Biomedical subjects

D A Low

Publications and source records attributed to D A Low.

At least 55 records · Page 3Linked to original sources

PubMed1998

A software tool for the quantitative evaluation of 3D dose calculation algorithms.

Current methods for evaluating modern radiation therapy treatment planning (RTP) systems include the manual superposition of calculated and measured isodose curves and the comparison of a limited number of calculated and measured point doses. Both techniques have significant limitations in providing quantitative evaluations of the large number of dose data generated by modern RTP systems. More sophisticated comparison techniques have been presented in the literature, including dose-difference and distance-to-agreement (DTA) analyses. A software tool has been developed that uses superimposed isodose plots, dose-difference, and DTA distributions to quantify errors in computed dose distributions. Dose-difference and DTA analyses are overly sensitive in regions of high- and low-dose gradient, respectively. The logical union of locations that fail both dose-difference and DTA acceptance criteria, termed the composite evaluation, is calculated and displayed. The composite evaluation provides a method for the physicist to efficiently identify regions that fail both the dose-difference and DTA acceptance criteria. The tool provides a computer platform for the quantitative comparison of calculated and measured dose distributions.

Algorithms↗

PubMed1998

Formation of DNA methylation patterns: nonmethylated GATC sequences in gut and pap operons.

Most of the adenine residues in GATC sequences in the Escherichia coli chromosome are methylated by the enzyme deoxyadenosine methyltransferase (Dam). However, at least 20 GATC sequences remain nonmethylated throughout the cell cycle. Here we examined how the DNA methylation patterns of GATC sequences within the regulatory regions of the pyelonephritis-associated pilus (pap) operon and the glucitol utilization (gut) operon were formed. The results obtained with an in vitro methylation protection assay showed that the addition of the leucine-responsive regulatory protein (Lrp) to pap DNA was sufficient to protect the two GATC sequences in the pap regulatory region, GATC-I and GATC-II, from methylation by Dam. This finding was consistent with previously published data showing that Lrp was essential for methylation protection of these DNA sites in vivo. Methylation protection also occurred at a GATC site (GATC-44. 5) centered 44.5 bp upstream of the transcription start site of the gutABD operon. Two proteins, GutR and the catabolite gene activator protein (CAP), bound to DNA sites overlapping the GATC-44. 5-containing region of the gutABD operon. GutR, an operon-specific repressor, was essential for methylation protection in vivo, and binding of GutR protected GATC-44.5 from methylation in vitro. In contrast, binding of CAP at a site overlapping GATC-44.5 did not protect this site from methylation. Mutational analyses indicated that gutABD gene regulation was not controlled by methylation of GATC-44.5, in contrast to regulation of Pap pilus expression, which is directly controlled by methylation of the pap GATC-I and GATC-II sites.

Bacterial Proteins↗

PubMed1997

Abutment region dosimetry for sequential arc IMRT delivery.

Arc-based intensity modulated radiation therapy (IMRT) planning and delivery is available as a commercial product (Nomos Corp.). The dose distribution is delivered to 1.68 cm thick regions, and the patient moved in a precise manner between treatments. Assuming accurate patient positioning, the abutment region dose distribution near the gantry isocentre is delivered with no undesired dose heterogeneities. However, for regions far from the isocentre, the dose distribution may exhibit high- or low-dose regions due to uncompensated beam divergence for arc treatments of less than 360 degrees gantry angle length. A study has been initiated to characterize abutment region dose distribution heterogeneities for sequential arc IMRT delivery. Five dose distributions were optimized, each using 8 cm diameter target volumes at different distances from the isocentre, and the arc delivery limited to 290 degrees symmetric about the vertical axis. The target lengths were sufficient to require a treatment consisting of five couch positions, yielding four abutment regions. The dose within the abutment regions was measured using film and analysed as a function of off-axis position along both the vertical and horizontal directions. Little dependence on the dose heterogeneity was seen along the horizontal axis passing through the isocentre. However, the abutment regions along the vertical axis contained 15% low and 7% high doses at 7 cm above and below the isocentre respectively. This dose heterogeneity is not predicted by the current clinical release of the treatment planning software due to limitations of the dose calculation algorithm. The intensity of dose heterogeneity is considered sufficient to warrant further study.

Humans↗

PubMed1997

The influence of angular misalignment on fixed-portal intensity modulated radiation therapy.

