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At least 631 records · Page 35Linked to original sources
PubMed1978

Pion radiotherapy: studies with nuclear emulsions.

Nuclear emulsions were used to provide information on (1) the pion star distribution for a therapy beam; (2) star production as a function of pion energy and residual range in nuclear emulsion; (3) the distribution of nuclear framgent ranges in emulsion; and (4) the neutron energy spectrum and fluence produced by negative pion capture in tissue, during treatment of a patient. This last item is important for determining the whole-body dose delivered to a patient undergoing pion radiotherapy.

Elementary Particles↗
PubMed1978

Photon absorptiometry of soft tissue and fluid content: the method and its precision and accuracy.

Dichromatic absorptiometry (DA) is a new, non-invasive method for determining soft tissue (STC) and fluid content (FC), and their changes. The technique uses linear transmission scans made across the limbs with a congruent 125I and 241Am dual-photon source and a NaI(TI)--PMT detector. Patient measurements could be made at the bedside with the mobile system. Factors affecting the precision and accuracy were evaluated theoretically and experimentally. Repositioning and subject motion were the greatest sources of variation and limited the precision to 2--3% in normals and 3--7% in patients; instrumental variation was 0.5%. The accuracy was 4% for typical (p10%) fluid changes observed in vivo. The ease, simplicity and small errors of the technique suggest its utility in monitoring the time course and degree of fluid changes in patients.

Americium↗
PubMed1979

Cavity ionisation theory applied to the design of a maximum permissible fluence instrument.

The maximun permissible fluence is proposed as a practical concept in radiological monitoring of photon fields. It is virtually this concept that is used for monitoring neutron fields. The practicability of designing instruments with a required response function over a large energy range is examined. Cavity isonisation theory is employed to examine the effect of gas filling, wall material, chamber size and wall thickness on response function and the optimum parameters for the design of an ionisation chamber are thus identified. An experimental chamber of parameters close to the optimum has been constructed and its response compared with the response calculated from cavity ionisation theory. It is concluded that it is possible to design an instrument, the energy response of which follows the maximum permissible fluence. The approach and data in this paper will facilitate any necessary design changes which would arise in the event of national regulation being modified.

Elementary Particles↗
PubMed1979

Influence of beam characteristics and detector size in negative-pion dose studies.

Dose calculations were performed for a tissue phantom irradiated by uniform circular beams of negative pions with an assumed gaussian momentum distribution. The mean momentum of the pions was varied from 104.4 to 171.5 MeV/c (mean range 5-20 cm in unit-density tissue) and the momentum spread from 0 to 5% of the mean. Depth-dose curves are shown for different mean momenta and momentum spreads. The radial distribution of dose as a function of distance from the beam axis was computed at different depths for a beam with a mean momentum of 153.4 MeV/c and spread of 2%. The responses of detectors of different sizes used to measure centre-line dose for this beam were shown by calculating depth-dose curves for detectors of radii 0.5, 1.0, 1.5, 2.0, 3.0 and 4.0 cm. Calculations were also performed for beams having radii of 1, 2 and 3 cm. Depending on particular conditions, it appears that considerable care may often be needed to infer the absorbed dose at a given location in a phantom irradiated by a negative-pion beam.

Elementary Particles↗
PubMed1979

Theoretical determination of scatter factors at depths in walls of cylindrical chambers.

A theoretical evaluation has been made of the contribution from scattered photons to dose at depths in the walls of cylindrical cavity chambers for photon beams in the energy range 2-30 MV. Scatter factors calculated for graphite, Perspex and water walls over a range of depths and thicknesses show satisfactory agreement with the experimental results of other workers.

Elementary Particles↗
PubMed1979

A modified lead attenuation method to determine the fast neutron sensitivity kU of a photon dosemeter.

Various lead attenuation techniques to determine the fast neutron sensitivity kU of a photon dosemeter are reviewed and a modified method is proposed to determine the kU value of a commercially available Geiger--Müller detector. The dependence on the 'effective' photon energy of the gamma-component of the mixed n--gamma field is illustrated and the most probable photon energy is deduced from measurements outside the radiation field. Using the relevant photon mass attenuation coefficient, measurements carried out with a well-collimated neutron beam at the MRC Cyclotron, Hammersmith Hospital, London, show that the Dg component contributes 2.41 +/- 0.13% to the total radiation field. Using a detector with a PTFE sleeve in the place of a Perspex sleeve, highly consistent results are obtained and the enhancement of the kU value by the Perspex is demonstrated. The neutron sensitivities of two MX 163 GM detectors with Perspex and PTFE sleeves were respectively determined as 0.73 +/- 0.07% and 0.98 +/- 0.14%.

Elementary Particles↗
PubMed1979

Measurement of stopping power ratios for 60 MeV positive or negative pions.

