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P J Mountford

Publications and source records attributed to P J Mountford.

At least 19 recordsLinked to original sources
PubMed2006

Effect of electron contamination of a 6 MV x-ray beam on near surface diode dosimetry.

In critical organ in vivo x-ray dosimetry, the relative contaminating electron contribution to the total dose and total detector response outside the field will be different to the corresponding contributions at the central axis detector calibration position, mainly due to the effects of shielding in the linear accelerator head on the electron and x-ray energy spectrum. To investigate these contributions, the electron energy response of a Scanditronix PFD diode was measured using electrons with mean energies from 0.45 to 14.6 MeV, and the Monte Carlo code MCNP-4C was used to calculate the electron energy spectra on the central axis, and at 1 and 10 cm outside the edge of a 4 x 4, 10 x 10 and a 15 x 15 cm(2) 6 MV x-ray field. The electron contribution to the total dose varied from about 8% on the central axis of the smallest field to about 76% at 10 cm outside the edge of the largest field. The electron contribution to the total diode response varied from about 7-8% on the central axis of all three fields to about 58% at 10 cm outside the edge of the smallest field. The results indicated that a near surface x-ray dose measurement with a diode outside the treatment field has to be interpreted with caution and requires knowledge of the relative electron contribution specific to the measurement position and field size.

Calibration↗
PubMed2006

A survey of MRI quality assurance programmes.

There are currently no national guidelines on appropriate quality assurance (QA) test frequencies for MRI equipment in clinical use. From a random selection of 45 hospitals in England, who were contacted by phone, 35 hospitals agreed to participate in a survey of MRI QA and were sent a questionnaire requesting information on the range and frequency of QA tests, as well as the staff groups who conduct these tests. Twenty-four completed replies were received, representing a 68% response rate from the distributed questionnaires. Of these, 79% undertook some form of QA, typically conducted by the radiographic staff. Tests were most often undertaken on the head coil, but there was a considerable variation in the frequency and range of tests undertaken at different hospitals. For example, exactly half of the respondents conducted signal to noise ratio (SNR) tests on both head and body coils, but only 13% of centres extended this test to other coils. Results from this survey should inform radiology departments regarding practice at other hospitals and should assist in formulating the frequency and scope of appropriate MRI QA programmes.

England↗
PubMed2005

The low energy X-ray response of the LiF:Mg:Cu:P thermoluminescent dosemeter: a comparison with LiF:Mg:Ti.

LiF:Mg:Cu:P thermoluminescent dosemeters (TLD) can be used for the same X-ray dosimetry applications as LiF:Mg:Ti, with each type having the disadvantage of a response dependent on energy, particularly at low energies. Measurements were made of the response per unit air kerma of LiF:Mg:Cu:P and LiF:Mg:Ti to nine quasi-monoenergetic X-ray beams with mean energies from 12 keV to 208 keV. Each measurement was normalized to the value produced by 6 MV X-rays. LiF:Mg:Cu:P was found to under-respond to a majority of these radiations whereas LiF:Mg:Ti over-responded to a majority. Their smallest relative measured response was produced by the lowest energy beam, and the maximum measured relative response of 1.15+/-0.07 and 1.21+/-0.07 for LiF:Mg:Cu:P and LiF:Mg:Ti, respectively, occurred at 33 keV. Energy response coefficients were derived from these measurements to estimate the error introduced by using either type of TLD to measure the dose from an X-ray spectrum different to that used for its absolute response calibration. It was calculated that if the response of either type of TLD was calibrated at 100 kVp, then an error of no more than +/-2% would be introduced into measurements of tube output at potentials of 50-130 kVp. LiF:Mg:Cu:P was found to introduce a larger error (up to 30%) into the measurement of body exit dose than LiF:Mg:Ti at tube potentials of 40-150 kVp, if its absolute response was calibrated using the corresponding body entrance beam. The method should allow this type of error to be estimated in other dosimetry applications for either type of TLD.

Copper↗
PubMed2004

Near surface photon energy spectra outside a 6 MV field edge.