A method has been developed to estimate potential dose errors due to linear accelerator angular setting misalignments of Intensity Modulated Radiation Therapy (IMRT) treatments. A first-order approximation to the dose error at a point is modeled as the dot product of the dose gradient and the shift vector of the point due to the rotational error. The analysis method is applied to a previously published set of optimized fluences for a 50 MV IMRT pelvis irradiation. Three of the published cases exhibiting a wide range of modulation are presented; a rectangular open field, a field optimized for a static multileaf collimator defining the portal outline coupled with a single broad bremsstrahlung profile modulation, and a fully modulated field using a physical modulator. To examine the energy dependence of angle setting errors, the study is repeated using the same fluence distributions, but with a dose-spread kernel appropriate for a 6 MV photon beam. The collimator angle error is set to 2 degree, and the dose error determined with both a centrally located isocenter and an isocenter chosen to model a split-field geometry. The dose error due to a 2 degree gantry setting error is assessed at a plane 10 cm distal to the isocenter. The mathematical form of the dose error due to couch motion is similar to the other two errors, so the dose error resulting from a couch angle missetting is not presented. The magnitude of the errors is largest for the 6 MV beam, while the volume encompassed by the errors is greater for the 50 MV beam. The gantry error yields the largest dose error values, with the 6 MV modulated case presenting dose errors of greater than 40%.

Humans↗

PubMed1996

An ultrasound system for simultaneous ultrasound hyperthermia and photon beam irradiation.

PURPOSE: An existing ultrasound system has been adapted for simultaneous use with external photon beam irradiation. The system is being used to investigate the potential for increased biological benefit of simultaneously combined hyperthermia and external beam irradiation with currently achievable temperature distributions. METHODS AND MATERIALS: An existing clinical ultrasound system has been modified for simultaneous operation with a 60Co teletherapy machine. The generator, thermometry system, computer, and applicators are located inside the treatment room, while the monitor and system control are located at the control console. Two approaches have been used clinically to combine the two modalities. In the first approach, an en-face setup is used in which the ultrasound beam and the photon beam travel through the same window of entry to the tumor. This is acheived by a reflecting system designed to deflect the ultrasound to the tumor while positioning the ultrasound transducer outside the radiation beam. The reflecting system consists of water and water-equivalent materials except for a 1 mm sheet of polished brass that is used as the reflector. The relative pressure fields were measured in water at the same distance from the ultrasound source using a scanning hydrophone with and without the reflector at the two operating frequencies of the device (1.0 and 3.4 MHz) for two applicators. Radiation dosimetry measurements were performed to determine the relationship between 60Co irradiation through the reflector and absorbed dose. In the second approach the ultrasound and the radiation beam travel into the tumor from different windows of entry such that the radiation beam passes through no portion of the water bolus prior to entering the patient. We have termed this approach the orthogonal approach. For both approaches, the radiation fraction is given in the middle of an uninterrupted 60-min hyperthermia treatment. RESULTS: The system modifications did not impair the ability to effectively deliver ultrasound hyperthermia or 60Co teletherapy. With the en-face approach the ultrasonic patterns generated with and without the reflector demonstrated that the ultrasound system maintained both a uniform and controllable heating pattern. The 60Co beam had no effect on the performance of the thermocouple thermometers. The radiation beam is attenuated nearly uniformly by the reflector system. To date, 10 patients have been treated with the en-face approach and 12 have been treated with the orthogonal approach (90 treatments). CONCLUSIONS: The clinical implementation of ultrasound hyperthermia simultaneous with 60Co irradiation is technically and clinically feasible without any complications or hazards to the patient. The implementation of a reflecting device allows en-face delivery of both the ultrasound and 60Co irradiation. Temperatures obtained during simultaneous treatments are comparable to those historically obtained during sequential treatments with the same commercial ultrasound device.

Calorimetry↗

PubMed1996

An analysis of intratreatment and intertreatment displacements in pelvic radiotherapy using electronic portal imaging.

