Biomedical subjects

C Duzenli

Publications and source records attributed to C Duzenli.

11 recordsLinked to original sources

PubMed2005

Technical considerations for implementation of x-ray CT polymer gel dosimetry.

Gel dosimetry is the most promising 3D dosimetry technique in current radiation therapy practice. X-ray CT has been shown to be a feasible method of reading out polymer gel dosimeters and, with the high accessibility of CT scanners to cancer hospitals, presents an exciting possibility for clinical implementation of gel dosimetry. In this study we report on technical considerations for implementation of x-ray CT polymer gel dosimetry. Specifically phantom design, CT imaging methods, imaging time requirements and gel dose response are investigated. Where possible, recommendations are made for optimizing parameters to enhance system performance. The dose resolution achievable with an optimized system is calculated given voxel size and imaging time constraints. Results are compared with MRI and optical CT polymer gel dosimetry results available in the literature.

Acrylic Resins↗

PubMed2004

Effects of gel composition on the radiation induced density change in PAG polymer gel dosimeters: a model and experimental investigations.

Due to a density change that occurs in irradiated polyacrylamide gel (PAG), x-ray computed tomography (CT) has emerged as a feasible method of performing polymer gel dosimetry. However, applicability of the technique is currently limited by low sensitivity of the density change to dose. This work investigates the effect of PAG composition on the radiation induced density change and provides direction for future work in improving the sensitivity of CT polymer gel dosimetry. A model is developed that describes the PAG density change (delta(rho)gel) as a function of both polymer yield (%P) and an intrinsic density change, per unit polymer yield, that occurs on conversion of monomer to polymer (delta(rho)polymer). %P is a function of the fraction of monomer consumed and the weight fraction of monomer in the unirradiated gel (%T). Applying the model to experimental CT and Raman spectroscopic data, two important fundamental properties of the response of PAG density to dose (delta(rho)gel dose response) are discovered. The first property is that delta(rho)polymer)depends on PAG %C (cross-linking fraction of total monomer) such that low and high %C PAGs exhibit a higher deltarho(polymer)than do more intermediate %C PAGs. This relationship is opposite to the relationship of polymer yield to %C and is explained by the effect of %C on the type of polymer formed. The second property is that the delta(rho)gel dose response is linearly dependent on %T. From the model, the inference is that, at least for %T < or = 2%, monomer consumption and delta(rho)polymer depend solely on %C. In terms of optimizing CT polymer gel dosimetry for high sensitivity, these results indicate that delta(rho)polymer can be expected to vary with each polymer gel system and thus should be considered when choosing a polymer gel for CT gel dosimetry. However, delta(rho)polymerand %P cannot be maximized simultaneously and maximizing %P, by choosing gels with intermediate %C and high %T, is found to have the greatest impact on increasing the sensitivity of PAG density to dose. As such, future research into new gel formulations for high sensitivity CT polymer gel dosimetry should focus on gels that exhibit an intrinsic density change and maximizing polymer yield in these systems.

Acrylic Resins↗

PubMed2002

Relative effectiveness of polyacrylamide gel dosimeters applied to proton beams: Fourier transform Raman observations and track structure calculations.

The feasibility of using polyacrylamide gel (PAG) to characterize a clinically relevant 74 MeV proton beam has been considered. Fourier transform Raman spectroscopy has been used to measure the response (i.e., consumption of monomer) of PAG exposed to the central and end portions of a spread out Bragg peak (SOBP), i.e., in two regions with measurable difference in proton linear energy transfer (LET). The response curve in each region was compared with a typical 6 MV x-ray irradiated gel response curve, thus arriving at a gel "relative effectiveness" (RE) in each of the two regions. In addition, the theory of track structure, which is typically used to calculate a detector RE, is shown to give reasonable agreement when compared with the experimental results. Both experimental and track structure results indicate a decrease in gel response when irradiated with protons, as compared with the x-ray response. In addition, both sets of results indicate a variation in gel response between the mid and end SOBP regions, thus illuminating the dependence of gel response to proton LET. The physical phenomenon causing a lower proton versus x-ray gel response may be understood by considering the track structure calculations, which indicate that gel radiosensitive elements close to the track of a proton (i.e., within 1.5 x 10(-6) cm) are saturated. This saturation is due to the high delta-ray doses deposited in these regions. The track calculations are extended to other situations (e.g., raw BP, different gel compositions) where experimental determination of RE is difficult or time consuming. Results again indicate a gel response dependent on position in the depth dose curve (i.e., LET). Overall, this study illuminates the difficulty in using polyacrylamide gel to extract quantitative dose maps when exposed to proton radiation.

Acrylic Resins↗

PubMed2002

CT gel dosimetry technique: comparison of a planned and measured 3D stereotactic dose volume.

