A new method of body habitus correction for total body potassium measurements.

This paper describes an accurate and time-efficient method for the determination of total body potassium via a combination of measurements in the Birmingham whole body counter and the use of the Monte Carlo n-particle (MCNP) simulation code. In developing this method, MCNP has also been used to derive values for some components of the total measurement uncertainty which are difficult to quantify experimentally. A method is proposed for MCNP-assessed body habitus corrections based on a simple generic anthropomorphic model, scaled for individual height and weight. The use of this model increases patient comfort by reducing the need for comprehensive anthropomorphic measurements. The analysis shows that the total uncertainty in potassium weight determination by this whole body counting methodology for water-filled phantoms with a known amount of potassium is 2.7% (SD). The uncertainty in the method of body habitus correction (applicable also to phantom-based methods) is 1.5% (SD). It is concluded that this new strategy provides a sufficiently accurate model for routine clinical use.

Response corrections for solid-state detectors in megavoltage photon dosimetry.

Solid-state detectors offer high sensitivity, stability and resolution and are frequently the dosimeter of choice for on-line dosimetry and small field therapies such as stereotactic radiosurgery. The departure from tissue equivalence of many solid-state devices, including diodes and MOSFETs, has to be carefully considered at lower energies and for Compton scattered radiation where the strongly Z-dependent photoelectric effect is significant. A modification of Burlin cavity theory is proposed that treats primary and scatter photon spectra separately and this has been applied to determine the correction factors for diode detector measurements of 6 and 15 MV linear accelerator beams. Uncorrected, an unshielded diode overestimates the dose at depth by as much as 15% for the 6 MV beam. The model predicts the effect to within 1% for both energies offering a basis for the correction of diodes for use in routine dosimetry.

The effects of delays in radiotherapy treatment on tumour control.

There is often a considerable delay from initial tumour diagnosis to the start of radiotherapy treatment, which may be due to factors such as waiting lists and referral delays. This paper uses widely published models and clinical parameters to calculate the effect of delays in treatment on local tumour control for four different types of tumour-squamous cell carcinoma (head and neck), breast, cervix and prostate. The Poisson model for tumour control probability (TCP), an exponential function for tumour growth and the linear quadratic model of cell kill are used to calculate the change in TCP for delays between diagnosis and treatment of up to 100 days. Typical values of the clinical parameters have been taken from the literature; these include alpha and beta, sigma(alpha), tumour size at diagnosis, pre-treatment doubling time, delay in onset of accelerated repopulation and doubling time during treatment. It is acknowledged that there are limitations in the reliability of these data for predicting absolute values of tumour control, but models are still useful for predicting how changes in treatment parameters are likely to affect the outcome. It is shown that for fast-growing tumours a delay of 1-2 months can have a significant adverse effect on the outcome, whereas for slow-growing tumours such as Ca prostate a delay of a few months does not significantly reduce the probability of tumour control. These calculations show the importance of ensuring that delays from diagnosis through to treatment are minimized, especially for patients with rapidly proliferating tumours.

Response of silicon diode dosemeters to scattered radiation from megavoltage photon beams.

Dose response effects of diodes due to the high atomic number of silicon relative to water are investigated. While quality chances in the primary component of a megavoltage beam with depth are minimal. Compton scattered photons are shown to have a substantial effect on the quality leading to their enhanced absorption in silicon via the photoelectric effect. Monte Carlo methods were used to study and model this phenomenon. Measurements of dose rate, depth and field size dependence are examined for commercially available diode detectors and ionisation chambers.

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 body's 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.

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

Experimental simulation of A-bomb gamma ray spectra for radiobiology studies.

Improved radiation protection of humans requires a better understanding of the mechanisms of radiation action and accurate estimates of radiation risk for both internal and external radiations. The Japanese atomic bomb survivors represent one of the most important sources of human data on the late carcinogenic effects of ionising radiations. The present study was undertaken to investigate whether it would be possible to use hospital radiotherapy/radiobiology equipment to mimic the spectra encountered in Hiroshima and Nagasaki. The estimated total gamma ray fluence spectra (including both prompt and delayed photons) at both Hiroshima and Nagasaki, for distances of 500, 1000, 1500 and 2000 m have been evaluated using DS86 data and previously unpublished information for delayed gamma radiations which constitute the major contribution to survivor doses. Monte Carlo (EGS4) simulations were performed to transport these photons through the body in order to investigate the variation in electron spectra for various body organs. The electron spectra obtained for these fluences at, for example, the colon, have been matched with combinations of electron spectra produced by linear accelerators to within 5% SD. These will, for the first time, enable a direct link to be made between radiobiological studies (for example, on mammography spectra) and the epidemiological data from Japan, which currently underpin radiation risk estimates.