The purpose of this study was to investigate the difference between a 6 MV linear accelerator x-ray energy spectrum outside the field edge near a phantom surface, and the corresponding spectrum on the central axis. The Monte Carlo code MCNP-4A was used to calculate the spectra on the central axis and at 1, 2, 5 and 10 cm from the edge of a 4 x 4 cm2, 10 x 10 cm2 and 15 x 15 cm2 field. Compared to the spectrum on the central axis, the spectra outside the field edge showed two distinct regions: a broad peak below about 0.5 MeV, and a lower amplitude, less rapidly changing region at higher energies from 0.5 to 6 MeV. The lower energy peak was due to scattered photons, and the higher energy component was due mainly to primary photons transmitted through the jaws of the secondary collimator. The potential impact of these spectral differences on critical organ photon dosimetry was determined by calculating the ratio of the sensitivity of a Scanditronix EDD-5 diode and of a LiF:Mg:Ti thermoluminescent dosimeter (TLD) outside the field edge to their respective sensitivity at the calibration position on the central axis. The lower energy peak combined with the non-uniform energy sensitivity of each detector produced up to a two-thirds overestimate of x-ray dose outside the field by the diode, whereas the response ratio of the TLD was about unity. These results indicated that a similar evaluation was required for profile measurements of a dynamic wedged field and measurements in an intensity modulated beam with either type of detector.

Algorithms↗
PubMed2004

Patient dose optimization in plain radiography based on standard exposure factors.

A computational technique for assessing patient dose in plain radiography is described allowing a large number of examinations to be assessed and enabling dose optimization to be promoted. Entrance surface dose (ESD) was calculated for more than 1500 standard exposure settings in an initial dose assessment. Validation of the technique showed good agreement with thermoluminescent dosimetry and showed broad agreement between the standard exposures and the exposure settings used in practice. The dose assessment was repeated 18 months later using the same techniques for almost 2000 standard exposure settings. In both cases, calculated doses showed good compliance with national diagnostic reference levels where available. Suggested investigation levels were established and set at twice the mean dose for each of 47 examinations. Radiology departments were encouraged to review and optimize doses exceeding these levels. The computed mean ESD in the review study was less than the corresponding value in the initial study in 37 of the 47 examinations. The dose reduction was attributable partly to equipment replacement, but primarily to optimization of exposure settings. The technique employed here provides a valid and cost effective method of complying with statutory requirements for the assessment of representative patient dose and is useful in assisting the ongoing process of dose optimization.

Humans↗
PubMed2004

Patient radiation doses during invasive cardiac procedures categorised by clinical code.

Patient radiation doses delivered during invasive fluoroscopic cardiology procedures at the University Hospital of North Staffordshire during a 3 year period from November 1999 to August 2002, and comprising 6189 patient records, have been analysed. Cases have been stratified using classification codes from the Office of Population Census and Surveys (OPCS-4 codes), allowing representative doses to be assessed for 34 distinct types of cardiac radiological procedure. In addition, local guidance levels have been derived for the eight most common procedures. This work represents one of the largest and most detailed published studies of patient radiation dose during cardiac procedures, and should assist in meeting the IR(ME)R regulations requirement for establishment of diagnostic reference levels, and in enabling dose optimization of individual exposures.

Cardiac Surgical Procedures↗
PubMed2001

Performance assessment of the Gulmay D3300 kilovoltage X-ray therapy unit.

A performance assessment was made of the Gulmay D3300 kilovoltage (combined superficial and orthovoltage) X-ray therapy unit. Results are presented for the key dosimetric beam parameters required for routine patient treatment. This unit is relatively new to the UK market and displayed similar properties to other existing equipment. Beam half-value layers were different from comparable published data, but were consistent with the actual values of external tube filtration employed. The applicator, system interlocks and dose monitor performance were satisfactory and the tube leakage was below the UK recommended maximum (air kerma rate 300 mGy h(-1) at 5 cm from the tube head). The variation of absorbed dose with stand-off distance from the applicator base followed the inverse-square law for all tested combinations of beam tube potential (kVp) and applicator, and the measured focus-to-surface distances were in acceptable agreement with the nominal values. A significant beam profile asymmetry was seen for field sizes greater than 10 cm at the upper tube potential (kVp) range (maximum ionization quotient 1.08), but this was an inherent property of the X-ray tube. The difficulties of obtaining percentage depth dose measurements are discussed, and it was concluded that the use of published data (appropriately verified) was acceptable. The methodology followed could form the basis of an acceptance and commissioning protocol. To address the relative lack of agreed standards for this type of equipment, performance test tolerances are proposed that are recommended for new installations.

