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Biomedical subjects

J A Purdy

Publications and source records attributed to J A Purdy.

At least 19 recordsLinked to original sources
PubMed1992

Acceleration of soft tissue repair by a thrombin-derived oligopeptide.

Augmentation of thrombin-modulated chemotaxis and mitogenic activity within the early phase of soft tissue repair is now possible. Identification of high-affinity thrombin receptor binding domains within thrombin has enabled the synthesis of a family of peptides which interact with thrombin receptors and enhance in vitro mitogenesis. A single (5.0 micrograms/wound) application of the thrombin receptor-activating peptide (P517-30) significantly increased wound breaking strength from Day 5 (31% over controls) to Day 12. Two models of impaired healing created by radiotherapy (RT) were used to elucidate possible mechanisms of P517-30 action. Although P517-30 did not completely overcome the RT-induced healing impairments, it increased breaking strength under conditions of penetrating whole body RT-induced pancytopenia by 22% and of nonpenetrating surface RT-induced dermal cell damage by 42%. This suggests that P517-30 directly stimulates resident endothelial cells, fibroblasts, or other cells to overcome dermal and circulating monocytic deficits. These results suggest a method to accelerate wound healing with potential clinical applications and emphasize the activity of thrombin as a growth factor.

Amino Acid Sequence↗
PubMed1992

Radiation oncology residents' computer workstation.

We are investigating the feasibility of using the Macintosh computer as a workstation platform for radiation oncology residents because of its ease of use, graphics capability, and low cost. Hypercard was chosen as the programming environment because it easily mixes graphics, text, and control functions in an integrated screen display. Furthermore, it results in a system that can be relatively easily extended and customized by individual users with varying degrees of computer skills. We have developed several software modules in order to test the ability of this environment to support the demands of such a workstation. Modules created thus far include various clinical physics aids and tutorials, treatment planning guides, oncology databases, and others. The software runs on all Macintosh configurations, but calculation speeds are improved when a 68020 or greater processor is used. In general, we have been pleased with the implementation thus far. Graphics display capability is good, but design and entry of graphics have proved labor-intensive. Searching is fast and text is easily entered and manipulated. Finished modules can be customized with minimal computer training, but implementing complex new functions requires familiarity with Hypercard's programming language. New modules, once developed, are easily integrated into the workstation universe, suggesting that cooperative development of the workstation by multiple contributors is realistically achievable.

Computer Graphics↗
PubMed1991

Numerical scoring of treatment plans.

This is a report on numerical scoring techniques developed for the evaluation of treatment plans as part of a four-institution study of the role of 3-D planning in high energy external beam photon therapy. A formal evaluation process was developed in which plans were assessed by a clinician who displayed dose distributions in transverse, sagittal, coronal, and arbitrary oblique planes, viewed dose-volume histograms which summarized dose distributions to target volumes and the normal tissues of interest, and reviewed dose statistics which characterized the volume dose distribution for each plan. In addition, tumor control probabilities were calculated for each biological target volume and normal tissue complication probabilities were calculated for each normal tissue defined in the agreed-upon protocols. To score a plan, the physician assigned a score for each normal tissue to reflect possible complications; for each target volume two separate scores were assigned, one representing the adequacy of tumor coverage, the second the likelihood of a complication. After scoring each target and normal tissue individually, two summary scores were given, one for target coverage, the second reflecting the impact on all normal tissues. Finally, each plan was given an overall rating (which could include a downgrading of the plan if the treatment was judged to be overly complex).

Humans↗
PubMed1991

Three-dimensional treatment planning for lung cancer.

The experience of four institutions involved in a three-dimensional treatment planning contract (NCI) for lung cancer is described. It was found that three-dimensional treatment planning has a significant potential for optimization of treatment plans for radiotherapy of lung cancer both for tumor coverage and sparing of critical normal tissues within the complex anatomy of the human thorax. Evaluation tools, such as dose-volume histograms, and three-dimensional isodose displays, such as multiple plane views, surface dose displays, etc., were found to be extremely valuable in evaluation and comparison of these complex plans. It is anticipated that with further developments in three-dimensional simulation and treatment delivery systems, major progress towards uncomplicated local regional control of lung cancer may be forthcoming.

Humans↗
PubMed1991

Three-dimensional treatment planning considerations for prostate cancer.

Over 300 treatment plans for a total of eight disease sites based on 3-D treatment planning considerations utilizing serial CT delineated target volumes were generated by four institutions as part of an NCI supported contract to both assess the current state-of-the-art capabilities and point directions for future efforts. Two patients with stage C prostate cancer were evaluated with protocol plans which required treatment of the prostate to 70 Gy and the pelvic lymph nodes to 46 Gy. When full 3-D target definition and multiple beam arrangements were employed, all institutions were able to submit plans which scored higher on tumor coverage and had lower normal tissue complication scores compared to traditional plans. The 3-D plans using standard beam arrangements, however, were often rated as highly as the 3-D unconstrained plans due to the multiple beam arrangements already selected to optimize standard plans at most institutions. For this site, heterogeneity corrections, beam energy changes and changes in CT number did not substantially change plan scores.

Aged↗
PubMed1991

Abutment of high energy electron fields.