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 patient's 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.

Body fat prediction from skinfold anthropometry referenced to a new gold standard: in vivo neutron activation analysis and tritium dilution.

Skinfold anthropometry is a widely practiced technique often with little appreciation of its limitations. The large residual error appearing in any regressions of body density versus sums of skinfolds is primarily due to biological causes, in particular the non-constancy of the ratio of subcutaneous to total body fat. Nevertheless this preliminary study shows that the residual error can be reduced by referencing to a gold standard other than body density. Using a difference technique involving in vivo neutron activation analysis and tritiated water dilution, this paper shows that at least in 20-29 year old normal subjects the residual error (expressed as a percentage of mean total body fat in the respective groups) can be reduced from 22% to 16% (p < 0.001) in males and from 17% to 9% in females (p < 0.0001). It is suggested that a large scale study could be initiated with this new gold standard to obtain accurate predictor relationships throughout the whole age range for both sexes.

Body fat: estimation or guesstimation?

Skinfold anthropometry is performed world-wide with little appreciation of its limitations. This paper shows that it is important to appreciate the magnitude of the biological component of the residual error in any regression of body density against sums of skinfolds; for example, for young adult males a biological error of around 21% (s.d.) is observed. Such 'errors' simply express nature's refusal to conform to imposed regression relationships, reflecting the inherent variability in the ratio of subcutaneous to total body fat.

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.

Absorbed dose measurements in dual energy X-ray absorptiometry (DXA).

In this study a predominantly film dosimetric method was used to measure the effective dose from posteroanterior (PA) lumbar spine and proximal femur scans performed on a Lunar DPX-L machine. Because of the very low dose rate in scanning mode, the depth dose data were determined using a stationary detector configuration. The characteristic curve for the film (Kodak TMAT-H) was obtained and depth dose measurements were made using slabs of "solid water". The film was calibrated using a superficial X-ray unit (calibrated against a standard traceable to a national standard). To assess the change in film response with beam hardening at depth, the film was exposed to calibration beams of different half value layer (HVL). The HVL of the DXA beam was determined for surface and depth doses using aluminium filters and a diamond detector (an energy independent device). All measurements were performed three times. Beam size was measured using film, and the scan areas and times were determined by scanning phantoms. The dose from a scan was calculated using Dsc = DTscAb/Asc, where D = dose rate (stationary), Tsc = scan time, Ab = beam area, and Asc = scan area. Organ doses were determined using an anatomical atlas and ICRP 23 female reference. All film measurements had good precision (coefficient of variation < 4%). There was little variation in film sensitivity with change in HVL (< 1% change for the first three HVLs) and consequently no corrections were applied to the depth dose data. Skin entrance dose was 11.5 microGy. Effective dose in females was 0.19 microSv for the PA lumbar spine. For the proximal femur scan, the effective dose was 0.14 microSv (ovaries included) and 0.023 microSv (ovaries excluded) for pre-menopausal and pos-menopausal women, respectively.

The 40K activity of one gram of potassium.

The activity of 40K in natural potassium as derived from the recent literature varies between 27.33 to 31.31 Bq g-1 of potassium. This paper reports measurements by gamma-ray spectrometry and compares these with independent calculations, yielding values of 30.18 +/- 0.72 Bq g-1 and 31.00 +/- 0.33 Bq g-1 respectively.

Microdosimetry of haemopoietic stem cells irradiated by alpha particles from the short-lived products of 222Rn decays in fat cells and haemopoietic tissue.

The Monte Carlo method is used to model fat cells and the nuclei of stem cells in haemopoietic tissue where 222Rn is dissolved in different amounts in the fat and tissue. Calculations are performed for fat cells of diameters 50 and 100 microns and for stem cell nuclei of 8 and 16 microns diameters for various fractions of fat filling the volume. Average doses (and their distributions) to stem cell nuclei from single passages of alpha particles are presented. In addition to dose, the relationship between LET and dose is obtained, illustrating the importance of 'stoppers' in the calculations. The annual average dose equivalent for a concentration of 1 Bq/m3 in air agrees well with other authors at 12 mu Sv/year. The method also allows the calculation of the fraction of stem cell nuclei hit annually. Here for 1 Bq/m3, stem cell nuclei of diameter 8 microns and 100% fat filing 15 x 10(-7) of the stem cell nuclei are hit.

