Biomedical subjects

P E Metcalfe

Publications and source records attributed to P E Metcalfe.

At least 19 recordsLinked to original sources

PubMed1998

Effects of read-out light sources and ambient light on radiochromic film.

Both read-out light sources and ambient light sources can produce a marked effect on coloration of radiochromic film. Fluorescent, helium neon laser, light emitting diode (LED) and incandescent read-out light sources produce an equivalent dose coloration of 660 cGy h(-1), 4.3 cGy h(-1), 1.7 cGy h(-1) and 2.6 cGy h(-1) respectively. Direct sunlight, fluorescent light and incandescent ambient light produce an equivalent dose coloration of 30 cGy h(-1), 18 cGy h(-1) and 0 cGy h(-1) respectively. Continuously on, fluorescent light sources should not be used for film optical density evaluation and minimal exposure to any light source will increase the accuracy of results.

Calibration↗

PubMed1998

Measurement of off-axis and peripheral skin dose using radiochromic film.

A radiotheraphy skin dose profile can be obtained with radiochromic film. The central axis skin dose relative to Dmax for a 10 x 10 cm2 field size was found to be 22%, 17% and 15.5% for 6 MV, 10 MV and 18 MV photon beams. Peripheral dose increased with increasing field size. At 10 MV the skin dose 2 cm outside the geometric field edge was measured as 6%, 10% and 17% for 10 x 10 cm2, 20 x 20 cm2 and 30 x 30 cm2 field sizes respectively. Off-axis skin dose decreased as distance increased from central axis for fields with Perspex block trays. For a 20 x 20 cm2 field, an approximately 5-8% drop in percentage skin dose was observed from central axis to the beam edge.

Biophysical Phenomena↗

PubMed1997

Skin dose from radiotherapy X-ray beams: the influence of energy.

Skin-sparing properties of megavoltage photon beams are compromised by electron contamination. Higher energy beams do not necessarily produce lower surface and basal cell layer doses due to this electron contamination. For a 5 x 5 cm field size the surface doses for 6 MVp and 18 MVp X-ray beams are 10% and 7% of their respective maxima. However, at a field size of 40 x 40 cm the percentage surface dose is 42% for both 6 MVp and 18 MVp beams. The introduction of beam modifying devices such block trays can further reduce the skin-sparing advantages of high energy photon beams. Using a 10 mm perspex block tray, the surface doses for 6 MVp and 18 MVp beams with a 5 x 5 cm field size are 10% and 8%, respectively. At 40 x 40 cm, surface doses are 61% and 63% for 6 MVp and 18 MVp beams, respectively. This trend is followed at the basal cell layer depth. At a depth of 1 mm, 18 MVp beam doses are always at least 5% smaller than 6 MVp doses for the same depth at all field sizes when normalized to their respective Dmax values. Results have shown that higher energy photon beams produce a negligible reduction of the delivered dose to the basal cell layer (0.1 mm). Only a small increase in skin sparing is seen at the dermal layer (1 mm), which can be negated by the increased exit dose from an opposing field.

Humans↗

PubMed1997

Skin dose reduction by a clinically viable magnetic deflector.

A variable magnetic deflector which attaches onto the treatment head of a linear accelerator has reduced skin dose by as much as 65% for 6MV x-rays. The magnetic deflector is constructed from Neodymium Iron Boron (NdFeB) rare earth magnets. It weighs approximately 15 kg and is designed to easily fit onto the accessory mount of a clinical linear accelerator. All field sizes are attainable up to 35 cm x 35 cm at 100 cm SSD. The gap between the magnetic poles can be adjusted, providing the highest field strength for each field size. Magnetic field strengths up to 0.55 Tesla are attainable. For a 6MV x-ray beam with a 10 mm perspex block tray, surface dose is reduced from 29% to 14% and from 59% to 37% for a 20 cm x 20 cm and 35 cm x 35 cm field size, respectively. Results at varying SSD's have shown at least 10 cm of space must be allowed between the magnets and patient for adequate reduction of skin dose through removal of electron contaminants.

Humans↗

PubMed1997

Conversion of an infrared densitometer for radiochromic film analysis.

By the simple incorporation of a high intensity red LED into a typical infrared film densitometer, radiochromic film can be analysed using existing detectors and scanning software. Results show an accurate dose measurement using radiochromic film and this system compared to conventional detectors for percentage depth dose and penumbral measurements in high and low energy x-ray beams. A small circuit including a red Light Emitting Diode (LED) was positioned inside the film densitometer which does not obscure the infrared source. The red and infrared diodes work independently. For 6 MV x-rays, the 80%/20% penumbral width at 15 mm depth for a 10 x 10 cm field at 100 cm SSD was measured to be 3.5 mm with radiochromic film as compared to 3.3 with corrected diode measurements. Percentage depth doses were measured to within +/- 3% of ionisation chamber data at 6MV and within +/- 2% for 250 kVp x-ray with the film placed parallel to the beam direction in both cases.