PURPOSE: To evaluate the relative frequency and magnitude of intratreatment and intertreatment displacements in the patient positioning for pelvic radiotherapy using electronic portal imaging. METHODS AND MATERIALS: Five hundred ninety-four electronic portal images of seven patients treated with a four-field pelvic technique were evaluated. All patients were treated prone without an immobilization device. Two fields were treated per day, from which an average of two electronic portal images were obtained for each field. No treatment was interrupted or adjusted on the basis of these images. Each image was aligned to the corresponding simulation film to measure the displacements in the mediolateral, craniocaudal, and anteroposterior directions relative to the simulated center. The intertreatment displacement was the displacement measured from the initial image for each daily treated field. For each daily treated field the intratreatment displacement was calculated by subtracting the displacement measured on the initial image from the displacement measured on the final image. RESULTS: The frequency of the intertreatment displacements exceeding 10 mm was 3%, 16%, and 23% for the mediolateral, craniocaudal, and anteroposterior translations, respectively. There were no intratreatment displacements exceeding 10mm (p < 0.001). The frequency of intertreatment displacements exceeded 5 mm was 40, 52, and 51% for the mediolateral, craniocaudal, and anteroposterior translations, respectively; whereas, the frequency of intratreatment displacements exceeding 5 mm was 1, 5, and 7% for the same translations, respectively (p < 0.001). The standard deviation of the intertreatment displacements was at least three times as great as the standard deviation of the intratreatment displacements for all translations. These deviations were greater than the precision limit of the measurement technique, which is approximately 1mm. Each patient had one direction where systematic error predominated in intertreatment positioning. Random error predominated for intratreatment positioning and for the other two directions in intertreatment positioning. CONCLUSIONS: During a course of pelvic radiotherapy, the frequency of intertreatment displacements exceeding 5 and 10 mm is significantly greater than the frequency of intratreatment displacements of these magnitudes. Errors in intertreatment positioning are predominantly systematic in one direction for each patient, whereas intratreatment error is predominantly random. Because patients do not move considerably during the daily treatment of a pelvic field, a single electronic portal image per daily field may be considered representative of the treated position.

Adenocarcinoma↗

PubMed1996

Commissioning and periodic quality assurance of a clinical electronic portal imaging device.

PURPOSE: An electronic portal imaging device (EPID) was recently installed on our dual-energy linear accelerator. Commissioning and quality assurance techniques were developed for the EPID. METHODS AND MATERIALS: A commissioning procedure was developed consisting of five parts: (a) physical operation and safety; (b) image acquisition, resolution, and sensitivity calibration; (c) image storage, analysis, and handling; (d) reference image acquisition; and (e) clinical operations. RESULTS: The physical operation and safety tests relate to the motions of the unit, stability of the unit supports, safety interlocks, and interlock overrides. Imager contrast and spatial resolutions are monitored by imaging a contrast-detail phantom. The imager calibration procedure consists of a no-radiation image to compensate for signal offsets, as well as a "flat-field image." The flat-field image is taken with 5.0 cm of homogeneous phantom material placed at isocenter to provide some photon scatter and to approximate the presence of a patient. Daily quality assurance procedures consists of safety tests and the acquisition and inspection of images of the contrast-detail phantom. After 1 year, the frequency of the daily procedure was reduced to weekly. Quarterly QA procedures are conducted by the physicist and consist of the same procedures conducted in the weekly test. The annual QA procedure consists of a duplication of the commissioning procedure. CONCLUSION: The procedures discussed in this article were applied to an ionization-chamber device. They have been useful in identifying difficulties with the EPID operation, including the need for recalibrating and monitoring the accelerator output stability.

Calibration↗

PubMed1996

A quality assurance program for ancillary high technology devices on a dual-energy accelerator.

Our facility has added high-technology ancillary devices to our dual-energy linear accelerator. After commissioning and acceptance testing of dual asymmetric jaws, dynamic wedge, portal imaging, and multileaf collimation (MLC), quality assurance programs were instituted. The programs were designed to be both periodic and patient specific when required. In addition, when dosimetric aspects were affected by these technologies, additional quality assurance checks were added. Positional accuracy checks (light and radiation) are done for both asymmetric jaws and MLC. Each patient MLC field is checked against the original simulation or digitally reconstructed radiographs. Off-axis factors and output checks are performed for asymmetric fields. Dynamic wedge transmission factors and profiles are checked periodically, and a patient diode check is performed for every new dynamic wedge portal. On-line imaging checks encompass safety checks along with periodic measurement of contrast and spatial resolution. The most important quality assurance activity is the annual review of proper operation and procedures for each device. Our programs have been successful in avoiding patient-related errors or device malfunctions. The programs are a team effort involving physicists, maintenance engineers, and therapists.

