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P R Almond

Publications and source records attributed to P R Almond.

At least 19 recordsLinked to original sources
PubMed1999

AAPM's TG-51 protocol for clinical reference dosimetry of high-energy photon and electron beams.

A protocol is prescribed for clinical reference dosimetry of external beam radiation therapy using photon beams with nominal energies between 60Co and 50 MV and electron beams with nominal energies between 4 and 50 MeV. The protocol was written by Task Group 51 (TG-51) of the Radiation Therapy Committee of the American Association of Physicists in Medicine (AAPM) and has been formally approved by the AAPM for clinical use. The protocol uses ion chambers with absorbed-dose-to-water calibration factors, N(60Co)D,w which are traceable to national primary standards, and the equation D(Q)w = MkQN(60Co)D,w where Q is the beam quality of the clinical beam, D(Q)w is the absorbed dose to water at the point of measurement of the ion chamber placed under reference conditions, M is the fully corrected ion chamber reading, and kQ is the quality conversion factor which converts the calibration factor for a 60Co beam to that for a beam of quality Q. Values of kQ are presented as a function of Q for many ion chambers. The value of M is given by M = PionP(TP)PelecPpolMraw, where Mraw is the raw, uncorrected ion chamber reading and Pion corrects for ion recombination, P(TP) for temperature and pressure variations, Pelec for inaccuracy of the electrometer if calibrated separately, and Ppol for chamber polarity effects. Beam quality, Q, is specified (i) for photon beams, by %dd(10)x, the photon component of the percentage depth dose at 10 cm depth for a field size of 10x10 cm2 on the surface of a phantom at an SSD of 100 cm and (ii) for electron beams, by R50, the depth at which the absorbed-dose falls to 50% of the maximum dose in a beam with field size > or =10x10 cm2 on the surface of the phantom (> or =20x20 cm2 for R50>8.5 cm) at an SSD of 100 cm. R50 is determined directly from the measured value of I50, the depth at which the ionization falls to 50% of its maximum value. All clinical reference dosimetry is performed in a water phantom. The reference depth for calibration purposes is 10 cm for photon beams and 0.6R50-0.1 cm for electron beams. For photon beams clinical reference dosimetry is performed in either an SSD or SAD setup with a 10x10 cm2 field size defined on the phantom surface for an SSD setup or at the depth of the detector for an SAD setup. For electron beams clinical reference dosimetry is performed with a field size of > or =10x10 cm2 (> or =20x20 cm2 for R50>8.5 cm) at an SSD between 90 and 110 cm. This protocol represents a major simplification compared to the AAPM's TG-21 protocol in the sense that large tables of stopping-power ratios and mass-energy absorption coefficients are not needed and the user does not need to calculate any theoretical dosimetry factors. Worksheets for various situations are presented along with a list of equipment required.

Biophysical Phenomena↗
PubMed1997

The investigation of 32P wire for catheter-based endovascular irradiation.

The dose distribution from a 32P source has been measured and calculated in order to evaluate its application in endovascular irradiation. The source dimension was 27 mm in length and 0.3 mm in diameter and was embedded in the end of a Ni-Ti wire. Dose measurements were performed using radiochromic film in several specially designed tissue equivalent phantoms. Loevinger's point dose kernel was used for the calculation. The approximate dose rate at a radial distance of 1.5 mm from the center of the source was found to be 6.75 cGy/s per GBq (0.25 cGy/s per mCi), which allows the delivery of a therapeutic dose in a short time interval with a satisfactory homogeneity without stepping the source. However, the dose rate falls off almost exponentially along the radial distance. Therefore it may not be suitable for treating large diameter vessel from a centrally located source. The effect of a curved 32P wire source on the radial dose distribution was also investigated. The results showed that for a maximum bend of 180 degrees the dose rate was increased by as much as 20% along the inner radial distance but decreased by as much as 20% along the outer radial distance compared to the dose along a straight wire. However, for curvatures normally encountered in a clinical situation, the dose rate was changed less than 5%.

Angioplasty, Balloon, Coronary↗
PubMed1996

The dose distribution produced by a 32P source for endovascular irradiation.