Absorptiometry, Photon↗

PubMed1996

Radiochromic film as a radiotherapy surface-dose detector.

Radiochromic film is shown to be a useful surface-dose detector for radiotherapy x-ray beams. Central-axis percentage surface-dose results as measured by Gafchromic film for a 6 MVp x-ray beam produced by a Varian 2100C Linac at 100 cm SSD are 16%, 25%, 35%, 41% for 10, 20, 30 and 40 cm square field sizes, respectively. Using a simple, uniform light source and a CCD camera connected to an image analysis system, quantitative 3D surface doses are accurately attainable in real time as either numerical data, a black-and-white image or a colour-enhanced image.

Biophysical Phenomena↗

PubMed1996

A new radiotherapy surface dose detector:the MOSFET.

Radiotherapy x-ray and electron beam surface doses are accurately measurable by use of a MOS-FET detector system. The MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is approximately 200-microns in diameter and consists of a 0.5-microns Al electrode on top of a 1-microns SiO2 and 300-microns Si substrate. Results for % surface dose were within +/- 2% compared to the Attix chamber and within +/- 3% of TLD extrapolation results for normally incident beams. Detectors were compared using different energies, field size, and beam modifying devices such as block trays and wedges. Percentage surface dose for 10 x 10-cm and 40 x 40-cm field size for 6-MV x rays at 100-cm SSD using the MOSFET were 16% and 42% of maximum, respectively. Factors such as its small size, immediate retrieval of results, high accuracy attainable from low applied doses, and as the MOSFET records its dose history make it a suitable in vivo dosimeter where surface and skin doses need to be determined. This can be achieved within part of the first fraction of dose (i.e., only 10 cGy is required.)

Electrons↗

PubMed1996

Magnetic repulsion of linear accelerator contaminates.

Neodymium Iron Boron (NdFeB) rare earth permanent magnets have unique properties that enable them to fit easily onto the accessory mount of a clinical linear accelerator to partially sweep away electron contamination produced by the treatment head and block trays and thus increase skin sparing. Using such magnets the central axis entrance surface dose has been reduced by 11% for a 20 x 30 cm field size from 32% to 21% of maximum dose by the magnetic device. A reduction of 14% from 32% to 18% was seen for a 20 x 20 cm field size with a 6 mm perspex block tray positioned above the magnet. The magnetic device is light weight and thus clinically usable.

Biophysical Phenomena↗

PubMed1996

6MV x-ray dose in the build up region: empirical model and the incident angle effect.

A simple and fast empirical model has been developed which accurately predicts central axis surface and build up dose for a 6MV radiotherapy x-ray beam. The model is based on fits to experimental data and accounts for open fields, block trays and wedges at normal incidence and at angle. The model separates the beam into components produced by primary photon interactions which have only interacted in the phantom at normal and oblique incidence and head scattered photons/electrons generated in the treatment head. The model quantifies these components for open unwedged fields and then the effect on each component by introducing beam modifying devices/ accessories or changing the angle of incidence is determined. Dose results at oblique incidence for Monte Carlo (electron contamination free) and experimental (electron contamination present) are sufficiently close to imply that the increase in build up dose with beam angle is mainly due to changes in photon interactions within the phantom and only a slight increase with angle is due to changes in the electron contamination. Electron contamination/ head scatter component was found to be measurable by three methods. These being TLD extrapolation in air, ionisation chamber measurements in air and Monte Carlo pure photon methods. These methods produced comparable electron contamination/head scatter dose results at all field sizes.

Biophysical Phenomena↗

PubMed1996

An analytical representation of equivalent square field size in relation to surface dose.

Analytical representation of the build up characteristics of megavoltage photon beams is achievable, producing accurate percentage dose results in the build up region for typical patient treatments. This approach requires conversion of an irregular shaped field to a equivalent square field for mathematical analysis. Surface dose for rectangular fields is expressed by an extension to Stirling's 4A/P formula while irregular shaped fields follow an area integration technique. The equivalent square for a typical irregular treatment field is modelled within +/- 0.5 cm2 for all measured fields.

Biophysical Phenomena↗

PubMed1996

Effect of block trays on skin dose in radiotherapy.

Percentage dose at the surface and at 1 mm depth for megavoltage photon beams are increased through the influence of block trays. This represents a decrease in skin sparing properties for both the epidermal and dermal layers. The increase in percentage dose varies with type and thickness of block tray material. At 6MVp, 20cm x 20cm field size, the percentage surface dose is 26%, 26.5%, 33% and 35% for open, steel honeycomb tray, 6mm perspex and 10 mm perspex block trays respectively. At 1 mm depth these values are 52%, 52.5%, 61%, 60% respectively. A similar effect is seen at higher energies. Results show that care should be taken when selecting an appropriate block tray if skin sparing is of importance.

