Search PubMed⌕ Search

Biomedical subjects

P Andreo

Publications and source records attributed to P Andreo.

14 recordsLinked to original sources
PubMed1992

On the calibration of plane-parallel ionization chambers for electron beam dosimetry.

The procedure recommended by different dosimetry protocols for the determination of the absorbed dose to air chamber factor, ND,pp, of plane-parallel chambers, comparing absorbed dose determinations in a high-energy electron beam with a reference cylindrical chamber having a known ND,cyl factor, has been investigated. Attention has been focused on the case that the chamber serving as reference has a solid aluminium central electrode. It has been found that using a wide spread Farmer-type chamber (NE 2571), together with recommendations which specifically take into account central electrode corrections for electron beam dosimetry, kcelpcel = pcel-global(IAEA) = 1.008, yields inconsistent results compared with those obtained from a fully homogeneous ionization chamber; for the NE 2571 chamber, a value kcelpcel = pcel-global(IAEA) congruent to 1.0 has been obtained. Analytical calculations of kmkatt for Farmer-type cylindrical chambers and experimental determinations of the product kmkattkcelpcel in electron beams agree within experimental uncertainties, with no evidence of statistical significance for the commonly used assumption pcel = 1, which yields a 0.8% correction (due to kcel only) for the effect of the NE 2571 aluminium electrode in electron beam dosimetry. The use of a 'NACP-chamber' specific factor (kpp or kmkatt) to obtain ND,pp from NK,pp in NACP plane-parallel chambers has been found unsatisfactory, and direct experimental determinations of ND,pp are recommended instead. It is suggested that Standard Dosimetry Laboratories provide ND,pp calibration factors in 60Co beams.

Calibration↗
PubMed1990

Uncertainties in dosimetric data and beam calibration.

Recent studies indicate that the calibration of therapeutic beams is one of the main sources of uncertainty in the mean absorbed dose to the target volume in radiotherapy. Interaction coefficients and data used through the different steps in the calibration are pointed out as the main contribution to this uncertainty. Procedures used to select dosimetric data, that is, input parameters used in the specification of the quality of the beam, cause another contribution. In this paper the actual status of the data used for the dosimetry of photon and electron beams is introduced first. Uncertainties along the dosimetric chain are analyzed according to the procedure and data used in recent publications. Uncertainties in stopping-power ratios, considered the main contribution, are discussed in detail starting from the basic electron stopping-power data. Overall uncertainties in the presently available set of stopping-power ratios are analyzed. Recent developments in the dosimetry of electron beams, related to the effect of energy and angular spread and electron and photon contamination, are discussed in connection with the procedure to select stopping-power ratios for clinical dosimetry. Uncertainties along the dosimetric chain are evaluated in terms of the present knowledge of error sources.

Calibration↗
PubMed1989

Determination of effective bremsstrahlung spectra and electron contamination for photon dose calculations.

A method is described for determining an effective, depth dose consistent bremsstrahlung spectra for high-energy photon beams using depth dose curves measured in water. A simple, analytical model with three parameters together with the nominal accelerating potential is used to characterise the bremsstrahlung spectra. The model is used to compute weights for depth dose curves from monoenergetic photons. These monoenergetic depth doses, calculated with the convolution method from Monte Carlo generated point spread functions (PSF), are added to yield the pure photon depth dose distribution. The parameters of the analytical spectrum model are determined using an iterative technique to minimise the difference between calculated and measured depth dose curves. The influence from contaminant electrons is determined from the difference between the calculated and the measured depth dose.

Computer Simulation↗
PubMed1987

Recent developments in basic dosimetry.

CCEMRI(I) (1985) has recommended that from January 1st 1986 the Primary Standard Dosimetry Laboratories (PSDLs) should adopt new values for W/e (33.97 J/C), stopping powers for electrons (ICRU Report 37, 1984), g value in air for 60Co (3.2 X 10-3), and energy absorption coefficients [17]. The consistency of the whole dosimetric chain requires the same basic physical data at the users' beam quality and PSDLs, but most of the existing dosimetry protocols are not generally based on such a set of data and in some cases old and new data have been employed together. A review of the basic data included in the dosimetry protocols is presented here, together with a comparison with experimental data. The most recent data include the recommendations of CCEMRI(I) and at the same time, some of the inconsistencies existing in dosimetry protocol have been eliminated. The new set of data is presented in this work. New dosimetry protocols and updated versions of protocols published before 1986 are discussed in terms of their basic data.

Clinical Protocols↗
PubMed1986

Chamber-dependent wall correction factors in dosimetry.

The calculation of the response of ionisation chambers to different photon beam qualities used in radiotherapy requires electron stopping-power data. These data have recently been revised and are used here to derive a consistent set of correction factors for ionisation chambers of different wall and build-up cap composition. Theoretically derived parameters km and katt which relate the exposure calibration of an ionisation chamber to the absorbed dose to the air of the cavity, are compared with the experimentally derived product kmkatt showing generally good agreement but also significant discrepancies for plastic-walled chambers with inner graphite coatings. A table of km values is given for a large number of commercial ionisation chambers. The new stopping-power data are also used to evaluate the wall-dependent correction factor (pwall) that enters into the determination of the absorbed dose to water in photon beams, results being given as a function of the quality of the beam. Our theoretical calculations of pwall are consistent with existing experimental data.