The problem of positioning multiple electron fields on the skin surface is complicated by the fact that the scattering properties of electrons result in constriction of the higher value isodoses and bulging out of the lower value isodoses. We have studied the dose distributions in the junction region for adjacent electron fields using our 3-dimensional radiation treatment planning system by investigating the effects of field separation for several field size, air gap, and beam energy combinations. Film dosimetry was used to measure specific test cases. Using our 3-dimensional radiation treatment planning system, we are able to display the calculated dose distribution, calculate dose-volume histograms, and compare calculations with the film dosimetry measurements. Results of our study are presented.

Electrons↗
PubMed1990

On-line radiotherapy imaging with an array of fiber-optic image reducers.

In the optical approach for on-line radiotherapy imaging, a large metal sheet-fluorescent screen combination is used to convert the radiation intensity distribution into a visible light image. Data are then captured via a mirror with a camera located out of the beam. Although usable portal images can be acquired, presence of the large mirror renders the system impractical in many treatment geometries. We have overcome this limitation by replacing the mirror with an array of 16 by 16 bundles of plastic fiber-optic image reducers. Each bundle, in turn, is made up of 16 by 16 individual optical fibers. The total of 256 by 256 fibers spans an input area of 40 cm by 40 cm with each individual fiber viewing an area of 1.6 mm by 1.6 mm. Within a height of 12 cm, each fiber is reduced to an area of 0.1 mm by 0.1 mm. The reduced portal image is then turned and "piped" to a final 3.0 cm by 3.0 cm output area. For data acquisition and digitization, the fiber output is directly coupled to the sensor of a TV camera interfaced to a small computer via a 512 by 512 frame grabber. In this initial evaluation, the imaging system has been characterized in terms of its line spread function, noise and resistance to radiation damage. Adequate phantom and patient images are presented.

Fiber Optic Technology↗
PubMed1979

Dosimetric evaluation of a computed tomography treatment system.

Pixel-by-pixel correction for tissue inhomogeneities was performed with a CT-interfaced treatment planning system. The CT number for each pixel was converted to a physical density. Comparison of the resulting data with dose measurements in a RANDO phantom shows that pixel calculation overestimates dose by 5--7% at locations adjoining the lung, where loss of scatter is not taken into account by the algorithm. However, the new system does have some advantages over conventional treatment planning systems.

Humans↗
PubMed1979

Field shaping for electron-beam radiation therapy.

A system for designing electron-beam treatment fields of any shape and size using the low-melting alloy Lipowitz's metal is described. An individualized shield is produced for each patient. The shields are designed to be mounted on any of the six cone sizes provided with the Varian Clinac 18 linear accelerator but should be adaptable to most accelerators which use cones for electron-beam collimation. Materials are reusable upon completion of treatment.

Electrons↗
PubMed1977

Extended field radiation therapy in Hodgkin's disease: analysis of failures.

A retrospective analysis of 116 primary cases of stage I, II and III-A Hodgkin's disease demonstrated 27 failures, fourteen confirmed by biopsy. Twenty of the 27 (74%) were in lymph node areas only and seven had extranodal extensions. The most frequent site of failure was the hilar nodes and contiguous lung. The majority of failures (78%) occurred within 30 months of treatment and the cause of failure determined in 23 (85%), sixteen of which were due to technical errors of irradiation. Analysis of the dosimetry in the mediastinal, hilar nodes and contiguous lung failures revealed three factors which may have contributed to a low dose. These factors are: 1) the equivalent square, 2) off-axis beam diminution, and 3) the anteroposterior dose profile. Combined radiation therapy and chemotherapy was more effective than either modality alone in the treatment of these failures.

Female↗
PubMed1977

Pseudoblocks and portal localization.

A new method of rapid simulation including design, verification and patient marking for shaped fields is described. This system utilizes a "pseudoblock" constructed from the Styrofoam components of the Cerrobend blocking system. The procedure is precise; errors in block aperture or patient positioning can readily be seen and corrected before the block is cast. In addition, the use of the pseudoblock does not significantly increase the time the patient spends in the simulator on his initial visit.

Humans↗
PubMed1975

"Measurements of surface dose using build-up curves" obtained with an extrapolation chamber.

Surface dose was measured in an extrapolation chamber and was found to be less than previously reported. The slope of the build-up curve was less steep with the Clinac 4 than with 60-Co and much steeper with the Clinac 35 at 25 MV than with the betatron at the same energy. The gradient of the depth-dose curve in the build-up region is probably the best indicator of skin damage.

Cobalt Radioisotopes↗
PubMed1975

Gonadal shield.

A secondary gonadal shield for use in the pelvic irradiation of males was designed and built using material and apparatus available with the Cerrobend blocking system. The gonadal dose was reduced to approximately 1.5 to 2.5% of the given dose.

Hodgkin Disease↗
PubMedDate not supplied

Build-up region of megavoltage photon radiation sources.

Measurements of surface dose and build-up have been measured for photon radiation sources with maximum energy from 1.2 to 25 MeV. A variable volume ionization chamber was used and the results extrapolated to what would be obtained with a zero volume chamber. The results are found to depend systematically on the plate separation of a parallel plate ionization chamber and an empirical method was derived for correcting measurements made with a fixed volume chamber. The relationship of dose build-up curves with skin reactions in radiation therapy patients is discussed.

Cobalt Radioisotopes↗