measurements were made using Presage 3D dosimeters, which were readout by an in-house optical-CT scanner. Three patient cases were chosen for the study: one head-and-neck VMAT treatment and two spine IMRT treatments. The transform method showed good agreement with the planned dose values for all three cases. Furthermore, the transformed DVHs adhered to the planned dose with more accuracy than the Delta4 DVHs. The similarity between the Delta4 DVHs and the transformed DVHs, however, was greater for one of the spine cases than it was for the head-and-neck case, implying that the accuracy of the Delta4 Anatomy software may vary from one treatment site to another. Overall, the transform method, which incorporates data from full-density 3D dose measurements, provides clinically intuitive results that are more accurate and consistent than the corresponding results from a semi-3D Delta4 system.

Purpose:

The feasibility of MRT has recently been demonstrated utilizing a new technology of Carbon-Nano-Tube (CNT) field emission x-ray sources. This approach can deliver very high dose (10's of Gy) in narrow stripes (sub-mm) of radiation, which enables the study of novel radiation treatment approaches. Here we investigate the application of high- resolution (50𝑢𝑚 isotropic) PRESAGE®/Optical-CT 3D dosimetry techniques to characterize the radiation delivered in this extremely dosimetrically challenging scenario.

Methods:

The CNT field emission x-ray source irradiator comprises of a linear cathode array and a novel collimating system. The device delivers small `stripe' beams of approximately X long and Y wide, at an energy of 160 kVp. To characterize the MRT beams, an ultra-high-resolution prototype 3D dosimetry system was constructed and optimized, consisting of two parts: a radiochromic 3D dosimetry material PRESAGE, and a high resolution small field-of-view optical-CT imaging system for dose-readout (DMicrOS - Duke Micro Optical-CT Scanner). Small PRESAGE cylindrical dosimeters (~2.2cm in height and ~2.5cm in diameter) were irradiated by CNT MRT delivering 3 stripes of radiation with a nominal entrance dose of 32 Gy (16Gy for the second batch). PRESGAE dosimeters (with same dimensions) were also irradiated with at 32 Gy entrance dose, with a regular x-ray irradiator collimated to microscopical stripe- beams using a customized cerrobend material collimator. 50𝑢𝑚 (isotropic) 3D dosimetry was performed on all dosimeters using an in-house optical-CT system designed and optimized for high-resolution imaging (including a stray light deconvolution correction). The Percentage Depth Dose (PDD), Peak-to-Valley Dose Ratio (PVR) and beam width (FWHM) data were obtained and analyzed in both cases. Independent verification against EBT2 radiochromic film is ongoing.

Results:

Basic testing of the DMicrOS system indicated the following performance: Modulated-Transfer-Function (MTF), dynamic range, resolution, largest Field-Of-View (FOV), Point-Spread-Function (PSF) were performed. When applied to the PRESAGE dosimeters irradiated with MRT stripe beams, high-resolution 3D images were successfully achieved with the prototype system, enabling extraction of dose profiles. The PDDs for the CNT irradiation showed pronounced attenuation in UNC_A and UNC_C (little attenuation in UNC_F), but less build-up effect than that from the multibeam irradiation. The beam spacing between the three strips has an average value of 0.9mm while that for the 13 strips is 1.5 mm at a depth of 16.5 mm. The spacing between the three strips' barely varies with depth, while the 13 beams exhibit clear divergence. The three stripes show consistent PVR values (the average value is 18 at all depth). The stray light corrected image shows line profiles with reduced noise and consistent PVR values.

Conclusion:

MRT dosimetry is extremely challenging mostly due to little beam divergence tolerance and high dose rate required associated with the ultra small geometry. As a result, various artifacts (ring, donut, "dirty" fluid, imperfection removing the were observed and cannot be removed easily) present in the data. This preliminary application of a novel, ultra-high resolution, optical-CT 3D dosimetry system showed promise (reduced dose diverging, more accurate dose delivery), but suggested extremely careful techniques (flood matching, mounting, rotation stability). Further work is required to further validate the accuracy of dose distribution and investigate the causes of the artifacts as well as their removal methods. Especially, the stray light correction is believed to have a substantial impact in this extreme geometry, further optimizing the correcting methods is necessary to be explored.