Cobalt Radioisotopes↗
PubMed1986

Stopping power data for high-energy photon beams.

Spencer-Attix stopping power ratios for the dosimetry of high-energy photon beams used in radiation therapy have been calculated using the Monte Carlo method. The stopping power ratios are calculated in a more consistent way than previously and are given as a function of the attenuation properties of the beam. The dependence of the stopping power ratio on the electron contamination of the beam as well as on depth and field size has also been investigated. Results are compared with stopping power ratios recommended in different dosimetry protocols and to experimental results. The agreement with most dosimetry protocols is within about one per cent and with recent experimental data is better than half a per cent.

Evaluation Studies as Topic↗
PubMed1985

Stopping-power ratio for a photon spectrum as a weighted sum of the values for monoenergetic photon beams.

Stopping-power ratios for water to air, Sw,air, have been calculated for 60Co and thin-target bremsstrahlung beams. The Monte Carlo method was used to generate depth-dependent Sw,air values and depth-absorbed-dose distributions in water for monoenergetic photon beams covering the energy range 0.1-50 MeV. An averaging procedure has been developed to derive the stopping-power ratios for a photon spectrum from the monoenergetic data. Values of the Spencer-Attix ratio for delta = 10 keV, SSAw,air (delta = 10 keV), and the Bragg-Gray ratio, SBGw,air, evaluated using the new I values and two different sets of values for the density effect correction, are given at the reference depths recommended in the dosimetry protocols.

Humans↗
PubMedDate not supplied

A study of interface effects in 60Co beams using a thin-walled parallel plate ionization chamber.

A large plane-parallel ionization chamber has been constructed to investigate interface effects in 60Co beam. The designed geometry yields negligible perturbation from the side walls, as opposed to the large effects existing in commercially available plane-parallel chambers. The chamber has been used to investigate interface phenomena in transition zones using a wide range of elements (Z = 4-82) as front- and back-scattering media and a clinically relevant 60Co gamma-ray field size. The effects of varying the chamber height discretely (0.5-11 mm) and increasing the wall thickness (1-9 mg/cm2) have been investigated. The variation of the measured ionization with the experimental setup (air gap between backscatter material and chamber wall, measurements at dmax and at 5-cm depth, varying the material both in front of and behind the chamber, etc.) has also been investigated. The simple geometry of the ion chamber has been found optimum for benchmark studies of Monte Carlo calculations. The ion chamber is suited for investigating experimentally the effects of varying transport parameters used in Monte Carlo simulations. The results presented show that the complex physical mechanisms governing 60Co interface dosimetry still make Monte Carlo condensed-history (macroscopic) techniques uncertain. It has been found that the EGS4 Monte Carlo system, together with the user code DOSRZ V4.0 and the PRESTA algorithm, yields good agreement with experiments for low and medium Z (main interest in dosimetry and radiotherapy), but may underestimate up to 10% the backscatter from high-Z materials even when transport parameters are optimized.

Algorithms↗
PubMedDate not supplied

Dosimetry and quality specification of high energy photon beams.

A number of quality descriptors are defined characterizing the photon attenuation and lepton contamination properties of high energy photon beams for radiation therapy. The dependence of the quality parameters on the design of the clinical beams such as the incident electron energy, target and filter thicknesses, field size and depth in the phantom are analyzed in some detail using analytical and Monte Carlo techniques. It is shown that the mean attenuation coefficient of the beam for a standard field size of 10 cm X 10 cm is related very accurately to the mean stopping power ratio for ionizing chamber dosimetry but also approximately to the equilibrium absorbed dose in the beam for a given photon energy fluence. This means that accurate photon dosimetry can be performed without knowing the acceleration potential, target design or filter thickness for the beam in use. Furthermore, the mechanism behind beam hardening and softening in the phantom are quantitized and suitable quality parameters for the lepton contamination are identified. The latter allow a determination of the lepton contamination for correction of the stopping power ratio near the surface if the contamination is large.

Monte Carlo Method↗
PubMedDate not supplied

Calculation and application of point spread functions for treatment planning with high energy photon beams.

A general dose calculation method for treatment planning with high energy photon beams, based on folding of the total energy released by primary photons per unit mass, the terma, with a fractional mean energy imparted point spread function is described. A set of point spread functions has been calculated with Monte Carlo technique for energies of primary photons between 100 keV and 20 MeV. Dose distributions have been calculated for a 6 MV bean using the method. The results clearly point out the considerably increased precision and flexibility achieved when calculating photon beam dose distributions from first principles using Monte Carlo generated point spread functions. The point spread functions calculated in this work are available on magnetic tape from the authors.

Humans↗