PURPOSE: Percutaneous transluminal coronary angioplasty (PTCA) is one of the most common therapies for obstructive coronary artery disease. Unfortunately, subsequent restenosis after percutaneous balloon angioplasty occurs in 30-50% of patients and remains one of the major unsolved problems of contemporary cardiology. The study of endovascular irradiation has been greatly stimulated by the discovery that the process of restenosis may be impaired by irradiation. The objective of this study was to examine a custom-made commercial 32P wire and to determine whether the present source presentation is suitable for this application. METHODS AND MATERIALS: Measurements of the dose distribution around a 3 mm long 32P source with an activity of 0.414 GBq (11.2 mCi) were made by using LiF thermoluminescent dosimeters and a scintillation detector. The source had the dimensions of 0.3 mm in diameter and 3 mm in length, and was first encapsulated by a plastic tube and then encapsulated in a specially manufactured Ni-Ti wire with a diameter of 0.4 mm and a length of 2.6 m. The detector size effect is removed from the measurements calculation. Loevinger's equation for the dose distribution around a 32P source was used for the calculations. RESULTS: The dose rate at a radial distance of 1.5 mm was 53 cGy/s per GBq (1.96 cGy/s per mCi) and fell off rapidly perpendicularly to the axis of the source in an approximately exponential manner, from 53-5.3 cGy/s per GBq (approximately 2 to 0.2 cGy/s per mCi) as radial distances increased from 0.2 to 0.4 g/cm2 (1.5 to 3.5 mm away from the center of the source). The treatment length parallel along the wire could be as long as 24 mm for eight source dwell positions with the average dose rate of 59 cGy/s per GBq (2.2 cGy/s per mCi) and a variation of +/- 2.3% at a radial distance of 1.5 mm. CONCLUSIONS: Our experiments show that the dose distribution is ideal for endovascular irradiation. The source was incorporated in the end of a flexible cable and with a half-life of 14.3 days is suitable for endovascular irradiation.

Brachytherapy↗
PubMed1996

Evaluation of several Dupont portal film systems.

The sensitometric curves (also known as the characteristic curves, or more commonly as the H-D curves after Hurler and Driffield who first used such curves in 1890 to describe the response of photographic film to light) of three DuPont portal films (CRONEX 10T, 10TL and Ultra-Vision C) in combination with two DuPont cassettes (CRONEX Radiation Therapy and Radiation Therapy Verification) were produced utilizing a 60Co beam and a 18 MV beam. The results are compared with that of a Kodak portal film system (T-Mat G/RA film and X-Onratic V cassette). The sensitometric curves of the DuPont films in the Kodak cassette were also measured. The results show that the Kodak system is superior to the DuPont systems with respect to the imaging quality and the feasibility of film techniques, and that if the DuPont films are used, significant improvements can be made by combining the DuPont films with the Kodak cassette.

Cobalt Radioisotopes↗
PubMed1996

Verification of absorbed dose determined with plane-parallel chambers in clinical electron beams following AAPM Task Group 39 protocol using ferrous sulphate dosimetry.

The absorbed dose values determined with the Exradin and PTW-Markus plane-parallel chambers were compared to the values obtained with the ferrous sulphate dosimetry for a number of the Philips SL25 and the Therac 20 electron beams. For the plane-parallel chambers, the cavity-gas calibration factor Ngaspp, was derived by a direct comparison with a calibrated cylindrical chamber using the three different calibration methods as proposed by the newly published AAPM TG 39 protocol. For the ferrous sulphate dosimetry, an epsilon mG value of 352 x 10(-6) m-2 kg-1 Gy-1 was adopted from ICRU Report No. 35. The average ratio of the dose values determined with the plane-parallel chambers and the dose values determined with the Fricke dosimetry system was 1.001 +/- 1.4%. These measurements are consistent with the AAPM TG 39 protocol.

Biophysical Phenomena↗
PubMed1996

Measured electron energy and angular distributions from clinical accelerators.

Electron energy spectra and angular distributions, including angular spreads, were measured using magnetic spectrometer techniques, at isocenter, for two clinical linear accelerators: one scanning beam machine, which achieves field flatness by scanning a pencil beam over the desired field at the patient plane, and one scattering foil machine, which disperses the electrons through a graded-thickness scattering foil. All measurements were made at isocenter (in the patient plane), in air, 1 m from the nominal accelerator source. The energy measurements were confined to electrons traveling along the central axis; any widely scattered electrons were effectively neglected. The energy spectra of the scanning beam machine are all of nearly Gaussian shape and energy full-width-at-half-maximum intensity (FWHM) of about 5% of the peak mean energy (denoted (E0)*). The energy spectra of the scattering foil machine have a variety of forms as a function of energy, including even spectra with double peaks, and spectra which changed with time. The FWHM values ranged from 9%-22% of (E0)*. The angular spread measurements, at isocenter, yielded sigma theta (x) x (E0)* approximately 295 mrad-MeV for the scanning beam machine, and 346 mrad-MeV for the scattering foil machine, where sigma theta x denotes the standard deviation of the plane-projected angular distribution. These angular spreads are 30%-40% smaller than angular spreads reported by others on a very similar machine using the penumbra method. Possible causes of this discrepancy are discussed.