Engineering↗

PubMed1996

Feasibility study of multileaf collimated electrons with a scattering foil based accelerator.

BACKGROUND AND PURPOSE: There is an ever evolving process to improve the technical aspects of electron beam delivery. Both the foil/applicator and scanning electron beam systems have gone through recent upheavals. Concomitantly, multileaf collimators are now a staple method for collimating photons. We undertook a study of multileaf collimated electron beam (MLCEB) using a dual scattering foil system. MATERIALS AND METHODS: We compared MLCEB with applicator collimated electron beams (AEB) by examining the dosimetric aspects of the two systems using 70 and 80 cm SSDs for the MLCEB, the minimum practical SSDs achievable. Percent depth dose, isodose profiles, output factors, leakage, surface dose, bremstrahlung, effective SSDs, etc. were measured using film and/or ion chamber. Clinical fields, such as posterior neck node (PNN), were compared. We also investigated the use of MLCEB for arc therapy using segments. RESULTS: In all cases, the MLCEB performed inferior, as judged by isodoses, uniformity index (UI) and penumbra analysis. The 80 cm SSD (minimum for PNN), low energy, small fields, was the worst case. For a 6 MeV beam, the UI/penumbra was 0.823/10 mm for the AEB, and 0.561/29 mm for the MLCEB at 80 cm SSD. The PNN multileaf fields exhibited narrow 90% and 80% isodose lines, and wide 20% and 10% lines. CONCLUSIONS: We conclude that multileaf for PNN fields could not be matched to adjacent off-cord photon fields. The "stair-stepping' effect associated with MLC photons was absent for electrons.

Electrons↗

PubMed1996

Verification of milled two-dimensional photon compensating filters using an electronic portal imaging device.

A computer-driven milling machine is being installed at the Mallinckrodt Institute of Radiology to fabricate photon compensating filters for conventional compensation and beam-intensity modulation. Commissioning and quality assurance procedures are being developed for the design, fabrication, and delivery systems prior to using the milled filters in the clinic. The portion of the quality assurance procedures governing the filter fabrication using a computer-driven milling machine includes, in part, a comparison of designed and fabricated filters. Test filters include geometrically regular filters, such as flat surfaces and steps. The verification of these shapes is accomplished using spatial measurements. However, to test the mill's ability to generate complex curved surfaces, filters with more complicated surfaces are generated. These filters do not lend themselves to precise verification using physical measurement of the thickness distribution. A method has been developed to verify the thickness distributions of these complex filters by irradiating the filters with high-energy x rays and comparing the scattered and transmitted fluence to the fluence calculated using the intended filter shape. The fluence measurement is made using a calibrated commercial liquid ionization chamber electronic portal imaging device. The calculated fluence is separated into transmitted primary and scattered fluences and is determined using a convolution of a distributed radiation source kernel with an exponential filter attenuation function. The attenuation coefficient is measured for the filter material (Lipowitz metal) and fit to a second-order polynomial in filter thickness and off-axis distance. The distributed source kernel is measured using a split-field technique and fit to a sum of three two-dimensional Gaussian distributions. The scattered radiation is modeled by the Klein-Nishina cross section. The algorithm is tested by comparing calculated fluences with measured fluences for a series of machined filters: an open field, flat filters of 9.6-, 15.8-, and 31.6-mm thickness, split-field filters, and a pyramid-shaped filter. In each case, the algorithm calculates the fluence to within 3% of the measured values over the entire irradiated field size to within 1.5 cm of the collimated field edges.

Algorithms↗

PubMed1995

Differential binding of Lrp to two sets of pap DNA binding sites mediated by Pap I regulates Pap phase variation in Escherichia coli.

Pyelonephritis-associated pili (Pap) expression in Escherichia coli is subject to a phase variation control mechanism that is regulated by the leucine-responsive regulatory protein (Lrp), PapI, and deoxyadenosine methylase (Dam). In previous work, we found that the differential Dam methylation of two target sites in pap regulatory DNA, GATC-I and GATC II, is essential for the transition between active and inactive pap transcriptional states. Here, we identify six Lrp binding sites within the pap regulatory DNA, each separated by about three helical turns. Lrp binds with highest affinity to three sites (1, 2 and 3) proximal to the papBAp promoter. A mutational analysis indicates that the binding of Lrp to sites 2 and 3 inhibits pap transcription, which is consistent with the fact that Lrp binding site 3 is located between the --35 and --10 RNA polymerase binding region of papBAp. The addition of PapI decreases the affinity of Lrp for sites 1, 2 and 3 and increases its affinity for the distal Lrp binding sites 4 and 5. Mutations within Lrp binding sites 4 and 5 shut off pap transcription, indicating that the binding of Lrp to this pap region activates pap transcription. The pap GATC-I and GATC-II methylation sites are located within Lrp binding sites 5 and 2, respectively, providing a mechanism by which Dam controls Lrp binding and Pap phase variation.

