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Biomedical subjects

P R Almond

Publications and source records attributed to P R Almond.

At least 19 recordsLinked to original sources
PubMed1977

Ionization chamber dosimetry for photon and electron beams. Theoretical considerations.

New Clambda-values (Cair,lambda) are proposed which should be applied for ionization chambers with an inner wall of air-equivalent material, air eq. plastic or graphite. The new values are up to approximately 3 per cent lower, and apply for a chamber with an inner wall lining of water-equivalent material.er cent higher than those published in the ICRU Report No. 14 and here named Cwater,lambda as the inner wall is considered water-equivalent. Also two sets of CE-values are proposed, namely Cair,E which is given in ICRU No. 21 and Cwater,E which is approximately 3 p

Cobalt Radioisotopes↗
PubMedDate not supplied

Dosimetry intercomparisons between fast-neutron radiotherapy facilities.

Neutron dosimetry intercomparison visits have been made by physicists from the M. D. Anderson Hospital-Texas A&M University Project to the Naval Research Laboratory, the University of Washington, and the MRC Cyclotron at Hammersmith Hospital. The Naval Research Laboratory and University of Washington physicists have made dosimetry intercomparisons at the Texas A&M Variable-Energy Cyclotron (TAMVEC). The parameters that are usually measured during these visits are tissue kerma in air, tissue dose at depth of dose maximum, relative central-axis depth dose, neutron/gamma ratios in air and in phantom, and photon calibrations of ionization chambers. In addition, beam profiles and dose buildup curves are sometimes measured. Other parameters that are compared are values of W, stopping power ratios, kerma corrections, and calculations that lead to the statement of tumor doses for patients. This paper presents some of the results of the intercomparisons and discusses the implications of the findings.

Fast Neutrons↗
PubMedDate not supplied

Lead shielding thickness for dose reduction of 7- to 28MeV electrons.

The relative percent dose reduction by lead of 7- to 18-MeV electrons with a Siemens betatron and of 19- to 28-MeV electrons with a Sagittaire linear accelerator has been measured with a thin-wall buildup chamber for 6.3 X 6.3- and 10.5 X 10.5-cm field sizes at the chamber position for the normal treatment source-to-skin distance (SSD) of each machine. The thickness of lead necessary to attenuate the open beam by 95-98% was then determined of 7- to 28-MeV electrons. The required thickness of lead to attenuate 95% of the 7- to 28-MeV electron beam ranged from 2.3 to 18 mm for the 6.3 X 6.3-cm field and from 2.4 to 23 mm for the 10.5 X 10.5-cm field, respectively. For 98% attenuation, thicknesses from 2.6 to 25.0 mm for the 6.3 X 6.3-cm field and from 2.8 to 27.5 mm of lead for the 10.5 X 10.5-cm field were necessary.

Electrons↗
PubMedDate not supplied

Computer dosimetry for flattened and wedged fast-neutron beams.

Beam flattening by the use of polyethylene filters has been developed for the 50-MeV d in equilibrium Be fast-neutron therapy beam at the Texas A&M Variable-Energy Cyclotron (TAMVEC) as a result of the need for a more uniform dose distribution at depth within the patient. A computer algorithm has been developed that allows the use of a modified decrement line method to calculate dose distributions; standards decrement line methods do not apply because of off-axis peaking. The dose distributions for measured flattened beams are transformed into distributions that are physically equivalent to an unflattened distribution. In the transformed space, standard decrement line theory yields a distribution for any field size which, by applying the inverse transformation, generates the flattened dose distribution, including the off-axis peaking. A semiempirical model has been constructed that allows the calculation of dose distributions for wedged beams from open-beam data.

Computers↗
PubMedDate not supplied

Fast-neutron dose rate vs energy for the d+Be reaction--a reanalysis.

