Biomedical subjects

James F Dempsey

Publications and source records attributed to James F Dempsey.

28 records · Page 2Linked to original sources

PubMed2003

Ionization chamber volume averaging effects in dynamic intensity modulated radiation therapy beams.

The commercial cylindrical ionization chamber ionization integration accuracy of dynamically moving fields was evaluated. The ionization chambers were exposed to long (14 cm), narrow (0.6, 1.0, 2.0, and 4.0 cm) 6 MV and 18 MV fields. Rather than rely on the linear accelerator to reproducibly scan across the chamber, the chambers were scanned beneath fixed portals. A water-equivalent phantom was constructed with cavities that matched the chambers and placed on a computer-controlled one-dimensional table. Computer-controlled electrometers were utilized in continuous charge integrate mode, with 10 samples of the charge, along with time stamps, acquired for each chamber location. A reference chamber was placed just beneath the linear accelerator jaws to adjust for variations in linear accelerator dose rate. The scan spatial resolution was selected to adequately sample regions of steep dose gradient and second spatial derivative (curvature). A fixed measurement in a 10 x 10 cm2 field was used to normalize the profiles to absolute chamber response. Three ionization chambers were tested, a microchamber (0.009 cm3), a Farmer chamber (0.6 cm3) and a waterproof scanning chamber (0.125 cm3). The larger chambers exhibited severe under-response at the small field's centers, but all of the chambers, independent of orientation, accurately integrated the ionization across the scanned portal. This indicates that the tested ionization chambers provide accurate integrated charges in regions of homogeneous dose regions. Partial integration (less than the field width plus the chamber length plus 2 cm), yielded integration errors of greater than 1% and 2% for 6 MV and 18 MV, respectively, with errors for the Farmer chamber of greater than 10% even for the 4 cm wide field.

Equipment Failure Analysis↗

PubMed2003

Initial evaluation of commercial optical CT-based 3D gel dosimeter.

We evaluated the OCTOPUS-ONE research laser CT scanner developed and manufactured by MGS Research, Inc. (Madison, CT). The scanner is designed for imaging 3D optical density distributions in BANG gels. The scanner operates in a translate-rotate configuration with a single scanning laser beam. The rotating cylindrical gel phantom is immersed in a refractive index matching solution and positioned at the center of a square tank made of plastic and glass. A stationary polarized He-Ne laser beam (633 nm) is reflected from a mirror moving parallel to the tank wall and scans the gel. Another mirror moves synchronously along the opposite side of the tank and collects the transmitted light and sends it to a single stationary silicon photodetector. A filtered backprojection algorithm is used to reconstruct projection data in a plane. The laser-mirrors-detector assembly is mounted on a horizontal platform that moves vertically for slice selection. We have tested the mechanical and optical setup, projection centering on the axis of rotation, linearity, and spatial resolution. We found the optical detector to respond linearly to transmitted light from control samples. The spatial resolution of the scanner was determined by employing a split field resolution technique. We obtained the horizontal and vertical full widths at half maxima of the laser beam intensity profiles as 0.6 and 0.8 mm, respectively. Dose calibration tests of the gel were performed using a nine-field (2 x 2 cm2 each) dose pattern irradiated at different dose levels. Finally, we compared gel-derived 2D planar dose distribution against radiochromic film measured dose distribution for both the nine-field and a uniform 5 x 5 cm2 field of 6 MV x rays. Very similar dose distributions were observed in gel and radiochromic film except in regions of steep dose gradient and highest dose. A dose normalization of 15.6% was required between the two dosimeters due to differences in overall radiation response. After normalization, analysis using the gamma evaluation showed that the radiochromic film and gel-measured dose distributions differed by a maximum gamma of 1.3 using 5% and 1.5 mm dose difference and distance-to-agreement criteria. The optical CT scanner has great potential as a 3D dosimeter, but a few refinements and further testing are necessary before its routine clinical use.

Algorithms↗

PubMed2003

Analysis of various beamlet sizes for IMRT with 6 MV photons.

