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Biomedical subjects

R D Zwicker

Publications and source records attributed to R D Zwicker.

36 records · Page 2Linked to original sources
PubMed1995

Performance evaluation of a prototype high resolution digital radiographic/near real-time fluoroscopic computerized tomographic system for radiotherapy simulation.

PURPOSE: A new prototype digital radiographic/near real-time fluoroscopic and computerized tomographic (CT) imaging system has been developed and its performance is evaluated for future implementation in a radiotherapy simulator. METHODS AND MATERIALS: The new imaging modality uses a slow scan cooled charge-coupled device (CCD) camera with 2 k x 2 k x 12 bit image resolution. X-ray images formed by a Gd2O2S:Tb flat screen are directly viewed by the CCD camera via a 45 degrees angled mirror and a high speed objective lens. For CT image reconstruction, digital data from a slit image on the CCD array obtained from time-controlled x-ray scans around the object are transferred to a second tomographic computer for processing and display. RESULTS: In the digital radiographic mode, and for a 43.2 x 43.2 cm field size at the phosphor plane, the spatial resolution of the system is 2.3 +/- 0.1 lp/mm (1 sigma) as measured at the 4% level of the modulation transfer function (MTF), whereas the contrast resolution has a value of 0.5%. In the fluoroscopic mode, the system may be operated at a maximum rate of eight frames/s at a relatively lower spatial and contrast resolution. For CT scans, although the potential intrinsic spatial resolution at the isocenter is 0.35 mm, display-limited spatial resolutions of 1.2 and 1.6 mm were obtained for 30 and 40 cm reconstruction circle diameters, respectively. A contrast resolution of 1% at 0.015 Gy entrance dose was achieved, and CT reconstruction circles of up to 51 cm were attainable. CONCLUSION: The new CCD-based imaging system is capable of delivering high-quality digital radiographic and CT images for radiotherapy simulation, whereas the near real-time fluoroscopic mode yields acceptable flicker-free lower quality images at object speeds typical of simulation motions.

Fluoroscopy↗
PubMed1995

Dose uniformity in a planar interstitial implant system.

PURPOSE: This work makes use of a volume-ratio technique to examine dose uniformity in a planar interstitial implant system based entirely on geometrical constraints. The rationale for determining an upper limit for acceptable dose variation is examined and discussed. Variation of ribbon spacing and interplanar separation is evaluated in terms of its effect on dose homogeneity. METHODS AND MATERIALS: Volume-dose curves were generated for a range of planar implant dimensions. The volume inside the target region and enclosed between the reference isodose and a higher isodose surface was calculated as a measure of dose uniformity. Studies of homogeneity, target coverage, and external tissue irradiation were carried out to evaluate the importance of flexible interplanar spacing in optimizing implants. New dose tables were generated to accommodate the frequent clinical need to minimize the number of catheter insertions. RESULTS: Implants carried out in accordance with specified geometric constraints were found also to provide optimal dose homogeneity as determined using the volume ratio method with a flexible high dose limit. For two-plane implants, the interplanar spacing should be determined specifically in each case to ensure accurate target coverage. Calculations for specific cases showed that the tissue volume treated to unnecessarily high dose levels can be reduced by a large factor by careful positioning of the implant planes. A smaller ribbon and seed spacing will, in general, lead to better dose uniformity when this is evaluated in terms of the volumes treated to very high dose levels. CONCLUSIONS: Our studies showed that implants carried out using simple and useful geometric guidelines will also provide an acceptably uniform dose distribution. For double plane implants, the separation of the implant planes should be optimized for each target thickness.

Brachytherapy↗
PubMed1994

Use of styrene monomer-modified polyester in the fabrication of tissue-equivalent phantoms.

Tissue equivalent materials are often required both for routine quality control and for specialized studies in radiological physics. This work describes techniques and materials for casting of dosimetry devices from a widely available commercial resin suitable for use in a radiotherapy clinic. Methods are described for mixing and curing the material in such a way as to ensure complete reaction of the resin while maintaining a safe exothermic temperature. The clear plastic form of the cured resin is particularly useful for aligning measurement devices in the radiation field. Its specific gravity and narrow beam attenuation characteristics are similar to those of acrylic. The dosimetric characteristics of the material are reported, and its use in the construction of a patient phantom is described.