The differences in the published information concerning tissue kerma in air vs deuteron energy for the d+Be reaction are analyzed in light of some recent measurements. The reason for the discrepancy is determined to be a lack of electron suppression on the Be target in some earlier measurements, and the relation ln(tissue kerma)=ln(1.356 X 10(-4)+2.97lnE is found to fit the measured data over the deuteron energy range 11-50 MeV.

Fast Neutrons↗
PubMedDate not supplied

In vivo measurements of lung corrections for fast-neutron therapy.

Silicon diodes were inserted into the esophagus and bronchus of anesthetized rhesus monkeys in order to measure the corrections to tumor dose resulting from intervening lung tissue during fast-neutron therapy. The derived corrections were applied to tumor doses for patients being treated for cancer of the esophagus on the fast-neutron beam at TAMVEC. In vivo dosimetry performed on these patients using silicon diodes in the esophagus confirmed the accuracy of the lung corrections. The measured dose and calculated dose agreed to within 4% for four different patients. Tha magnitude of the correction is on the order of 16% for the typical esophageal cancer patient. These studies were also done with Cobalt-60 in order to test, against other data, the results obtained with this animal model.

Animals↗
PubMedDate not supplied

Scattered photons as the cause for the observed dmax shift with field size in high-energy photon beams.

Measurements on the Sagittaire linear accelerator and Allis-Chalmers betatron at M. D. Anderson Hospital indicate that the observed dmax shift with field size is due to the presence of Compton-scattered photons in the therapy beam, and not electrons as proposed by others. Separating the primary from other radiation components indicates that the secondary fraction represents a percentage contribution to the overall beam that increases as the collimators are opened. This is consistent with what would be expected from Compton scattering and explains the effective softening of the beam as field size increases.

Radiotherapy Dosage↗
PubMedDate not supplied

Silicon diode detectors used in radiological physics measurements. Part I: Development of an energy compensating shield.

Silicon diode detectors have the advantages of high resolution, large signal, and fast response, but lack the flat energy response of the Farmer ion chamber. A study was undertaken to develop a compensating shield for a diode which would make it suitable for use in the spectrum of energies produced by a high-energy radiation beam at depth in a phantom. The energy response of the unshielded diode was quantitated over a range of energies from 18.5 keV to 8 MeV. Shields of different thickness, density, and design were tested experimentally. A partial shield of high-Z material over a diode with miniaturized contacts produced a probe which duplicated the relative dose measurements of the Farmer chamber with less than 1% variation. Typical central axis depth-dose curves and a beam profile, measured with the chamber and the shielded and unshielded probe, are illustrated.

Cobalt Radioisotopes↗
PubMedDate not supplied

Composition of A-150 tissue-equivalent plastic.

In recent years, the use of tissue-equivalent materials has become quite common in fast-neutron dosimetry, with the A-150 plastic developed by Shonka et al. probably the most popular. Information on this specific plastic is scantily reported in the literature and as a consequence a preponderance of authors unknowingly reference an article by Shonka describing an early version of a tissue substitute plastic but having a different elemental composition than the present A-150 formulation. We have reviewed the results of 21 chemical analyses which have occurred over a time span of four years on a total of 14 samples of A-150 plastic and based on these data and the formulation of the plastic, have arrived at a suggested composition for A-150 tissue-equivalent plastic. The ambiguities of water absorption by nylon, one of the components of the plastic, and the uncertainty this reflects in the composition of the plastic were evaluated.

Fast Neutrons↗
PubMedDate not supplied

Evaluation of the Therac 6 linear accelerator for radiation therapy.

The Therac 6 is new generation of low-energy linear accelerator. It incorporates a PDP-11/05 computer for beam control, treatment-factor input, and beam shutdown in the event of failure of the system. The performance of the unit has not been hindered by computer or software malfunction, and the computer has provided an excellent means for preventive maintenance and repair. Dosimetry parameters are similar to other 6 MV x-ray beams, and comparison to 60Co therapy beams shows that this unit may be more like 60Co units in penumbra and absence of off-axis peaking than other low-energy accelerators.

Cobalt Radioisotopes↗