Application of intensity modulated radiation therapy (IMRT) using multileaf collimation often requires the use of small beamlets to optimize the delivered radiation distribution. Small-beam dose distribution measurements were compared to dose distributions calculated using a commercial treatment planning system that models its data acquired using measurements from relatively large fields. We wanted to evaluate only the penumbra, percent depth-dose (PDD) and output model, so we avoided dose distribution features caused by rounded leaf ends and interleaf leakage by making measurements using the secondary collimators. We used a validated radiochromic film dosimetry system to measure high-resolution dose distributions of 6 MV photon beams. A commercial treatment planning system using the finite size pencil beam (FSPB) dose calculation algorithm was commissioned using measured central axis outputs from 4.0x4.0 to 40.0x40.0 cm2 beams and radiographic-film profile measurements of a 4.0x4.0 cm2 beam at twice the depth of maximum dose (dmax). Calculated dose distributions for square fields of 0.5x0.5 cm2, and 1.0x1.0 cm2, to 6.0x6.0 cm2, in 1.0x1.0 cm2, increments were compared against radiochromic film measurements taken with the film oriented parallel to the beam central axis in a water equivalent phantom. The PDD of the smaller field sizes exhibited behavior typical of small fields, namely a decrease in dmax with decreasing field size. The FSPB accurately modeled the depth-dose and central axis output for depths deeper than the nominal dmax of 1.5 cm plus 0.5 cm. The dose distribution in the build-up and penumbra regions was not accurately modeled for depths less than 2 cm, especially for the fields of 2.0x2.0 cm2 and smaller. Using the gamma function with 2 mm and 2% criteria, the dose model was shown to accurately predict the penumbra. While for single small beams the compared dose distributions passed the gamma function criteria, the clinical appropriateness of these criteria is not clear for a composite IMRT plan. Further investigation of the cumulative impact of the observed dose discrepancies is warranted. We speculate that the observed differences in the penumbra regions arise from some energy dependent artifact in the radiographic-film profiles used for commissioning. In the future, radiochromic film based commissioning might provide a more accurate data set for dose modeling.

Calibration↗

PubMed2003

Evaluation of the gamma dose distribution comparison method.

The gamma tool was developed to quantitatively compare dose distributions, either measured or calculated. Before computing gamma, the dose and distance scales of the two distributions, referred to as evaluated and reference, are renormalized by dose and distance criteria, respectively. The renormalization allows the dose distribution comparison to be conducted simultaneously along dose and distance axes. The gamma quantity, calculated independently for each reference point, is the minimum distance in the renormalized multidimensional space between the evaluated distribution and the reference point. The gamma quantity degenerates to the dose-difference and distance-to-agreement tests in shallow and very steep dose gradient regions, respectively. Since being introduced, the gamma quantity has been used by investigators to evaluate dose calculation algorithms, and compare dosimetry measurements. This manuscript examines the gamma distribution behavior in two dimensions and evaluates the gamma distribution in the presence of data noise. Noise in the evaluated distribution causes the gamma distribution to be underestimated relative to the no-noise, condition. Noise in the reference distribution adds noise in the gamma distribution in proportion to the normalized dose noise. In typical clinical use, the fraction of points that exceed 3% and 3 mm can be extensive, so we typically use 5% and 2-3 mm in clinical evaluations. For clinical cases, the calculation time is typically 5 minutes for a 1 x 1 mm2 interpolated resolution on an 800 MHz Pentium 4 for a 14.1 x 15.2 cm2 radiographic film.

Algorithms↗

PubMed2002

Determination and delineation of nodal target volumes for head-and-neck cancer based on patterns of failure in patients receiving definitive and postoperative IMRT.