Humans↗
PubMed1991

Interstitial IR-192 implants of the oral cavity: the planning and construction of volume implants.

The success of radioactive implant therapy for head and neck carcinomas depends critically on careful planning and execution of the implant procedure. In this paper we discuss our experience with oral tongue and floor of mouth implants, and some innovations introduced to facilitate these procedures. Implants were carried out using standard angiocatheters modified with magnetic caps at the open end and terminated with Teflon spacers and lead shot at the closed end. The importance of accurate source placement and careful determination of the target dose rate is discussed with numerical examples. Differential hot-loading of sources is clearly indicated in cases of extension of the lesion to the dorsal or lateral tongue surface. For dorsal surface extension the use of Teflon spacers at the closed ends of the catheters helps to ensure adequate coverage of the target volume with the higher dose region enclosing the demonstrable tumor. All 10 patients implanted with this technique are controlled without recurrence at a median follow-up of 38 months. The two complications observed appeared to be associated with excessive hot-loading of edge plane sources.

Brachytherapy↗
PubMed1985

Planning of Ir-192 seed implants for boost irradiation to the breast.

The conservative management of early stage breast cancer with tumor excision and irradiation of the breast is becoming increasingly accepted as an alternative to modified radical mastectomy. The radiotherapy typically consists of 45 to 50 Gy delivered with external beam irradiation, followed by boost irradiation of 15 to 20 Gy to the tumor bed using electron beams or interstitial implantation. Pathological evaluation of the excised tumor, clinical assessment, and mammography are used to determine the tissue volume potentially containing a residual tumor burden and therefore requiring boost irradiation. In this paper we describe planning and implantation procedures for Quimby-type breast implants using Ir-192 seeds encapsulated in nylon tubing. This system deviates in several important respects from the requirements of the standard brachytherapy systems. For double-plane implants, optimized values of the interplanar spacing are given for a range of implant sizes, along with the corresponding target dose rates for 1.0 mCi seeds. We also describe a modification of the angiocatheter implantation technique, which allows the radioactive sources to be secured in place by a magnetic cap and washer, thus greatly facilitating the removal of the sources at the end of treatment.

Brachytherapy↗
PubMedDate not supplied

UV-induced second readout of LiF thermoluminescent dosimeters in the therapeutic dose range.

A previously developed technique for reading out LiF thermoluminescent dosimeters (TLD) a second and subsequent time by means of ultraviolet (UV)-induced repopulation of dosimetry traps was studied to determine the feasibility of maintaining a second readout capability in a radiotherapy clinical setting. It was found that with grouping of the TLDs according to second readout sensitivity, the UV-induced readout attained a precision comparable to that of the initial readout. However, the UV-induced TL response retained a strong previous dose memory following a standard high-temperature anneal, and routine calibration before each reuse is recommended.

Fluorides↗
PubMedDate not supplied

Thermal energy deposition from a single-loop rf whole-body applicator.

Whole-body hyperthermia at moderate temperatures (40.0 +/- 0.5 degrees C) is currently being investigated to assess its effectiveness as an adjuvant to other cancer therapies. At our institution, the whole-body heating is achieved by a combination of hot air circulation and rf inductive heating at 27.1 MHz. This paper reports the results of a study of the thermal energy deposition pattern associated with the rf applicator. The applicator consists of a single loop of nearly elliptical shape with dimensions 20 x 46 cm. The loop is embedded in a mattress and is fed by lead wires entering the side of the mattress. Temperature mapping in a heated gelatin phantom placed just above the rf loop showed maximum heating near the lead wires and a cool region above the center of the loop. Calculations of the magnetically induced and capacitive contributions to the electric field near the applicator indicate that the heating near the lead wires is largely capacitive in nature. This feature of the rf heating is of interest because capacitive fields result in preferential heating of fatty tissues. Further calculations showed that the capacitive heating contribution falls away much more rapidly than the induced field contribution with increasing distance from the plane of the rf loop.