Electrons↗
PubMed1995

The calibration and use of plane-parallel ionization chambers for dosimetry of electron beams.

The AAPM TG 39 protocol has proposed three different methods of calibrating plane-parallel ionization chambers, i.e., in-phantom irradiation with a high-energy electron beam and in-phantom and in-air 60Co irradiation. To verify the consistency of the three methods, we have measured Ngaspp values using each of these techniques for the five most commonly used plane-parallel chambers considered by the protocol. Our results demonstrate that the measured Ngaspp values for the three different methods for any of the chambers agree to within +/- 0.6%. Once Ngaspp was measured, the determination of absorbed dose for electron beams with different energies for an AECL Therac 20 and Philips SL25 was carried out according to the AAPM TG 39 protocol. The results show that the determination of the absorbed dose outputs for any of the five chambers agree to within +/- 0.7% for electron-beam energies of 4-20 MeV if all five chambers had Ngaspp values determined by the electron-beam method. The uncertainties are well within the expected error for these approaches.

Cobalt Radioisotopes↗
PubMed1994

The spectral dependence of electron central-axis depth-dose curves.

Electron linac fields are usually characterized by the central-axis practical range in water, Rp, and the depth of half maximum dose, R50, for dosimetry, quality assurance, and treatment planning. The quantitative relations between the range parameters and the intrinsic linac beam's energy structure are critically reviewed. The spectral quantity * is introduced which is defined as the mean energy of the incident spectral peak, termed the "peak mean energy." An analytical model is constructed to demonstrate the predicted relation between polyenergetic spectral shapes and the resulting depth-dose curves. The model shows that, in the absence of electrons at the patient plane with energies outside about * +/- 0.1 *, Rp and R50 are both determined by *. This analytical approximation is confirmed by a Monte Carlo calculation comparing two different idealized incident spectra. The effect of contaminant lower energy or wide-angle scattered electrons is also discussed. The effect of the width of the intrinsic energy spread on the shape of the depth-dose curve is investigated using Monte Carlo depth-dose simulations based on measured linac energy spectra having energy spreads (full width at half maximum) as large as 20%. These simulations show that the energy spread has only a small effect on the shape of the central-axis depth-dose curve.

Electrons↗
PubMed1994

A simple magnetic spectrometer for radiotherapy electron beams.

A small, lightweight, single-focusing magnetic spectrometer was designed, assembled, and tested for analysis of electron beams from radiotherapy electron linacs. The objective was to develop a low cost, simple device that could be easily replicated in other medical centers, and to demonstrate the practicality of individual electron counting for precise analysis of electron spectra. Two methods of spectroscopy have been developed. One method consists of counting electrons individually as a function of magnetic field setting. Electrons are deflected through 90 degrees in the magnetic spectrometer, through an exit slit, and into a scintillation detector. A second method consists of recording the complete spectrum of electron energies from the accelerator on a strip of film at a single magnetic field setting. A critical design element is the 10-cm long collimator for electrons entering the magnet gap, with defining apertures and scraper slits. The spectrometer's cleanliness of transmission, energy calibration, and resolution were all tested at 10 and 16 MeV using the nearly monoenergetic electron beam of the accelerator at the National Research Council of Canada (NRCC). These accelerator tests, and also Monte Carlo trajectory simulations, both show that contamination of the transmitted spectrum due to scattered or knock-on electrons is negligible. Low-energy characteristics were tested using a 90Sr + 90Y beta-particle source. The energy calibration of the 90 degree spectrometer mode was based on mapping the magnetic field and also electron trajectory computer simulations. That calibration agrees with the NRCC's own calibrated scale to 0.8% for the single-particle counting method and to 1.3% for the film method. The energy resolution was measured to be 2% at 10 MeV, which is adequate for radiotherapy linac measurements. The acceptance half angle is 0.5 degrees or less, depending on the aperture size, which is adequate for electron angular distribution measurements within the forward cone of the electron beam. Used with film, the spectrometer is a simple, accurate, and highly transportable device for measuring radiotherapy electron energy spectra.

Biophysical Phenomena↗
PubMed1994

The calibration and use of plane-parallel ionization chambers for dosimetry of electron beams: an extension of the 1983 AAPM protocol report of AAPM Radiation Therapy Committee Task Group No. 39.