Equipment Design↗
PubMed1999

Radiation exposure of the families of outpatients treated with radioiodine (iodine-131) for hyperthyroidism.

Patients who receive radioiodine (iodine-131) treatment for hyperthyroidism (195-800 MBq) emit radiation and represent a potential hazard to other individuals. Critical groups amongst the public are fellow travellers on the patient's journey home from hospital and members of the patient's family, particularly young children. The dose which members of the public are allowed to receive as a result of a patient's treatment has been reduced in Europe following recently revised recommendations from ICRP. The annual public dose limit is 1 mSv, though adult members of the patient's family are allowed to receive higher doses, with the proviso that a limit of 5 mSv should not be exceeded over 5 years. Unless the doses received during out-patient administration of radioiodine can be demonstrated to comply with these new limits, hospitalisation of patients will be necessary. The radiation doses received by family members (35 adults and 87 children) of patients treated with radioiodine at five UK hospitals were measured using thermoluminescent dosimeters mounted in wrist bands. Families were given advice (according to current practice) from their treatment centre about limiting close contact with the patient for a period of time after treatment. Doses measured over 3-6 weeks were adjusted to give an estimate of values which might have been expected if the dosimeters had been worn indefinitely. Thirty-five passengers accompanying patients home after treatment also recorded the dose received during the journey using electronic (digital) personal dosimeters. For the "adjusted" doses to infinity, 97% of adults complied with a 5-mSv dose limit (range:0.2-5.8 mSv) and 89% of children with a 1-mSv limit (range: 0.2-7.2 mSv). However 6 of 17 children aged 3 years or less had an adjusted dose which exceeded this 1 mSv limit. The dose received by adults during travel was small in comparison with the total dose received. The median travel dose was 0.03 mSv for 1 h travel (range: 2 microSv-0.52 mSv for 1 h of travel time). These data suggest that hyperthyroid patients can continue to be treated with radioiodine on an out-patient basis, if given appropriate radiation protection advice. However, particular consideration needs to be given to children aged 3 years or younger. Admission to hospital is not warranted on radiation protection grounds.

Adolescent↗
PubMed1999

Exposure of critical groups to nuclear medicine patients.

When a radiopharmaceutical has been administered to a patient, assessment of the risk to critical groups from emitted photon radiation is by measurement of the integral dose received by an individual, or by measurements of the dose rate external to the patient coupled with appropriate occupancy factors. Estimations have been made from the available data of the dose to critical groups exposed to patients who have undergone diagnostic or therapeutic procedures. These dose estimations can be used to assess the impact of the proposed changes in statutory requirements, and to allow appropriate recommendations to be formulated. Two areas for consideration are that pregnant staff exposed to nuclear medicine patients will require an abdominal surface dose limit lower than 2 mSv to restrict their foetal dose to 1 mSv, and the current UK restrictions for the behaviour of patients who have undergone 131I treatment are either already adequate or can even be relaxed in order to restrict the exposure of members of the public to the proposed lower dose limits. Agreement is needed on the value (e.g. 95th percentile) from a study of the dose to a number of individuals which should serve as the basis for radiation protection recommendations.

Breast Feeding↗
PubMed1999

Effective dose to children and adolescents from radiopharmaceuticals.

Published values of tissue weighting factors for adolescents and children derived from the life-span study of the atomic bomb survivors have been used to calculate the effective dose to patients aged 1, 5, 10 and 15 years undergoing a common paediatric procedure requiring one of the following radiopharmaceuticals: 99Tcm-mercaptoacetyltriglycine (MAG3), 99Tcm-diethylenetriaminepentaacetic acid (DTPA), 99Tcm-dimercaptosuccinic acid (DMSA), 99Tcm-pertechnetate, 99Tcm-iminodiacetic acid (IDA) derivatives, 99Tcm-hexamethylpropyleneamineoxine (HMPAO), 99Tcm-labelled leukocytes, 99Tcm-labelled erythrocytes, 99Tcm-phosphates, 99Tcm-methyloxyisobutylisonitrile (MIBI), 201Tl-chloride, sodium 123I-iodide, 123I-metaiodobenzylguanidine (MIBG) and 67Ga-citrate. Administered activities for each age group were based on ARSAC maximum usual values for adult patients and scaling factors listed by the European Association of Nuclear Medicine for different body weights. These effective doses were compared to values derived from ICRP whole-population tissue weighting factors and found to differ by -33% to +71% of these values, and by less than +/- 20% for two-thirds of the procedures. Because these differences were considerably less than the uncertainties in the estimates of organ absorbed dose, we conclude that these published age-specific tissue weighting factors should not be used for the estimation of effective dose to children and adolescents following the administration of radiopharmaceuticals, and that whole-population factors should continue to be used for these estimations.