Humans↗

PubMed1995

Dose characteristics of a new 300kVp orthovoltage machine.

The dose characteristics of a relatively new type of orthovoltage machine which displays different beam qualities compared to other orthovoltage machines has been studied. Surface dose, depth dose and dose profiles have been measured with various ionization chambers as well as diodes and thermoluminescent dosemeters. Profiles produced by the 300kVp x-ray machine's fixed and variable collimators show no significant differences. A slight depth dose build up effect of 2% is seen over the first 1mm with the variable collimator at 250kVp but this is not seen with the fixed collimators. Surface charge is increased to 110%, normalised to 100% at 0.3mm depth in solid water when the front perspex plate of the 10x10cm 50cm FSD fixed collimator is removed. The use of a magnetic field placed directly under the fixed collimator to sweep away any electrons produced has shown that the extra charge and thus dose is caused mainly by electrons produced from the fixed collimator material. The percentage surface charge was reduced from 110% to 102% at 50cm FSD with the magnetic field energised.

Electrons↗

PubMed1994

A comparison of three electron planning algorithms for a 16 MeV electron beam.

PURPOSE: We report results of a comparison of three electron planning algorithms, an Age-Diffusion Pencil beam algorithm and two (2-D) and three dimensional (3-D) Hogstrom pencil beam algorithms, using simple 2 x 2 cm air and hard bone inhomogeneities and a complex anthropomorphic head and neck phantom. METHODS AND MATERIALS: The simple inhomogeneities have variable dimensions outside the plane of calculation to test the effects of out of plane scattering on 2-D algorithms, compared with dose measured by film below the inhomogeneity in the dose fall-off range. Comparisons are also made of a parotid treatment field for 16 MeV electrons, and the dose measured by high sensitivity thermoluminescent dosimeters in the head and neck phantom. RESULTS: Behind the simple inhomogeneities, the electron algorithms are found to underestimate the dose behind the air cavity by up to 40% and overestimated the dose behind bone by up to 30%. In the head phantom, the presence of inhomogeneities also presents problems for the algorithms, with overestimations of dose of up to 20% found behind bone-tissue interfaces, apparently due to shielding by high density bone. Overestimations of up to 17% are also found beside interfaces parallel to the beam. Underestimations of dose of up to 10% are found on the beam-side of interfaces, due to under-prediction of backscattered electrons. All three investigated algorithms underestimate the dose by up to 20% behind extreme surface curvature. One algorithm is found to underestimate the dose in the falloff region while another overestimates the dose around the 90% isodose. CONCLUSION: Clinicians should be aware of the limitations of their planning systems.

Algorithms↗

PubMed1994

Surface doses from combined electron/photon fields in a radiotherapy.

Using mixed modality treatments of photon and electron beams, some skin sparing can be acquired whilst administering a safer dose to crucial structures such as the spine or lung. The combination of 6MV X-rays and 12MeV electron beams in the treatment of breast nodes is a clinical example where such treatments are beneficial. By weighting the photon and electron beams accordingly, the surface dose and dose at depth can be changed whilst not dramatically varying the depth at which the 90% dose level is maintained. In order to accurately predict near surface dose, build up results were obtained using TLD extrapolation, Markus parallel plate and Attix parallel plate ionisation chambers in a solid water phantom. This data was then used to predict surface dose due to different beam weights. Depending on the weightings given to the photon and electron beams, the surface dose and dose at depth varies. For example, when 6MV X-rays and 12MeV electrons are combined the percentage dose at surface and 20cm depth is 46%/23%, 54%/20%, 61%/15% for 60/40, 50/50 and 40/60 X-ray/electron weightings respectively. For these weightings, the depth of the 90% level remained at 30mm. From a clinical point of view this data is important, showing that the 90% level of radiation does not vary in depth significantly provided the ratio of photon/electron weights is kept within a range of 60/40 to 40/60. However by varying the weightings, the ability to control dose to skin in particular to produce the optimum level for both areas whilst still delivering the required tumour dose is obtained.

Electrons↗

PubMed1993

Radiotherapy X-ray beam inhomogeneity corrections: the problem of lateral electronic disequilibrium in lung.