Bacterial Proteins↗

PubMed1995

Clinical implementation of a commercial multileaf collimator: dosimetry, networking, simulation, and quality assurance.

PURPOSE: Clinical implementation of multileaf collimation (MLC) includes commissioning (including leaf calibration), dosimetric measurements (penumbra, transmission, calculation parameters), shaping methods, networking for file transfer, verification simulation, and development of a quality assurance (QA) program. Differences of MLC and alloy shaping in terms of penumbra and stair-step effects must be analyzed. METHODS AND MATERIALS: Leaf positions are calibrated to light field. The resultant decrement line, penumbras, leaf transmission data, and isodoses in various planes were measured with film. Penumbra was measured for straight edges and corners, in various media. Ion chambers were used to measure effects of MLC on output, scatter, and depth dose. We maintain midleaf intersection criteria. MLC fields are set 7 mm beyond planning target volumes. After shaping by vendor software or by our three-dimensional planning system, files are transferred to the MLC workstation by means of sharing software, interface cards, and cabling. A MLC emulator was constructed for simulation. Our QA program includes file checks, monthly checks (leaf position accuracy and interlock tests), and annual review. RESULTS: We found the MLC leaf position (light field) corresponds to decrement lines ranging from 50 to 59%. Transmission through MLC (1.5-2.5%) is less than alloy (3.5%). Multileaf penumbra is slightly wider than for alloy. Relative penumbra did not increase in the lung, and composite field dosimetry exhibited negligible differences compared with alloy. Verification simulations provide diagnostic image quality hard copies of the MLC fields. Monitor unit parameters used for alloy held for MLC. DISCUSSION: Clinical implementation for MLC as a block replacement was conducted on a site-by-site basis. Time studies indicate significant (25%) in-room time reductions. Through imaging and dosimetric analysis, the accuracy of field delivery has increased with MLC. The most significant impact of MLC is the ability to increase the number of daily treatment fields, thereby reducing normal tissue dosing, which is vital for dose escalation.

Calibration↗

PubMed1995

Computer-aided design and fabrication of an electron bolus for treatment of the paraspinal muscles.

PURPOSE: Demonstrate the technology for the design, fabrication, and verification of an electron bolus used in the preoperative irradiation of a mesenchymal chondrosarcoma in the paraspinal muscle region (T8-T12), in which the target volume overlay a portion of the spinal cord, both lungs, and the right kidney. METHODS AND MATERIALS: An electron-bolus design algorithm implemented on a three dimensional (3D) radiotherapy treatment planning system designed the bolus to yield a dose distribution that met physician-specified clinical criteria. Electron doses were calculated using a 3D electron pencil-beam dose algorithm. A computer-driven milling machine fabricated the bolus from modeling wax, machining both the patient surface and the beam surface of the bolus. Verification of the bolus fabrication was achieved by repeating the patient's computed tomography (CT) scan with the fabricated bolus in place (directly on the posterior surface of the prone patient) and then recalculating the patient's dose distribution using the 3D radiotherapy treatment planning system. RESULTS: A treatment plan using a 17-MeV posterior electron field with a bolus delivered a superior dose distribution to the patient than did the same plan without a bolus. The bolus plan delivered a slightly increased dose to the target volume as a result of a slightly broader range of doses. There were significant reductions in dose to critical structures (cord, lungs, and kidney) in the bolus plan, as evidenced by dose-volume histograms (DVHs). The patient dose distribution, calculated using CT scan data with the fabricated bolus, showed no significant differences from the planned dose distribution. CONCLUSIONS: A bolus can provide considerable sparing of normal tissues when using a posterior electron beam to irradiate the paraspinal muscles. Bolus design and fabrication using the tools described in this paper are adequate for patient treatment. CT imaging of the patient with the bolus in place followed by calculation of the patient's dose distribution demonstrated a useful method for verification of the bolus design and fabrication process.