This report is an extension of the 1983 AAPM protocol, popularly known as the TG-21 Protocol. It deals with the calibration of plane-parallel ionization chambers and their use in calibrating therapy electron beams. A hierarchy of methods is presented. The first is to calibrate the plane-parallel chamber in a high energy electron beam against a cylindrical chamber which has an Ncylgas value that has been obtained from a NIST traceable 60Co beam calibration. The second method, which is recommended for implementation by the ADCLs is an in-air calibration against a NIST-traceable calibrated cylindrical chamber in a Cobalt-60 beam to obtain a plane-parallel-chamber calibration factor in terms of exposure or air kerma. The third method places the two chambers in a phantom in a Cobalt-60 beam, and leads to an Nppgas value for the plane-parallel chamber. This report also gives Nppgas/NxAion)pp and Nppgas/(NkAion)pp values for five commonly used commercially available plane-parallel chambers: the Capintec PS-033, the Exradin P-11, the Holt, the NACP and the PTW-Markus. The calculation of these Ngas ratios introduces a Kcomp factor which is also calculated for the five parallel plate chambers. The use of the plane-parallel chambers follows the 1983 AAPM protocol for absorbed dose calibrations of electrons, except that new energy-dependent Prepl values are given for the Capintec PS-033 and PTW-Markus chambers consistent with the consensus of reports in the literature. For all the chambers, however, Prepl is unity for 20 MeV electrons. This report does not address the issue of the use of plane-parallel chambers in calibrating photon beams.

Cobalt Radioisotopes↗
PubMed1993

Tissue compensation and verification of dose uniformity.

The problem of nonuniform dose distribution due to irregular and sloping surfaces is frequently encountered during treatment planning whose ultimate goal is to deliver uniform dose to a specific target volume. In order to overcome this problem, various tissue compensation systems have often been employed. The purpose of the present study was to evaluate the computerized tissue compensation system, Autocomp, manufactured by Nuclear Associates, for its ability to obtain dose uniformity both in phantoms and in clinical use. For the phantom studies, a film was placed below a tissue-equivalent phantom with a 45 degrees sloping surface and exposed to either a compensated or an uncompensated beam of 60Co. The field profiles scanned by a computerized film scanning densitometer showed significant improvement in dose uniformity when the compensator was used. For the clinical study, a left buccogingival sulcus carcinoma treated with 60Co gamma ray was chosen to verify the dose uniformity at a depth over the entire field of irradiation. A comparison of two isodose distributions, obtained with and without tissue compensation, indicated the elimination of the nonuniform dose distribution.

Humans↗
PubMed1993

Imaging of radiation dose for stereotactic radiosurgery.

The distributions of radiation dose for stereotactic radiosurgery, using a modified linear accelerator (Philips SL-25 and SRS-200), have been studied by using three different dosimeters: (1) ferrous-agarose-xylenol orange (FAX) gels, (2) TLD, and (3) thick-emulsion GafChromic dye film. These dosimeters were loaded into a small volume of defect in a phantom head. A regular linac stereotactic radiosurgery treatment was then given to the phantom head for each type of dosimeter. The measured radiation dose and its distributions were found to be in good agreement with those calculated by the treatment planning computer.

Humans↗
PubMed1993

Radioneurosurgery using the LINAC scalpel: technique, indications, and literature review.

Two available commercial units for radiosurgery are the modified linear accelerator (LINAC scalpel) and the gamma knife. Advantages of the LINAC scalpel over the gamma knife are its greater accuracy, the availability of a wide range of collimator sizes that allow for a more homogeneous field of radiation for large lesions, state-of-the-art computer software programs, and lower expense. Radiosurgery does not require an incision, is painless, and can be performed on an outpatient basis. It is ideally suited for the treatment of inaccessible, deep intracranial lesions that are radioresistant to conventional forms of radiotherapy, such as arteriovenous malformations, meningiomas, vestibular schwannomas, selected primary brain tumors, and cerebral metastases. Radiosurgery is an attractive treatment alternative to conventional neurosurgery for several intracranial lesions.

Brain Diseases↗
PubMed1990

Shielding considerations for an operating room based intraoperative electron radiotherapy unit.

The leakage radiation characteristics of a dedicated intraoperative radiotherapy linear accelerator have been measured on a machine designed to minimize the shielding required to allow it to be placed in an operating room suite. The scattering foil design was optimized to produce a flat beam for the field sizes employed while generating minimal bremsstrahlung contamination over the available energy range. More lead shielding was used in the treatment head than is used in conventional accelerators. A small amount of borated polyethylene shielding was also employed since neutron production was present at measurable levels. The room shielding installed in the operating room was demonstrated to be adequate to treat at least 20 patients each month to an average dose of 20 Gy. The worst case exposure was found to be 73% maximum permissible exposure. Administrative control was required for adjoining areas when calibrations and maintenance were performed.

Intraoperative Period↗