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P L Roberson

Publications and source records attributed to P L Roberson.

30 records · Page 2Linked to original sources
PubMed1992

Personnel dosimeter angular response properties and the adoption of ICRU Report 39 quantities.

A new set of quantities for applied health physics has been proposed in the International Commission on Radiation Units and Measurements publication 39 (1985), with the recommendation that they be based on the ICRU sphere phantom model. The quantities proposed for individual monitoring, which incorporate specific non-isotropic angular response properties, are designed to provide an estimate of an individual's effective dose equivalent (HE). They are the individual dose equivalent superficial (Hs(d)) and penetrating (Hp(d)). Our study of typical dosimeter-wearing practices indicated that there were no consistent locations on or angular orientations of dosimeters to the wearer's body. This demonstrated the difficulties in the practical implementation of personnel dosimeters having an angular response approximating Hp(d) for a specifically selected point on a body, rather than the traditionally assumed design goal of dosimeters utilizing an isotropic response. It also indicated the importance that dosimeters utilizing a Hp(d) response have an adequate means of mounting the dosimeter on a body to assure the required dosimeter orientation to the body under a wide range of conditions of application. Questions also remain as to how the specified ICRU sphere reference phantom (or an approximation thereof) can be used as a practical testing laboratory phantom [corrected].

Humans↗
PubMed1992

Three-dimensional tumor dosimetry for hepatic yttrium-90-microsphere therapy.

The nonuniformity of dose deposition for hepatic 90Y-microsphere therapy is believed to play an important role in the relative sparing of normal liver tissues. To help study this issue, three-dimensional dose calculations have been performed for the VX2 tumor model in the rabbit treated with hepatic arterial administration of 90Y-glass microspheres (90Y-MS). Colored, nonactivated spheres of similar size to 90Y-MS were injected into the hepatic artery to mimic the treatment deposition of 90Y-MS. Sample blocks of treated liver were serially sectioned (200 microns thickness), fixed and photographed showing the position of the colored microspheres. The microsphere positions were digitized into a three-dimensional treatment planning system, and three-dimensional dose calculations were performed. A 2-mm diameter liver tumor nodule receiving 15 times more microspheres than nearby normal liver resulted in tumor-to-normal-tissue (TNT) calculated dose ratios of 2.6 (average dose) and 1.9 (minimum dose). The nonuniform microsphere distribution resulted in a dose gradient over the nodule with a minimum value which was less than one half the average dose. The relative dose deposition in the vicinity of the tumor nodule does not fully reconcile the known liver tolerance dose derived from uniform irradiation with the large calculated average doses tolerated with this type of therapy.

Animals↗
PubMed1989

Determination of photon conversion factors relating exposure and dose for several extremity phantom designs.

Dosimetric measurements were performed to determine the exposure-to-dose conversion factors (Cx) for simple extremity phantoms suitable for extremity dosimeter performance testing. The phantoms studied represented the forearm or lower leg and the finger. Measurements were performed for solid plastic phantoms and for phantoms containing simulated bone material to determine the effect of backscattered radiations from the simulated bone to the phantom surface. Photon beam energies used for the measurements ranged from 16 keV to 1.25 MeV (average). The Cx factors for the finger phantoms did not vary significantly with phantom composition. The Cx factors in the arm/leg phantoms with the bone simulant material differed significantly from those for the solid plastic phantom over the energy range of 40-100 keV. This effect was attributed to the preferential absorption of the lower energy backscattered photons by the higher atomic number material that was contained in the bone-simulant insert. The position of the bone-simulating material below the surface of the phantom was more important than its size or level of bone equivalency. For calibrations and dosimeter testing, Al was found adequate as a bone-simulating material.

Health Physics↗
PubMed1988

Improving health physics measurements by performance testing.

Testing standards have been written to establish minimum performance requirements for personnel dosimeters, radiation survey instruments and bioassay laboratory measurements. These standards were subjected to experimental evaluation and to a consensus review to ensure that the requirements were practical. Programs have been established, using the personnel dosimeter standards, to improve and control the performance of dosimeter processors. Similar implementation of performance testing of instruments will improve reliability of field measurements. Initiation of a program based on the bioassay standard will improve the accuracy, precision, and detectability levels for measuring radioactivity in occupationally exposed workers. Evaluating the performance of health-physics measurement tools will improve the accuracy and precision of measurement and add to our knowledge of personnel exposures. Performance testing will become a more widely recognized tool for maintaining high-quality programs, and the concept will be applied to other measurements as well.

Biological Assay↗
PubMed1985

Dose to the contralateral breast due to primary breast irradiation.