Adolescent↗

PubMed1995

Changes in electron beam dosimetry with a new scattering foil-applicator system on a CL2100C.

PURPOSE: The optimization of clinical electron beams is a challenge to accelerator manufacturers. There are numerous variations and reports of scattering-foil and applicator configurations. The accelerator at our facility was recently updated with new foils and applicators. We conducted many dosimetric tests to critically evaluate dosimetric changes and their clinical effects. METHODS AND MATERIALS: The new dual foil systems are thicker and have shaped disks seated on the lower foils. The 12 MeV beam no longer shares a common foil used for 6 and 9 MeV. The applicators now have denser collimating plates, and Fiberglas no longer connects the plates. The new applicator set includes a rectangular 10 x 6 cm applicator that uses one photon jaw setting for all energies. After the electron beam energies were tuned to previous specifications (energy according to ionization depths, symmetry to +/- 2%, and flatness to +/- 6%), recommissioning took place. Electron beam output checks at various source-to-skin distances (SSD) were conducted for all energies and applicators. Computer-driven water scanning provided percent depth dose, profile, isodose, and Bremsstrahlung data. Surface doses, in-air electron dispersion, effective SSDs, and leakage were also measured. All results compared the previous and updated systems. RESULTS: We found little change in relative percent depth doses for 100 cm SSD between the two systems. The differences in PDD due to increasing SSD, however, decreased with the updated system. Surface doses decreased in most cases, while Bremsstrahlung increased in all cases (typically by a factor of two). Beam uniformity indices increased significantly, while penumbra widths decreased. Diagonal profiles are now quite flat for large fields. For a 20 MeV beam, the 90% width along the diagonal axis for a 25 x 25 applicator at dmax depth has increased from 25 to 32 cm. There was little or no change in 'effective SSD' or in-air dispersion. Leakage outside the applicators was reduced by a factor of two to three. The flatness characteristics of the 10 x 6 cm applicator were poor in comparison to the improved flatness of the new square applicators. CONCLUSIONS: The updated scattering foil-applicator electron beam system has yielded many dosimetric changes. Major improvements have been made in beam flatness and leakage. These positive changes have not been accompanied by any clinically significant dosimetric deficiencies.

Calibration↗

PubMed1995

Dosimetry and clinical implementation of dynamic wedge.

PURPOSE: Wedge-shaped isodoses are desired in a number of clinical situations. Physical wedge filters have provided nominal angled isodoses with dosimetric consequences of beam hardening, increased peripheral dosing, nonidealized gradients at deep depths, along with the practical consequences of filter handling and placement problems. Dynamic wedging uses a combination of a moving jaw and changing dose rate to achieve angled isodoses. The clinical implementation of dynamic wedge and an accompanying quality assurance program are discussed in detail. METHODS AND MATERIALS: The accelerator at our facility has two photon energies (6 MV and 18 MV), currently with dynamic wedge angles of 15 degrees, 30 degrees, 45 degrees, and 60 degrees. The segmented treatment tables (STT) that drive the jaw in concert with a changing dose rate are unique for field sizes ranging from 4.0 cm to 20.0 cm in 0.5 cm steps, resulting in 256 STTs. Transmission wedge factors were measured for each STT with an ion chamber. Isodose profiles were accumulated with film after dose conversion. For treatment-planning purposes, dmax orthogonal dose profiles were measured for open and dynamic fields. Physical filters were assigned empirically via the ratio of open and wedge profiles. RESULTS: A nonlinear relationship with wedge factor and field size was found. The factors were found to be independent of the stationary field setting or second order blocking. Dynamic wedging provided more consistent gradients across the field compared with physical filters. Percent depth doses were found to be closer to open field. The created physical filters provided planned isodoses that closely resembled measured isodoses. Comparative isodose plans show improvement with dynamic wedging. CONCLUSIONS: Dynamic wedging has practical and dosimetric advantages over physical filters. Table collisions with physical filters are alleviated. Treatment planning has been solved with an empirical solution. Dynamic wedge is a positive replacement for physical filters, and a first step for commercial introduction of dynamic conformal therapy.

Humans↗

PubMed1995

The Electronic View Box: a software tool for radiation therapy treatment verification.