Pion stopping power ratios are essential parameters for pion radiotherapy treatment planning. The validity of scaling proton stopping powers to pions is called into question since the pion mass is intermediate between the electron and proton masses. Direct measurements of stopping power ratios with respect to water were made for 60 MeV pions of both charges in Teflon, Plexiglas, nylon, paraffin, gelatine, tissue-equivalent plastic (Shonka A150), graphite, aluminium, steel and copper. Corrections for multiple scattering and energy dependence of the stopping power are applied. Measured stopping power ratios at an accuracy of 0.6% are in agreement to within the limits of experimental error with stopping power ratios calculated from the Bethe-Bloch equation using elemental I-values and Bragg additivity.

Elementary Particles↗
PubMed1979

Uniform depth dose distribution for biological irradiation using negative pions.

A simple, flexible technique has been developed to generate uniform depth dose profiles for the biomedical pion beam at TRIUMF using dynamic momentum control and linear programming. Either the entrance dose or the irradiation time required for a certain dose over the uniform region can be minimised. The dynamic momentum control can operate automatically under computer control even with a highly unstable beam. Cell survival profiles have been obtained for this uniform dose distribution using the gelatin technique. The RBE increases with increasing depth through the uniform dose region.

Animals↗
PubMed1980

A theoretical study of cavity chamber correction factors for photon beam absorbed dose determination.

Theoretical expressions have been derived for the contributions to ionisation density in cavity chambers from secondary electrons arising from photon absorption in wall, build-up cap and surrounding medium. The method allows the derivation of C lambda and displacement factors, which are evaluated for the Farmer graphite chamber for a range of photon beams. Differences between C lambda values calculated for air-walled and water-walled chambers are found to be consistent with those determined by Nahum and Greening. The currently recommended displacement factor for the Farmer chamber with cap, however, is found to be in error by about 0.8%. On the basis of a comparison between theoretically predicted air doses in cavities and the experimental results of Fregene it is proposed that no further correction to C lambda is required for the chamber 'effective point of measurement'.

Elementary Particles↗
PubMed1980

Thermoluminescent response of LiF and Li2B4O7:Mn to pions.

LiF (TLD-100, TLD-600, and TLD-700) and Li2B4O7:Mn (TLD-800) have been exposed to pi+ and pi- beams over the clinically useful range of 0.05-4 Gy (5-400 rad). Thermoluminescence response curves were determined for the 200 degrees C and 260 degrees C and 260 degrees C glow peaks in LiF and for the 200 degrees C peak in Li2B4O7 when the dosemeters were exposed in the peak and plateau regions of the Bragg curve. TLD-700 appears to be suited for use in pion dosimetry since it is capable of measuring both total and high-LET dose by utilising the 200 and 260 degrees C peaks. TLD-100, TLD-600, and TLD-800 will not be useful because the neutrons emitted from pi- capture result in an enhanced dose to the thermoluminescence dosemeters via (n, alpha) reactions which cannot be extracted from the data. Linearity and fading of each glow peak was also investigated.

Elementary Particles↗
PubMed1980

Depth dose and off-axis characteristics of TLD in therapeutic pion beams.

The thermoluminescent (TL) response of LiF (TLD-100, TLD-600, TLD-700) and Li2B4O7 (TLD-800) has been measured as a function of depth and off-axis position in a therapeutic negative-pion beam in order to evaluate their usefulness in pion radiotherapy. TLD-100, TLD-600, and TLD-800 have been shown to be of little use as in vivo dosemeters because the neutron kerma relative to that in tissue changes grossly with depth. The neutron source comes primarily from pion absorption in the lead-alloy collimator. The 200 degrees C TLD-700 response agrees well with the depth dose spectra, except for small changes due to the varying linear energy transfer (LET) distributions. This variation can be partially accounted for by incorporating the known LET response of LiF. The 260 degrees C peak of TLD-700 has been found to be approximately four times more sensitive than the 200 degrees C peak to high LET dose. Using a simple model of the LET responses, the measured 200 degrees C and 260 degrees C peaks predict total dose within +/- 4% and high LET dose within +/- 50%, therefore indicating TLD-700 to be a good in vivo dosemeter for total dose but only an indicator of high LET dose.

Elementary Particles↗
PubMed1980

Pion dosimetry with thermoluminescent materials.

Various thermoluminescent materials (6LiF, 7LiF, Li2B4O7:Mn, CaSO4:Dy, CaF2:Mn, CaF2-Dy, CaF2:Tm) were irradiated with pi- mesons throughout the depth dose distribution of a beam with central momentum of 176 MeV/c. Doses from 0.05 to 2 Gy were used and linearity and reproducibility were investigated. For LiF, CaF2:Dy and CaF2:Tm, low- and high-temperature peaks were analysed separately. CaF2:Tm was found to be the most suitable for pi- dosimetry since the total dose and the high LET component could be determined using the integral TL signal and the peak height ratio of the 150 degrees C and 240 degrees C peaks. In the dose range used, both peak responses are linear with dose and the sensitivity of the materials is 10 times that of LiF. The reproducibility was +/- 3%. The high temperature peak of 7LiF is too small to be determined accurately and the high temperature peak of CaF2:Dy decreases with increasing LET in the same way was the low temperature peak does, so that no LET information can be extracted.

Calcium Fluoride↗