Purpose: Clinical brachytherapy systems are capable of delivering very high doses with high dose gradients. It is important therefore to be able to accurately verify the doses calculated by brachytherapy treatment planning. Current dose verification methods are limited by poor resolution, and in the presence of large dose gradients, may give non-­representative results [1]. This thesis aims to evaluate the feasibility of a novel radiochromic dosimetry system for in-­vivo dose verification in organs at risk (bladder and rectum) in high dose rate (HDR) intracavitary gynecological brachytherapy through a comparison with a gold standard.

Methods: A novel dosimeter PRESAGE®-­IV designed for in-­vivo dosimetry is investigated. PRESAGE®-­IV dosimeters are small cylinders 4mm in diameter by 20mm in height. When irradiated, the dosimeters change color in proportion to the local absorbed dose. The dosimeters were irradiated to doses between 1-­15 Gy. Two methods were investigated for readout of this radiochromic response: (i) a volume averaged readout by conventional spectrophotometer, and (ii) a line profile readout by a novel 2D projection imaging method utilizing a high-­resolution (50 micron) telecentric optical system. Method (i) is considered the gold standard, as it is has been extensively used with PRESAGE® in well-­defined optical-­cuvettes. The feasibility of PRESAGE®-­IV was evaluated by comparison to standard PRESAGE® in optical-­cuvettes. The feasibility of the high-­resolution readout (method ii) was evaluated by direct comparison against method (i). Dosimeters were also tested in-­vivo on six patients undergoing Iridium-­192 HDR intracavitary brachytherapy treatments and dose measurements were compared to Eclipse Treatment Planning System (Varian Medical Systems).

Results: When compared to the gold standard (optical-­cuvettes), the sensitivity and noise of PRESAGE®-­IV shows a linear relationship in sensitivity between 1-­15 Gy with a 95% confidence interval in the slope (0.8703 +/-­ 0.0192). The feasibility of the high-­resolution readout (method ii) evaluated by direct comparison against method (i) resulted in a sensitivity of 0.0136 ± 0.0002 and for the spectrophotometer 0.0135 ± 0.0002, which is a 0.74% difference in sensitivity within the 95% confidence interval. Examination of patient data showed large differences, and on average gave 19% and 22% differences in measured doses vs. Eclipse measurements in the bladder and rectum, respectively.

Conclusions: Results show that a novel radiochromic dosimetry system for in-­vivo dose verification in organs at risk is feasible. The conventional spectrophotometer readout method had the limitation that it averages the change in optical density over a 10 mm area of the dosimeter. The novel, high-­resolution 2D readout technique was found to have the advantage of producing images that could be further analyzed through line profiles in any area of the dosimeter. Due to the large differences in measured doses for organs at risk, further work is needed to validate dosimeter-­positioning technique.

The purpose of this research is to validate the use of a PRESAGE dosimeter as a method to quantitatively measure dose distributions of injectable brachytherapy based on elastin-like polypeptide (ELP) nanoparticles. ELP has several useful properties for treatment purposes, including the ability to be tagged with a radioactive element, an inverse temperature phase transition useful for self-assembly into hydrophobic aggregation upon injection, and a highly tunable threshold temperature based on the amino acid composition and concentration. PRESAGE is a solid, transparent polyurethane-based dosimeter whose dose is proportional to a change in optical density, making it useful for visualizing the dose from a radionuclide-tagged-ELP injection. Initial experiments with the gel phantoms demonstrate viability for assessing I-125 dose deposition, as the image analysis showed the similar relative dose distributions compared with a MATLAB simulation.