Diathermy↗
PubMedDate not supplied

Dosimetry characteristics of large wedges for 4- and 6-MV x rays.

Two sets of newly designed large wedge filters for field sizes up to 20 X 20 cm2 have become commercially available for use with 4- and 6-MV linear accelerators. Such field sizes are sometimes required to ensure adequate coverage in certain treatment techniques. This work reports base line data resulting from an investigation of the dosimetric properties of these wedges. Measurements of wedge angles, transmission factors, and beam hardening effects are described, and comparisons are made with the smaller standard wedges.

Humans↗
PubMedDate not supplied

Electron contamination due to Lucite in a 45-MV photon beam.

The presence of the Lucite blocking tray in a 45-MV betatron beam leads to a considerable loss in the skin-sparing properties of this unit. Measurements conducted with a parallel-plate chamber show that the surface dose is approximately doubled (up to 58% of the maximum absorbed dose) and the depth of maximum dose is reduced by as much as 1.5 cm by the Lucite. These results suggest that separate dosimetry data should be kept for blocked and unblocked fields. The penetration of the electron contamination to the open field depth of maximum dose can lead to erroneous results if attenuation measurements are made at that depth. Studies of the effect of the lead sheet and leaded acrylic on the surface dose showed that under normal circumstances the presence of high-Z materials increases the surface dose, hence these materials cannot in general be used as electron filters on very high energy x-ray units. A Luciteless blocking tray with universal triangular corner blocks has been designed for use in special cases requiring a reduced surface dose.

Electrons↗
PubMedDate not supplied

A beam-edge modifier for abutting electron fields.

Abutment of unmodified electron fields to irradiate large areas can lead to significant dose inhomogeneities in the region of junction of the fields. In this paper we describe the design and dosimetric characteristics of a device developed to broaden the electron beam penumbra and thereby to improve the dose uniformity in the overlap region. The device is a high-density triangular-toothed comb capable of reducing the beam intensity without seriously degrading the beam energy. The effect of the comb is such that a single device will generate a beam penumbra which is broad and very nearly linear at all depths for all clinically used beam energies. Results are shown for various field configurations and energies. With a gap of 5.0 cm between the treatment cone and phantom surface the dose "ripple" in the region beneath the teeth was found not to exceed +/- 5% at 0.5-cm depth.

Electrons↗
PubMedDate not supplied

Effective wedge angles for 6-MV wedges.

Measurements were undertaken with 30 degrees and 45 degrees large wedges on a 6-MV linac to determine the effective wedge angle for various combinations of open and wedged fields. The validity of Tatcher's equation, relating effective wedge angle to maximum dose weightings, was examined over a range of field sizes from 5 X 5 to 20 X 20 cm. An alternative equation involving only central axis quantities was also investigated. The results obtained from an analysis of point dose measurements indicate that, for these wedges, either equation yields sufficient accuracy for clinical purposes.

Animals↗
PubMedDate not supplied

Tumor dose specification of I-125 seed implants.

A new calculation procedure for interstitial implants with I-125 seeds is presented. The total activity of I-125 seeds required to deliver a net minimum dose of 1 Gy is also given as a function of implant volume for various seed spacings. This will permit clinicians to prescribe a specific dose for patients as a primary or boost treatment. Elongation and shape corrections are studied. Discussions and comparisons with other dosimetric calculations such as "matched" peripheral dose, average minimum dose, and minimum peripheral dose are made. The results indicate that for a fixed total activity the net minimum dose is relatively insensitive to variations in seed spacing and individual seed activity.

Brachytherapy↗
PubMedDate not supplied

Comparison of dose calibration by ionometric and chemical dosimetry for 6- and 45-MV x-ray beams.

This work reports the results of an experiment carried out to investigate the compatibility of TG-21 ion chamber calibration with Fricke dosimetry near the extremes of the clinically available megavoltage photon energy range. Doses determined from ion chamber measurements and from chemical dosimetry are compared for a 6-MV linear accelerator x-ray beam and 45-MV betatron beam. The Fricke dosimetry was carried out by the National Institute of Standards and Technology. Agreement within 1% between the two techniques was obtained at both photon energies using a currently accepted G-value.

Calibration↗