This study presents a 3D dose mapping of complex dose distributions using an x-ray computed tomography (CT) polymer gel dosimetry technique. Two polyacrylamide gels (PAGs) of identical composition were irradiated with the same four arc stereotactic treatment to maximum doses of 15 Gy (PAG1) and 8 Gy (PAG2). The PAGs were CT imaged using a previously defined protocol that involves image averaging and background subtraction to improve image quality. For comparison with the planned isodose distribution, the PAG images were converted to relative dose maps using a CT number-dose calibration curve or simple division. The PAG images were then co-registered with the planning CT images in the BrainLab treatment planning software which automatically provides reconstructed sagittal and coronal images for 3D evaluation of measured and planned dose. The hypo-intense high dose region in both sets of gel images agreed with the planned 80% isodose contour and was shifted by up to 1.5 and 3.0 mm in the axial and reconstructed planes, respectively. This demonstrates the ability of the CT gel technique to accurately localize the high dose region produced by the stereotactic treatment. The resulting agreement of the measured relative dose volume for PAG1 was within 3.0 mm for the 50% and 80% isodose surfaces. However, the dose contrast was too low in PAG2 to allow for accurate definition of measured relative dose surfaces. Thus, a PAG should be irradiated to higher doses if quantitative relative dose information is required. Unfortunately, this implies use of an additional PAG and its CT number dose response since doses greater than 8-10 Gy fall outside the linear regions of the response.

Acrylamides↗

PubMed2001

The treatment of choroidal melanoma with 198 Au plaque brachytherapy.

BACKGROUND: Seventy-nine consecutive patients with primary choroidal melanoma were treated with 198 Au plaque brachytherapy at the British Columbia Cancer Agency (BCCA) between 1992 and 1998 with perioperative ultrasound to confirm plaque placement. Seventy-seven of the 79 patients were analyzable for this study. RESULTS: Five year actuarial disease specific survival, enucleation free survival, and local control are 95, 94, and 98%, respectively. There were four melanoma related deaths, all secondary to liver metastases. CONCLUSIONS: The BCCA experience in selected patients with choroidal melanomas treated with 198Au plaque brachytherapy has resulted in excellent survival and local control with minimal significant toxicity while preserving the globe. Our results using 198 Au seeds are comparable to other series using 125I, 60Co, and 106Ru at other centers.

Adult↗

PubMed2001

Characterization of monomer/crosslinker consumption and polymer formation observed in FT-Raman spectra of irradiated polyacrylamide gels.

Fourier transform Raman spectroscopy was undertaken in the study of irradiated polyacrylamide gels (PAGs) used in 3D radiation dosimetry. By employing correlation techniques, monomer and crosslinker consumption were characterized in the spectra as a function of absorbed dose. The consumption of both monomer and crosslinker is monoexponential up to 13 Gy, although the rates of consumption differ for the two molecules. A sensitivity parameter, D0, in the exponential function has been used to characterize this difference. Up to 13 Gy, D0(acr) = 12 +/- 2 Gy while D0(bis) = 8.0 +/- 0.5 Gy, indicating that bis is consumed at a greater rate than acrylamide and that bis is the limiting factor in the onset of gel saturation, for a gel composition of 6% by weight total monomer (6%T) and where 3% of the total monomer is crosslinker (50%C). Direct evidence of polymer formation was observed in the Raman spectra of irradiated PAG. Polymer formation is monoexponential to a dose of 13 Gy, with a sensitivity parameter of D0(poly) = 14 +/- 2 Gy. This is in good agreement with the consumption rate of acrylamide. The exponential nature of the polymer formation observed here is compared with existing MRI and x-ray CT dose response measurements previously reported to be linear. The results confirm previous studies indicating that Raman spectroscopy provides a direct and useful tool for characterization of irradiated PAG.

Acrylamide↗

PubMed2001

Effects of crosslinker fraction in polymer gel dosimeters using FT Raman spectroscopy.

Radiation-induced chemical changes in polyacrylamide gels (PAGs) used in 3D radiation dosimetry have been studied using Fourier transform Raman spectroscopy. Consumption of monomer and crosslinker were characterized for polymer gel dosimeters with initial fraction of crosslinker (%C) varying from to 100%. Results indicate that the rate of monomer/crosslinker consumption is highly nonlinear over %C. This indicates that maximum polymer gel dosimeter sensitivity (i.e. slope of the monomer consumption curves) is dependent not only on initial %C but also on dose range. For low-dose regions (<5 Gy) 30%C PAGs are the most sensitive dosimeters, their maximum sensitivity being a factor of 1.8 times greater than 50%C PAGs. In mid-dose range (S-15 Gy), however, 50%C PAGs are the most sensitive dosimeters, being potentially a factor of 1.3 more sensitive than any other PAG studied. In high-dose regions (> 15 Gy) 70%C PAGs are at least 1.5 times more sensitive than any other PAG studied. A qualitative model is presented which helps to explain the variation in overall monomer consumption rate as a function of initial %C. The variation in the shapes (i.e. mathematical course) of the consumption curves for different %C PAGs is, in part, qualitatively explained by consideration of the average number of monomer to crosslinker molecules available for reaction in each PAG as it is irradiated to higher doses. Finally, we demonstrate the importance of an existing polymer network on the subsequent monomer consumption rate in a 50%C PAG.