Adolescent↗
PubMed1999

The X-ray and electron benchmarking of the Monte Carlo codes MCNP-4A and 4B on different computers.

MCNP (Monte Carlo N-Particle) is a Monte Carlo transport code which has been of widespread use in modelling the dosimetry of ionizing radiations. The most recent version (4B) features improved electron transport compared with the previous version 4A. The processing time required by a number of computing systems to carry out X-ray and electron transport calculations using both versions of the code was compared. Version 4A was installed onto a Dec Alpha Server 8200, a personal computer (Pentium 90 MHz), and a Sun Sparc20, 10, 4 and 1+. MCNP-4B was also installed onto the Sun Sparc20. The benchmark tests consisted of determining the transmission of 2 MeV X-rays and 30 MeV electrons through lead. It was found that the Dec Alpha Server 8200 was the fastest computing platform, and the Sun Sparc1+ was the slowest for both tests. The difference in computational speed between different platforms was not matched by the corresponding differences in price. The time required by version 4B to complete the X-ray and electron benchmark tests was found to be 1.4 and 2.3 times greater than version 4A, respectively, without any difference in the results of the calculation for each type of radiation. This suggests that in cases where computing time is important, it may be preferable to use version 4A instead of 4B.

Benchmarking↗
PubMed1997

The response of a MOSFET, p-type semiconductor and LiF TLD to quasi-monoenergetic x-rays.

A metal oxide semiconductor field effect transistor (MOSFET), p-type semiconductor and a TLD can all be used for x-ray dosimetry, with each system having the common disadvantage of a response which is dependent upon the incident photon energy, particularly for energies < 1 MeV. A Pantak HF-320 quasi-monoenergetic x-ray unit was used to determine the response of two Thomson and Nielson TN-502RD MOSFETs, a Scanditronix EDP-10 semiconductor (build-up cap 10 mm: tissue equivalence), an EDD-5 semiconductor (build-up cap 4.5 mm: tissue equivalence) and an Lif:Mg:Ti TLD over the energy range 12-208 keV. The sensitivity of each detector was normalized to the value produced by exposure to 6 MV x-rays. The maximum relative sensitivities of the two MOSFET detectors were 4.19 +/- 0.25 and 4.44 +/- 0.26 respectively, occurring at an incident x-ray energy of 33 keV. The maximum relative sensitivity of the Scanditronix EDP-10 of 2.24 +/- 0.13 occurred at 65 keV, and for the EDD-5, it was 7.72 +/- 0.45 at 48 keV. The TLD produced a maximum relative sensitivity of 1.31 +/- 0.09 at 33 keV. Compared with available data based on heteroenergetic x-ray sources, these measurements have identified a more representative response for each detector to low-energy x-rays.

Equipment Design↗
PubMed1997

A survey of current in vivo radiotherapy dosimetry practice.

A questionnaire was sent out to 57 radiotherapy physics departments in the United Kingdom to determine the type of dosemeters used for in vivo measurements inside and outside X-ray treatment fields, and whether any correction is made for energy dependence when the dose to critical organs outside the main beam is estimated. 44 responses were received. 11 centres used a semi-conductor for central axis dosimetry compared with only two centres which used thermoluminescent dosimetry (TLD). 37 centres carried out dosimetry measurements outside the main beam; 25 centres used TLD and 12 centres used a semi-conductor detector. Of the 16 centres measuring the dose at both sites. 11 used a semi-conductor for the central axis measurement, but only four of those 11 changed to TLD for critical organ dosimetry despite the latter's lower variation in energy response. None of the centres stated that they made a correction for the variation in detector energy response when making measurements outside the main beam, indicating a need for a more detailed evaluation of the energy response of these detectors and the energy spectra outside the main beam.

Humans↗