PURPOSE: We present the guidelines for target volume determination and delineation of head-and-neck lymph nodes based on the analysis of the patterns of nodal failure in patients treated with intensity-modulated radiotherapy (IMRT). METHODS AND MATERIALS: Data pertaining to the natural course of nodal metastasis for each head-and-neck cancer subsite were reviewed. A system was established to provide guidance for nodal target volume determination and delineation. Following these guidelines, 126 patients (52 definitive, 74 postoperative) were treated between February 1997 and December 2000 with IMRT for head-and-neck cancer. The median follow-up was 26 months (range 12-55), and the patterns of nodal failure were analyzed. RESULTS: These guidelines define the nodal target volume based on the location of the primary tumor and the probability of microscopic metastasis to the ipsilateral and contralateral (Level I-V) nodal regions. Following these guidelines, persistent or recurrent nodal disease was found in 6 (12%) of 52 patients receiving definitive IMRT, and 7 (9%) of 74 patients receiving postoperative IMRT had failure in the nodal region. CONCLUSION: On the basis of our clinical experience in implementing inverse-planning IMRT for head-and-neck cancer, we present guidelines using a simplified, but clinically relevant, method for nodal target volume determination and delineation. The intention was to provide a foundation that enables different institutions to exchange clinical experiences in head-and-neck IMRT. These guidelines will be subject to future refinement when the clinical experience in head-and-neck IMRT advances.

Follow-Up Studies↗

PubMed2002

Toward automated quality assurance for intensity- modulated radiation therapy.

PURPOSE: To investigate whether high-quality, relatively inexpensive, document and transparency scanners used as densitometers are sufficiently quantitative for routine quality assurance (QA). METHODS AND MATERIALS: The scanner we investigated used a linear amplifier, digitizing gray-scale images to 12-bit resolution with a user-selected spatial resolution of 0.170 mm(2) pixels. To reduce Newton's rings artifacts, the standard glass platen was replaced by glass with an antireflective coating. Conversion of reading to transmission was conducted by permanently placing a calibrated photographic step tablet on the scanner platen. After conversion to light transmission, a zero-phase two-dimensional Wiener filter was used to reduce pixel-to-pixel signal variation. Light-scatter artifacts were removed by deconvolution of a measured light-spread kernel. The light-spread kernel artifacts were significant along the scanner's detector axis, but were insignificant along the scanning axis. RESULTS: Pixel-to-pixel noise was better than 2% for optical densities, ranging from 0.4 to 2.0 and 0 to 2.7 for the unfiltered and filtered images, respectively. The document scanning system response was compared against a confocal scanning laser densitometer. A series of IMRT dose distribution and dose calibration film sets were scanned using the two scanners, and the measured dose was compared. The maximum mean and standard deviation of the measured dose difference between the document scanner and confocal scanner was 1.48% and 1.06%, respectively. CONCLUSION: While the document scanners are not as flexible as dedicated film densitometers, these results indicate that, using the intensity and scatter corrections, the system provides accurate and precise measurements up to an optical density of 2.0, sufficient for routine IMRT film QA. For some film types, this requires the reduction in monitor units to limit the dose delivered to the film. The user must be cautious that the delivered IMRT dose is scaled appropriately. This inexpensive and accurate system is being integrated into an automated QA program.

Artifacts↗

PubMed2002

Applicator-guided intensity-modulated radiation therapy.

PURPOSE: We are introducing a novel method for delivering highly conformal dose distributions to cervical cancer tumors using external beam intensity-modulated radiation therapy. The method, termed applicator-guided intensity-modulated radiation therapy (AGIMRT), will use an applicator substitute placed in the vagina and uterus to provide spatial registration and immobilization of the gynecologic organs. The main reason for the applicator substitute will be to localize the fornices, cervix, and uterus with the expectation that the other nearby organs will also be reproducibly positioned with respect to the applicator substitute. Intensity-modulated radiation therapy (IMRT) dose distributions will be used as a substitute for high-dose-rate intracavitary brachytherapy procedures. The flexibility of IMRT will enable customized dose distributions that have the potential to reduce complications and improve local control, especially for locally advanced disease. METHODS AND MATERIALS: To test the advantages of IMRT over intracavitary brachytherapy, volumetric scans of three cervical cancer patients were obtained with implanted CT-compatible applicators. IMRT dose distribution simulations using tomotherapy, were compared against intracavitary brachytherapy using cesium tubes to investigate the dosimetric differences of the two modalities. Because these tumor volumes do not image well on CT, the target volumes were defined as the isodose surface containing the traditional point A, defined as 2 cm superior to the vaginal fornices and 2 cm lateral to the intrauterine canal. One patient had a uterus that wrapped superior and anterior to the bladder. For this case, the cervix and uterus were selected as the target volume. To determine the potential for using an applicator substitute to localize internal organs, the posterior bladder and anterior rectal surfaces were localized relative to the colpostats. Comparisons of the colpostat-localized surfaces were conducted for two scan studies for 3 patients. RESULTS: The IMRT distributions covered the point-A isodose surfaces while reducing doses to the bladder and rectum. Brachytherapy showed extensive underdose regions in the target volume for the wrapped-around target. Spatial positioning was better than 0.7 and 1.3 cm in the rectum and bladder, respectively, indicating the potential that an applicator substitute may be able to localize these structures. CONCLUSIONS: AGIMRT has the potential for improving critical structure avoidance while maintaining highly reproducible and accurate internal organ registration found with brachytherapy.

