Alun H. Beddoe

Application of diamond detectors to the dosimetry of 45 and 100 kVp therapy beams: comparison with a parallel-plate ionization chamber and Monte Carlo

Diamond detectors have become an increasingly popular dosimetric method where either high spatial resolution is required or where photon or electron spectra are likely to change with depth or field size. However, little work has been previously reported for superficial energies. This paper reports the response of a commercially available diamond detector (PTW Freiburg/IPTB Dubna) at 45 kVp (0.55 mm Al first HVL) and 100 kVp (2.3 mm Al first HVL) including dose and dose-rate linearity, percentage depth-dose and output factors as a function of applicator size. Comparisons are made with Br J. Radiol. supplement 25 data, measurements using a PTW parallel-plate chamber and Monte Carlo simulations based on spectra determined from transmission measurements in aluminium. Excellent agreement was obtained for percentage depth-dose curves between Monte Carlo and diamond after correcting for sublinearity of the dose-rate response and energy dependence of the diamond detector. However, significant differences were noted between diamond/Monte Carlo and the parallel-plate chamber, which is attributed to the perturbation caused by the polyethylene base of the chamber

Dose responses of diamond detectors to monoenergetic X-rays

The characterisation of a detectors response in the kilovoltage range is necessary to understand its response to scattered radiation in the megavoltage range. Scattered radiation is absorbed in the detector by the highly Z-dependent photoelectric process. Measurements of diamond detector response to highly filtered quasi-monoenergetic X-rays and synchrotron-generated monoenergetic photons have been performed revealing effects that relate to the presence of copper and silver used to form electrical contact with the crystal. A three-component model of energy absorption, utilizing tabulated cross-sections for C, Cu and Ag, is proposed and a calculation of phantom scatter factors for diamond detector is given.

The measurement of thermal neutron flux depression for determining the concentration of boron in blood.

Boron neutron capture therapy (BNCT) is a form of targeted radiotherapy that relies on the uptake of the capture element boron by the volume to be treated. The treatment procedure requires the measurement of boron in the patients blood. The investigation of a simple and inexpensive method for determining the concentration of the capture element 10B in blood is described here. This method, neutron flux depression measurement, involves the determination of the flux depression of thermal neutrons as they pass through a boron-containing sample. It is shown via Monte Carlo calculations and experimental verification that, for a maximum count rate of 1 x 10(4) counts/s measured by the detector, a 10 ppm 10B sample of volume 20 ml can be measured with a statistical precision of 10% in 32 +/- 2 min. For a source activity of less than 1.11 x 10(11) Bq and a maximum count rate of less than 1 x 10(4) counts/s, a 10 ppm 10B sample of volume 20 ml can be measured with a statistical precision of 10% in 58 +/- 3 min. It has also been shown that this technique can be applied to the measurement of the concentration of any element with a high thermal neutron cross section such as 157Gd.

Gamma dosimetry at surfaces of cylindrical containers

Gamma-ray dose rates on the exterior surfaces of cylindrical vessels containing radioactive solutions are calculated using a model based on the distributed point source approximation. A cylinder is subdivided into a number of annular sector segments of equal volume and the dose rate from each segment is combined to give the total dose rate at a point on the exterior surface of the cylindrical container. Calculated results for the method are compared with experimentally determined results for (a pure -emitter) and (a mixed and -emitter) in acrylic containers of various wall thicknesses, as well as for single containers made from polycarbonate and polypropylene; good agreement was obtained. Calculated results for the -ray dose rates to the skin of the fingers, for partially filled plastic syringes, are compared with other published results, for , , , , and in syringes of various diameters and wall thicknesses; good agreement was obtained. The calculations are extended to provide results for the -ray dose rate distribution along the external surfaces of partially filled syringes for and . These results are used to objectively derive guidelines for the safe handling of cylindrical vessels containing -emitting radionuclides, without the use of extra shielding. It was found that the weekly dose limit to the skin, of 10 mSv, is exceeded if the fingers are in contact with the container, over the active volume, for periods greater than about one minute. However, if handled at the rear of the syringe barrel a typical weekly work load can be managed without exceeding dose limits. It is recommended, when using syringes without syringe guards, that the fingers should never approach the active volume closer than the rear end of the syringe barrel, and that syringes should not be filled beyond 75% of their capacity.

Investigation into the relationship between body surface area and total body potassium using Monte Carlo and measurement

The use of body surface area (BSA) as a means of indexing chemotherapy doses is widespread even though the value of this practice is uncertain. In principle, the body cell mass (BCM) more closely represents the bodys metabolic size and this is investigated here as an alternative to BSA; since 98% of body potassium is intracellular the derivation of total body potassium (TBK) via the measurement of 40K in a whole body counter (WBC) will provide a useful normalizing index for metabolic size, potentially avoiding toxicity and underdosing. The Queen Elizabeth Hospital WBC has been used in this study, initially involving single geometrical phantoms and then combinations of these to simulate human body habitus. Monte Carlo N-particle (MCNP) codes were constructed to model the phantoms and simulate the measurements made in the WBC. Efficiency corrections were derived by comparing measurement and modelled data for each detector separately. A method of modelling a person in the WBC as a series of ellipsoids was developed. Twenty-four normal males and 24 females were measured for their 40K emissions. Individual MCNP codes were constructed for each volunteer and the results used in conjunction with the measurements to derive TBK, correcting for body habitus effects and detector efficiencies. An estimate of the component of error arising from sources other than counting statistics was included by analysing data from the measurement of phantoms. The total residual errors (expressed as coefficients of variation) for males and females were 10.1% and 8.5% respectively. The measurement components were determined to be 2.4% and 2.5%, implying that the biological components were 9.8% and 8.1% respectively. These results suggest that the use of BSA for indexing chemotherapy doses is likely to give rise to clinically significant under- or overdosing.

2003 Roberts Prize

The publishers of Physics in Medicine and Biology (PMB) in association with the Institute of Physics and Engineering in Medicine jointly award an annual prize for an article published in PMB during a given year. Members of the Editorial and International Advisory Boards are asked to assess papers in their own fields of expertise. The marks are averaged and the authors with the highest average are awarded the prize. The 2003 Roberts Prize is awarded to: Nagel M, Richter F, Haring-Bolivar P and Kurz H 2003 A functionalized THz sensor for marker-free DNA analysis Phys. Med. Biol. 48 3625-36 (The PDF file below contains the full text of this editorial, including the top ten papers of 2003.)

Measurement and prediction of total body fat

Though it has been established that skinfold anthropometry has severe limitations as a method of deriving total body fat (TBF), the possibility that the problem might be related more to the assumptions implicit in densitometry has to be addressed. This paper suggests that smaller residual errors might be obtained if sums of skinfolds are regressed against TBF measured by a difference technique (IVNAA + 3H2O dilution), suggesting that the latter may perhaps be a better standard than densitometry.