Accurate dose calculations in lung are important to assess lung and tumour dose in various radiotherapy cancer patients. Those patients of particular relevance are Ca lung and Ca Oesophagus patients because large volumes of lung are irradiated to high doses. In this paper, dosimetry results for megavoltage X-ray beams obtained in a lung phantom are compared with dose computations produced by (1) effective path length, (2) equivalent tissue-air ratio, (3) super-position/convolution and (4) Monte Carlo dose calculation methods. The mid-lung dose error at 10 MV for a 5 x 5 cm field is 10.0%, 6.7%, 1.9% and 0.6% respectively. Tests at the lower energy of 6 MV with a field size of 10 x 10 cm show a mid-lung error of only 2.0% for the equivalent tissue air ratio method. At this energy it appears that central axis dose voids are sufficiently small to enable the routine use of the equivalent tissue air ratio method. At the higher energies tested, 10 and 18 MV, this method is accurate. Superposition and Monte Carlo methods are presented which show good agreement with experimental results in a lung phantom even in regions of lateral electron disequilibrium.

Algorithms↗

PubMed1992

Radiotherapy x-ray dose distribution beyond air cavities.

Radiation oncologists are particularly concerned about tumours growing on the surface of air cavities in the head and neck regions, which involve treatment with small x-ray fields. An inhomogeneous dose distribution exists within and beyond the cavity. This is caused by the loss of electron equilibrium and the attenuation of both the primary and scattered photons is altered. The scatter function photon beam models for tissue inhomogeneity, such as the ETAR correction algorithm, currently implemented in commercial treatment planning systems do not predict the dose distribution accurately in many situations where lateral electron equilibrium does not exist. Using a Markus ionization chamber and different solid water slabs to simulate different air cavities, it is found that internal body cavities, depending upon their sizes, experience underdose or overdose on the distal surfaces of the cavities when compared with the results predicted by an ETAR correction method for 6 MV and 18 MV x-ray beams. For an infinitely long air passage of dimensions 2 cm x 2 cm, the error in the ETAR correction for a 6 MV x-ray beam is 4.8%, 0.5% and 1.1% for the field size of 5 cm x 5 cm, 7 cm x 7 cm and 10 cm x 10 cm respectively. The ETAR correction is accurate to within 1.6% for a 6 MV x-ray beam provided that the field size is 5 cm across the cavity and greater than 7 cm along it.

Absorption↗

PubMed1991

Electron contamination in 4 MV and 10 MV radiotherapy x-ray beams.

A thin window parallel-plate ionization chamber was constructed for dose measurement in the build-up region of high energy radiotherapy photon beams. The chamber is an integral part of a perspex block. The entrance window is 12 microns Melinex foil with a thin aluminium surface. Cavity thickness is 1.45 mm. Surface doses for varying field sizes were found to increase almost linearly with the side length of a square field. The surface dose for a 10x10 cm 4 MV photon beam is 12.1% for an open field and this increases to 14.1% with a polycarbonate block tray in the beam. Similarly for a 10 MV photon beam the surface dose is 10.6% for an open field and this increases to 12.4% with a polycarbonate block tray. The difference between the dose for an open field and a field with a polycarbonate block tray inserted becomes more significant for larger field sizes. Electron contamination depth dose curves are determined for a 4 MV and 10 MV photon beam. This is achieved by subtracting a pure photon beam build-up curve generated by an EGS4 Monte Carlo simulation from the experimental build-up curve. The EGS4 curve is a theoretical, electron contamination free curve. The electron contamination curve (of the 10 MV photon beam) has depth dose characteristics similar to that of a broad low energy electron beam.

Electrons↗

PubMed1990

Beam hardening of 10 MV radiotherapy x-rays: analysis using a convolution/superposition method.

Total and primary polyenergetic dose spread arrays (PDSA) have been generated for a high energy 10 MV radiotherapy photon beam using the electron gamma shower (EGS) Monte Carlo code. By considering the attenuation of fluence per energy interval, PDSA have been produced at radiological depths of 0 cm (the surface PDSA) and 40 cm (the beam hardened PDSA). By comparing primary PDSA produced at these different depths, the effect of beam hardening on the PDSA has been quantified. Calculations show that the mean electron range due to the surface primary PDSA is 6.67 mm and the mean electron range of the beam hardened primary PDSA is 8.24 mm. In comparison, a 3 MeV primary monoenergetic dose spread array (MDSA) has a much smaller mean electron range of 4.81 mm. A radiotherapy x-ray beam computation method is introduced which involves a single superposition of the surface generated PDSA or beam hardened PDSA with a polyenergetic TERMA. The mean percentage difference between depth-dose curves obtained using super-position of surface and beam hardened PDSA is only 0.1%. The mean percentage difference from experimental data for these superposition curves is 2.8% down to 40 cm in a homogeneous phantom. The superposition process is shown to be forgiving to spectral differences when calculating the PDSA, but sensitive to the incident photon energy spectrum used to calculate the TERMA.

Biophysical Phenomena↗