PURPOSE: We have developed a software tool for interactively verifying treatment plan implementation. The Electronic View Box (EVB) tool copies the paradigm of current practice but does so electronically. A portal image (online portal image or digitized port film) is displayed side by side with a prescription image (digitized simulator film or digitally reconstructed radiograph). The user can measure distances between features in prescription and portal images and "write" on the display, either to approve the image or to indicate required corrective actions. The EVB tool also provides several features not available in conventional verification practice using a light box. METHODS AND MATERIALS: The EVB tool has been written in ANSI C using the X window system. The tool makes use of the Virtual Machine Platform and Foundation Library specifications of the NCI-sponsored Radiation Therapy Planning Tools Collaborative Working Group for portability into an arbitrary treatment planning system that conforms to these specifications. The present EVB tool is based on an earlier Verification Image Review tool, but with a substantial redesign of the user interface. A graphical user interface prototyping system was used in iteratively refining the tool layout to allow rapid modifications of the interface in response to user comments. RESULTS: Features of the EVB tool include 1) hierarchical selection of digital portal images based on physician name, patient name, and field identifier; 2) side-by-side presentation of prescription and portal images at equal magnification and orientation, and with independent grayscale controls; 3) "trace" facility for outlining anatomical structures; 4) "ruler" facility for measuring distances; 5) zoomed display of corresponding regions in both images; 6) image contrast enhancement; and 7) communication of portal image evaluation results (approval, block modification, repeat image acquisition, etc.). CONCLUSION: The EVB tool facilitates the rapid comparison of prescription and portal images and permits electronic communication of corrections in port shape and positioning.

Hospital Records↗

PubMed1995

Leucine-responsive regulatory protein plays dual roles as both an activator and a repressor of the Escherichia coli pap fimbrial operon.

The expression of the pap pilus operon of Escherichia coli is under a phase-variation control mechanism in which cells undergo a reversible transition between transcriptionally active (phase ON) and inactive (phase OFF) states. In this study, we explore the roles of leucine-responsive regulatory protein (Lrp) and the histone-like protein H-NS in the regulation of pap phase variation. Our data indicate that the phase OFF state results from repression of the intrinsically active papBA promoter by Lrp and H-NS, each of which can act independently as transcriptional repressors. Lrp requires pap DNA sequences upstream of the papBA promoter for its repressor activity whereas H-NS does not. In contrast, in the ON state, Lrp, in conjunction with PapI, activates pap transcription. This activation is not merely a result of alleviating the H-NS mediated repression, but induces a level of transcription that is eightfold higher than the basal level of transcription from the papBA promoter measured in the absence of both H-NS and Lrp. Analysis of Lrp activation mutants indicates that binding of Lrp to pap DNA sequences is not sufficient for transcription activation, consistent with a model in which an additional domain of Lrp interacts with the transcriptional apparatus. Together, our results show that Lrp functions as a transcriptional activator in phase-ON cells and as a repressor of basal transcription in phase-OFF cells. Because pap phase variation occurs in the absence of H-NS, it is not clear what role this regulatory protein plays in pap gene regulation.

Bacterial Outer Membrane Proteins↗

PubMed1995

Minimization of target positioning error in accelerator-based radiosurgery.

The stereotactic radiosurgery system used at the Mallinckrodt Institute of Radiology is patterned after that developed at the Joint Center for Radiation Therapy (Brigham & Women's Hospital, Boston, MA) and uses the Brown-Roberts-Wells computed tomography (CT) stereotactic system. The patient's head is attached to a stand that rotates with the treatment couch. The irradiation is conducted using a set of converging arcs of irradiation. Because of mechanical limitations, no accelerator or treatment couch is capable of placing the center of the radiation beam at precisely the same point for all gantry and couch angles and a compromise must be made when locating the nominal isocenter. The stand settings are checked by placing a radiopaque QA sphere at the desired target location. The QA sphere is imaged using a series of eight films exposed at a set of couch and gantry angles that encompass the treatment angles. The distances between the QA sphere image and the center of the radiation field indicate if the correct coordinates were set on the stand and if the radiation beam converges to a sufficiently small region (< 0.1-cm diameter) for treatment. A mathematical procedure has been developed to use the film-measured position errors to determine a stand offset that will minimize the distance between the accelerator isocenter and the target. The technique is capable of reducing the average placement error, as measured by imaging the QA sphere, to 0.035 cm with a maximum deviation of 0.07 cm.

Humans↗