Biomedical subjects

David I Thwaites

Publications and source records attributed to David I Thwaites.

6 recordsLinked to original sources

PubMed2006

Calibration of an x-ray cabinet unit for radiobiology use.

A Faxitron sealed x-ray cabinet, operated at 100 kV, was modified to irradiate monkey testicles, to a uniform, accurately calibrated dose, for work aimed at investigating spermatogenesis in children undergoing radiotherapy. An aluminium filter was added to increase the beam quality and a lead collimating system manufactured to reduce the beam size to between 1 and 4 cm diameter. Percentage depth doses and profiles were analysed and relative in-air outputs measured with a selection of small (0.2 cc, 0.015 cc) ion chambers. The absolute calibration of the unit was carried out in a 10 x 10 cm2 beam with a 0.6 cc chamber. Backscatter factors were based on standard tables, but then modified according to experimental results with thermoluminescent dosimeters (TLD) in a phantom to account for reduced scatter in the irradiation situations. A suitable irradiation set-up was devised for the monkeys, to ensure accuracy of delivered dose to the target volume and minimize the dose to the surrounding healthy tissue. The homogeneity throughout the testes was calculated to be well within +/-5%, using a parallel-opposed irradiation technique. The TLD measured doses to the testes on three monkeys were lower than the calculated doses by 3 to 6%. Following modifications to the standard percentage depth doses to account for changes in scatter conditions, these differences became +/-3%. The uncertainties on both calculated and measured dose were estimated to be approximately +/-3.2% at 1 SD.

Aluminum↗

PubMed2006

Notes on the construction of solid-state detectors.

BACKGROUND AND PURPOSE: Solid-state detectors such as diodes and diamonds are useful radiation detectors, particularly for small field measurements, such as in stereotactic dosimetry or in the small sub-field measurements required for intensity modulated radiotherapy (IMRT) . In all small field measurements, positioning of the detector is critical. Generally it is assumed that the center of the sensitive volume of each solid-state detector is coincident with the geometric center of the outer casing. PATIENTS AND METHODS: X-rays were taken of four detectors (diamond, three types of diode) to determine the physical characteristics of each, particularly the position of the centre of the sensitive volume. RESULTS: The results showed that the position of the centre of the sensitive volume is not always coincident with the geometric centre of a solid-state detector. CONCLUSIONS: The construction of a detector used for precise measurements should be investigated prior to use, by taking high resolution x-ray images.

Diamond↗

PubMed2006

Back to the future: the history and development of the clinical linear accelerator.

The linear accelerator (linac) is the accepted workhorse in radiotherapy in 2006. The first medical linac treated its first patient, in London, in 1953, so the use of these machines in clinical practice has been almost co-existent with the lifetime of Physics in Medicine and Biology. This review is a personal selection of things the authors feel are interesting in the history, particularly the early history, and development of clinical linacs. A brief look into the future is also given. One significant theme throughout is the continuity of ideas, building on previous experience. We hope the review might re-connect younger radiotherapy physicists in particular with some of the history and emphasize the continual need, in any human activity, to remain aware of the past, in order to make best use of past experience when taking decisions in the present.

Animals↗

PubMed2006

Development and commissioning of a Monte Carlo photon beam model for Varian Clinac 2100EX linear accelerator.

Monte Carlo modeling of a linear accelerator is the first and most important step in Monte Carlo dose calculations in radiotherapy. We developed a photon beam model for Varian 2100EX for dose calculation purposes using MCNP4C Monte Carlo code. Results of our modeling were in close agreement with our measurements. The effect of beam width on percentage depth doses and beam profiles was studied. Our results showed that electron beam width could be tuned using large field beam profile at the depth of maximum dose.

Monte Carlo Method↗

PubMed2004

Dosimetric verification of a commercial collapsed cone algorithm in simulated clinical situations.

BACKGROUND AND PURPOSE: This work reports a detailed study carried out in two UK radiotherapy centres of the dosimetric accuracy of the collapsed cone algorithm of a commercial treatment planning system (Helax-TMS) in simulated clinical situations. MATERIALS AND METHODS: Initially the accuracy of the collapsed cone algorithm in homogeneous media is evaluated for an extensive set of simple and complex fields. Water, lung and bone substitute epoxy resin material were then used to assess the algorithm in inhomogeneous media and compare its accuracy with the pencil beam algorithm currently in clinical use. Finally a semi-anatomic phantom and an anthropomorphic phantom were employed to assess the dosimetric accuracy using simulated clinical set ups. Thermoluminescence dosimeter (TLD) measurements were made with the anthropomorphic phantom and ionisation chambers otherwise. Nominal 4, 6 and 15 MV photon beams were studied. RESULTS: For most homogeneous cases agreement between measured and calculated dose is within +/-2% or +/-2 mm. In cases with heterogeneities and simulated clinical situations it is observed that the accuracy is also generally within +/-2% or +/-2 mm. Specific instances where the difference between measured and calculated values exceed this are highlighted. CONCLUSIONS: It can be concluded that in clinical treatment planning situations where lung is present the collapsed cone algorithm should be considered in preference to pencil beam algorithms normally used but that there may still be some discrepancy between calculations and measurement.

Algorithms↗

PubMed2002

Verification of the dose to the isocentre in stereotactic plans.

BACKGROUND AND PURPOSE: The aim of the study was: (a) to develop a simple, reproducible, technique to verify the dose to the isocentre, in a typical stereotactic treatment plan, for collimators from 12.5 to 40 mm in diameter; (b) to investigate a variety of detectors to compare different approaches; and (c) to introduce the technique into a quality assurance programme. MATERIAL AND METHODS: The symmetry, directional response and stability of calibration of a small 0.125 cm(3) ion chamber, a diamond and three types of diode (photon, electron and stereotactic) were tested. Correction factors were calculated to account for directional dependence, where appropriate and calibration factors were obtained to convert each reading to absorbed dose in water. Single arcs and typical four arc treatments were planned on XKnife and the dose to the isocentre verified in phantom with each usable detector. RESULTS: The ion chamber showed no asymmetry, the stereotactic diodes exhibited 4% and the others 1-2%. Maximum directional dependence was 1% for the ion chamber and diamond and 7-20% for the diodes. Correction factors were calculated to account for this. Only the response of the diodes decreased with cumulative dose; the response of the other detectors remained constant. The ion chamber, electron diode and diamond measured the dose in single arcs to within 1.5% of calculation, in the 40 and 12.5 mm collimators. The photon diode was within 3.5 and 2.5% in the largest and smallest collimators, respectively. CONCLUSION: A simple method of verification was developed. The ion chamber, the diamond and the electron diode were found to be the best detectors to verify the dose to the isocentre in a typical multiple arc treatment for collimators between 40 and 12.5 mm in diameter. The technique has been incorporated into a quality assurance programme, using the ion chamber and diamond, on a twice yearly basis.

Humans↗