Acrylamide↗

PubMed2000

Polymer gel dosimetry using x-ray computed tomography: a feasibility study.

A new three-dimensional dosimetry technique using x-ray computed tomography (CT) to analyse polymer gels is proposed. The CT imaging is sensitive to radiation-induced density changes that occur within irradiated polyacrylamide gel (PAG). In this preliminary study, a CT imaging protocol is developed to optimize CT images of PAG; the response of PAG CT number to dose (N(CT)-dose response) and the reproducibility of the response are investigated, and the use of CT to analyse PAG is compared with MRI. Experiments were conducted using two 1.5 l cylindrical PAG phantoms (3% acrylamide, 3% bis and 5% gelatin by weight), one irradiated with four intersecting 10 MV photon beams and the other with 10 sets of 6 MV parallel opposed circular radiosurgery fields. The final imaging protocol involves using optimum CT parameters (120 kVp and 200 mAs for our GE HiSpeed CT/i scanner), image averaging and background subtraction. The N(CT)-dose response is reproducible, linear up to 800-1000 cGy and is relatively insensitive to the gel temperature during imaging. The dose resolution is approximately 50 cGy for an image thickness of 10 mm. Despite the low dose resolution, preliminary results indicate that this CT technique provides accurate localization of high dose gradients such as those observed in stereotactic radiosurgery. Thus, given the availability and speed of CT scanners, the technique has the potential to be a valuable and practical 3D dose verification tool in radiation therapy.

Acrylic Resins↗

PubMed1995

Correcting for rf inhomogeneities in multiecho pulse sequence MRI dosimetry.

The growing interest in using MRI to measure distributions of dose in specialized materials has prompted the need to improve existing methods for measuring NMR relaxation rates throughout large volume phantoms. The change in transverse relaxation rate (R2) has proven to be a more sensitive measure of dose than longitudinal relaxation rate (R1) in some materials; however, the accuracy of R2 measurements is limited by the presence of artifacts resulting from imperfect MRI tip angles throughout large phantoms. A novel echo quotient technique allowing accurate measurement of changes in R2 (delta R2) in regions of imperfect tip angle is presented. Application of this technique to the measurement of a dynamically wedged photon beam dose distribution demonstrates the improved accuracy of the echo quotient technique compared with other methods. Agreement of the MRI data with film and ion chamber data is within 2.5% throughout the volume of interest, indicating the potential for delta R2 MRI dosimetry to become a clinically useful tool.

Gelatin↗

PubMed1994

A spin-spin relaxation rate investigation of the gelatin ferrous sulphate NMR dosimeter.

Spin-spin NMR relaxation rate in the ferrous sulphate gelatin dosimeter has been studied in terms of pH, gelatin concentration, the addition of benzoic acid, and sample size. It is demonstrated that R2 is more sensitive to changes in Fe3+ ion concentration than R1 when measuring at frequencies of 64 and 100 MHz. pH has an important effect on dose response curves, and oxygen depletion occurs significantly more rapidly in FeSO4 gelatin than in the liquid FeSO4, resulting in a saturation dose of approximately 80 Gy at depths greater than approximately 3 mm in phantom. The concentration of gelatin can be increased to 12% by weight, and the dosimeter will continue to exhibit a linear dose response. Sensitivity is maintained at higher gel concentrations by pH compensation. Addition of low-concentration benzoic acid to the system does not alter the dose response of the gelatin FeSO4 system. Finally, spontaneous oxidation of Fe2+ ions does not significantly alter the shape of dose response curves but does result in increases in R2 by up to 4% per day.

Algorithms↗

PubMedDate not supplied

Backscatter into the beam monitor chamber: implications for dosimetry of asymmetric collimators.

Backscatter from the asymmetric collimators of a linac into the beam monitor chamber (BMC) has been investigated for two accelerators having different BMC configurations. The effect has been quantified as a function of field size and collimator jaw position for 6 and 18 MV beams. The results indicate a maximum 2.5% (6 MV) and 4% (18 MV) decrease in output in one case and a negligible effect in the other case. The experiments indicate that the difference can be attributed to the different construction of the BMC's for the two accelerators.

Humans↗