Female↗

PubMed2002

Performance evaluation of an 85-cm-bore X-ray computed tomography scanner designed for radiation oncology and comparison with current diagnostic CT scanners.

INTRODUCTION: The demand for computed tomography (CT) virtual simulation is constantly increasing with the wider adoption of three-dimensional conformal and intensity-modulated radiation therapy. Virtual simulation CT studies are typically acquired on conventional diagnostic scanners equipped with an external patient positioning laser system and specialized planning and visualization software. Virtual simulation technology has matured to a point where conventional simulators may be replaced with CT scanners. However, diagnostic CT scanner gantry bores (typically 65-70 cm) can present an obstacle to the CT simulation process by limiting patient positions, compared to those that can be attained in a conventional simulator. For example, breast cancer patients cannot always be scanned in the treatment position without compromising reproducibility and appropriateness of setup. Extremely large patients or patients requiring special immobilization or large setup devices are often unable to enter the limited-bore gantry. A dedicated 85-cm-bore radiation oncology CT scanner has the potential to eliminate these problems. The scanner should provide diagnostic-quality images at diagnostic-comparable dose levels. The purpose of this study was to independently evaluate the performance of a novel 85-cm-bore CT X-ray scanner designed specifically for radiation oncology and compare it against diagnostic-type, 70-cm-bore scanners that may be used in the same setting. MATERIALS AND METHODS: We performed image quality and dosimetric measurements on an 85-cm-bore CT scanner (AcQSim CT, Marconi Medical Systems, Inc., Cleveland, OH) and a 70-cm-bore, diagnostic-type scanner (UltraZ CT, Marconi Medical Systems, Inc.). Image quality measurements were performed using a manufacturer-supplied phantom (Performance Phantom, Marconi Medical Systems, Inc.), following the manufacturer's suggested testing procedures, and an independent image quality phantom (CATPHAN 500, The Phantom Laboratory, New York, NY). The standard image quality parameters evaluated for the purpose of comparison were as follows: slice thickness accuracy, high-contrast resolution (limiting spatial resolution), low-contrast resolution, uniformity and noise, and CT number accuracy and linearity. Standard head and body protocols were employed throughout most of our measurements and were kept equal between the two scanners. The computed tomography dose index was measured for standard head and body imaging protocols using accepted methods and procedures. For comparison purposes, data for a diagnostic-type, 70-cm-bore scanner (GE HighSpeed CT/i) were extracted from the literature. The results obtained for the 85-cm-bore scanner are compared with the following: (1) manufacturer-provided autoperformance phantom test results (validating its use for routine quality assurance), (2) a similar set of measurements performed on a conventional 70-cm-bore, diagnostic-type CT scanner (UltraZ CT, Marconi Medical Systems, Inc.), and (3) current available data on other diagnostic-type CT scanners (GE HighSpeed CT/i). RESULTS: Head and body doses seem on average to be slightly (1-2 cGy) higher for the 85-cm-bore unit than for conventional 70-cm units. Measured slice thickness was within acceptable criteria, +/-0.5 mm. There does not seem to be any significant difference in high-contrast resolution. Both units render a limiting value of approximately 7-8 lp/cm for slice thickness 8-10 mm. Both units exhibit comparable CT number uniformity, accuracy, and linearity. Noise levels seem to be slightly increased (by approximately 0.05-0.2%) for the large-bore geometry. Low-contrast resolution for both units was comparable (4.5-5.5 mm @ 0.35%). All image quality parameters are well within diagnostic acceptable levels. CONCLUSION: The overall imaging performance and mechanical integrity of the 85-cm-bore scanner are comparable to those of conventional diagnostic scanners that may be employed in a radiation oncology setting.