The radiation dose received by the contralateral breast during primary breast irradiation is of concern because breast tissue is subject to cancer induction from low to moderate doses of radiation. In this paper the dose to the opposite breast has been studied in detail for common breast treatment techniques. Measurements have been made on 16 patients, a water phantom, a polystyrene phantom with cork inserts to simulate lung tissue, and a body-shaped phantom with wax breasts. Thermoluminescent dosimeters (TLD), ion chambers, diodes, and film have been used in the various configurations. The patient measurements have shown that there is a wide variation in the opposite breast dose received by patients, even when all are treated with, for example, tangential fields alone. Addition of more radiation fields, such as supraclavicular/axillary and internal mammary fields, may increase the dose to the opposite breast for a particular patient. Variations in the details of the technique such as what wedges are used, the use of blocks, and the orientation of the field edges are all important to the final dose received by the patient's contralateral breast. With the phantom measurements, it has been possible to determine the contributions to the opposite breast dose from each of the relevant factors. This makes it possible to explain the wide variation in patient dose measurements, and to make some relatively simple recommendations that will allow the reduction of the dose to the opposite breast from several hundred cGy to about 50 cGy for a typical treatment course dose of 5000 cGy.

Breast↗
PubMed1983

Dose to lung in primary breast irradiation.

Using anatomic data derived from computerized tomography (CT) scans of the torso, the volume of lung irradiated during primary breast treatment has been measured for a variety of irradiation techniques. Two-field tangential plans which are angled into lung to treat also the internal mammary nodes have been compared to three-field plans which include a separate internal mammary field (IMF). The volume of lung achieving high dose (greater than 3000 rad) is similar in both techniques when photons only are used. Electron beam treatment of the IMF is successful in lowering the lung dose. Additional treatment plans that angle the IMF parallel to the tangential fields may offer some theoretical advantage.

Breast Neoplasms↗
PubMed1983

Whole-skin electron treatment: patient skin dose distribution.

Low-energy electron irradiation of the whole skin is used to treat skin diseases such as mycosis fungoides. The literature on the related dosimetry concentrates almost exclusively on idealized conditions, such as the "in-air" distribution of radiation and the dose received by body-shaped phantoms. The results of a detailed study of dose to five patients, using measurements from thermoluminescent dosimeters, are reported. The dose to different points on the trunk was fairly uniform, while there were significant deviations from uniformity for the arms, legs, and head. The data show that in-air measurements are of limited value as a measure of the uniformity of the dose received by the patient.

Electrons↗
PubMed1981

The stability of teletherapy-beam symmetry with gantry angle.

The authors describe a rapid means of evaluating the stability of beam symmetry as the gantry angle is varied. Two radiation detectors, a ratio circuit, and an X-Y recorder are used. The teletherapy machine is operated in the rotational mode, and the ionization-current ratio is recorded as a function of the gantry angle. This test can provide an early warning of beam-symmetry instabilities.

Particle Accelerators↗
PubMedDate not supplied

A sensitivity study of micro-TLDs for in vivo dosimetry of radioimmunotherapy.

The sensitivity and precision of teflon-imbedded CaSO4:Dy microthermoluminescent dosimeters (micro-TLDs) were determined. The micro-TLDs were sectioned from miniature TLDs (200 microns x 400 microns x 5 mm) that were fabricated using standard techniques. In order to measure absorbed dose, the miniature TLDs can be implanted directly into tissues (e.g., tumor xenografts) that have received injections of radiolabeled monoclonal antibodies. Micro-TLDs recovered from tissue sections cut with a microtome can be read out to determine local absorbed dose. The precision of dose estimation was quantified for uniformly irradiated 32-, 96-, and 192-microns TLD chips; coefficients of variation ranged from 22% to 41%, depending on chip size. The coefficients of variation were reduced to less than 12% using individual relative sensitivity factors for each micro-TLD. The spatial resolution of the micro-TLDs was studied by placing miniature TLDs across the sharp penumbral region of a linear accelerator x-ray field. TLDs were sectioned into 32-microns chips which were read out to determine the relative absorbed dose. The sharpness of the penumbra was readily quantified by the micro-TLDs.

Animals↗
PubMedDate not supplied

Multicellular dosimetry for beta-emitting radionuclides: autoradiography, thermoluminescent dosimetry and three-dimensional dose calculations.

Inhomogeneities in activity distributions over distances from 10 to 10(4) microns are observed in many tumors treated with radiolabeled antibodies. Resulting nonuniformities in absorbed dose may have consequences for the efficacy of radioimmunotherapy. Activity variations may be directly studied with quantitative autoradiography (ARG). Converting these data to absorbed dose distributions requires additional information about pharmacokinetics, the use of a point source function and consideration of the complete three-dimensional activity distribution, as obtained from sequential autoradiographic slices. Thermoluminescent dosimetry with specially prepared CaSO4:Dy dosimeters implanted into tissue can directly measure absorbed dose in selected regions. The conditions under which thermoluminescent dosimeters (TLD) are used differ markedly from "normal" use conditions in external beam radiotherapy. Therefore special calibration and quality assurance precautions are needed to assure the precision of this technique. Procedures and pitfalls in the use of both techniques in radioimmunotherapy are described.

Autoradiography↗