Computer Simulation↗

PubMed2002

PET-guided three-dimensional treatment planning of intracavitary gynecologic implants.

PURPOSE: Positron emission tomography (PET) provides physiologic information that is not available from computed tomography (CT) or magnetic resonance studies. PET images may allow more accurate delineation of three-dimensional treatment planning target volumes of brachytherapy gynecologic (GYN) implants. This study evaluates the feasibility of using PET as the sole source of target, normal structure, and applicator delineation for intracavitary GYN implant treatment planning. MATERIALS AND METHODS: Standard Fletcher-Suit brachytherapy tandem and colpostat applicators were used for radiation delivery. After insertion of the applicator in the operating room, the patient was taken to a PET scanner, where 555 MBq (15 mCi) 18F-fluorodeoxyglucose (18F-FDG) was administered intravenously. Forty-five minutes later, three localization tubes containing 18F-FDG were inserted into the source afterloading compartments of the tandem and colpostat. A whole-pelvis scan was performed, and the images were transferred to a commercial brachytherapy three-dimensional treatment planning system. A Foley catheter was inserted into the urinary bladder while the patient was in the operating room. The regions of radioactivity in the three applicator tube image were contoured for reconstruction of the applicator, along with the bladder, rectum, and 18F-FDG-defined target volumes. A treatment plan was generated that included dose-volume histograms and three-dimensional dose distribution displays, allowing the physician an opportunity to determine if adequate target coverage and normal-tissue sparing had been obtained. For a more conservative approach, three-dimensional dose distributions and dose-volume histograms delivered with conventional source arrangements and loading could be observed. The accuracy of applicator localization from the PET images was verified using a water phantom containing two aluminum CT-compatible tandems. The PET-defined and CT scan applicator reconstructions were compared. RESULTS: Feasibility of using PET images for treatment planning of brachytherapy intracavitary GYN implants has been demonstrated. A phantom study demonstrated applicator reconstruction accuracy in the axial direction to be better than 2 mm. Reconstruction accuracy in the longitudinal direction (principally craniocaudal) was similar to the PET scanner's voxel size of 4.3 mm. CONCLUSIONS: Brachytherapy intracavitary GYN implant design has traditionally been based on patient tumor staging, palpation, and clinical experience. PET images have the potential to provide better spatial information about the relationship of tumor and normal structures to the applicator. This information can be used to optimize the delivery of radiation therapy treatments. Thus far, six patients have been scanned using this process.

Brachytherapy↗

PubMed2002

Dosimetry of therapeutic photon beams using an extended dose range film.

For intensity modulated radiation therapy (IMRT) dose distribution verification, multidimensional measurements are required to quantify the steep dose-gradient regions. High resolution, two-dimensional dose distributions can be measured using radiographic film. However, the photon energy response of film is known to be a function of depth, field size, and photon beam energy, potentially reducing the accuracy of dose distribution measurements. The dosimetric properties of the recently developed Kodak EDR2 film were investigated and compared to those of Kodak XV film. The dose responses of both film types to 6 MV and 18 MV photon beams were investigated for depths of 5 cm, 10 cm, and 15 cm and field sizes of 4x4 cm2 and 15x15 cm2. This analysis involved the determination of sensitometric curves for XV and EDR2 films, the determination of dose profiles from exposed XV and EDR2 films, and comparison of the film-generated dose profiles to ionization chamber measurements. For the combinations of photon beam energy, depth, and field size investigated here, our results indicate that the sensitometric curves are nearly independent of field size and depth of calibration. For a field size of 4x4 cm2, a single sensitometric curve for either EDR2 and XV film can be used for the determination of relative dose profiles. For the larger field size, the sensitometric curve for EDR2 film is superior to XV film in regions where the dose falls below 20% of the central axis dose, due to the effects that the increased low energy scattered photon contributions have on film response. The limited field size and depth dependence of sensitometric data measured using EDR2 film, along with the inherently wide linear dose-response range of EDR2 film, makes it better suited to the verification of IMRT dose distributions.

Calibration↗