A comparison of dosimetry techniques in stereotactic radiosurgery.

Accurate dosimetry of small-field photon beams used in stereotactic radiosurgery (SRS) can be made difficult because of the presence of lateral electronic disequilibrium and steep dose gradients. In the published literature, data acquisition for radiosurgery is mainly based on diode and film dosimetry, and sometimes on small ionization chamber or thermolominescence dosimetry. These techniques generally do not provide the required precision because of their energy dependence and/or poor resolution. In this work PTW diamond detectors and Monte Carlo (EGS4) techniques have been added to the above tools to measure and calculate SRS treatment planning requirements. The validity of the EGS4 generated data has been confirmed by comparing results to those obtained with an ionization chamber, where the field size is large enough for electronic equilibrium to be established at the central axis. Using EGS4 calculations, the beam characteristics under the experimental conditions have also been quantified. It was shown that diamond detectors are potentially ideal for SRS and yield more accurate results than the above traditional modes of dosimetry.

Body composition - what is measurable?

In 1878 Behnke noted that 'nothing is measured with greater error than the human body'. Over the intervening period measurement techniques have been developed which range from the relatively simple anthropometric methods to those based on sophisticated radiation and nuclear physics technologies. Nevertheless, despite the undoubted progress, it is important that we ask ourselves whether Behnke's observation might still have some validity. Many reviews have concentrated on the problems and limitations of given techniques with particular emphasis on achievable precision (because precision is easy to measure). However a straight-forward analysis of published data, especially of that relating to indirect techniques, shows that measurement precision is frequently small in magnitude compared to biological precision, the latter being simply a reflection of Nature's refusal to conform to imposed regression relationships. In this paper the body composition technologies are reviewed in the context of achievable accuracy and precision.

Dose rate dependence of a PTW diamond detector in the dosimetry of a 6 MV photon beam.

The dose rate dependence and current/voltage characteristics of a PTW Riga diamond detector in the dosimetry of a 6 MV photon beam have been investigated. Diamond detectors are radiosensitive resistors whose conductivity (i) varies almost in proportion to dose rate and (ii) is almost independent of bias voltage for a constant dose rate. At the recommended bias of +100 V, and also at +200 V, the detector is operating with incomplete charge collection due to the electron-hole recombination time being shorter that the maximum time for an electron to be collected by the anode. As dose rate is varied by changing FSD or depth (changing dose per pulse), detector current and dose rate are related by the expression i alpha Ddelta where delta is approximately 0.98. This manifests itself in an overestimate in percentage depth-dose at a depth of 30 cm of approximately 1% when compared to ionization chamber results. A similar sublinearity is seen when pulse repetition frequency is varied, indicating that the dependence is an on average rather than an instantaneous dose rate. The dose rate dependence is attributed to the reduction in recombination time as dose rate increases.

Monte Carlo modelling of neutron depth dose distributions in optimizing prompt gamma in vivo neutron activation analysis systems.

Optimization of prompt gamma in vivo neutron activation analysis systems is best achieved using Monte Carlo simulation. In this study the modelling of the dimensions and materials for source holders and collimators is described and compared with experimentally derived results where feasible. Results show that valid depth doses are obtained by modelling only the central part of an IVNAA system and that the use of borated paraffin as a reflector provides acceptable thermal fluence and depth dose.

Absorbed dose in the skin from beta emitters in medical and laboratory containers.

The International Commission on Radiation Protection have recently recommended an annual dose limit for the skin of radiation workers of 500 mSv at a depth of 20-100 microns averaged over any 1 cm2 regardless of the area exposed. It has previously been shown by the authors that beta dose rates on the outer surfaces of typical laboratory containers (vials, test tubes) or on medical syringes can exceed 100 mSv h-1 for radionuclide concentrations of the order of 1 MBq g-1, depending on container diameter, wall thickness and material and the beta particle energy spectrum. Since the fingers are frequently in contact with such containers it is of some importance to extend these dose calculations to depths below the skin surface, taking into consideration the anatomy of skin on the fingers. Using an extension of a Monte Carlo method previously described, dose rates have been calculated for the clinically useful radionuclides 90Y, 32P, 198Au, 153Sm and 131I. For polypropylene syringes the beta dose rates at a depth of 270 microns (a typical basal cell depth in the fingers) range from 77 to 135 mGy h-1 per MBq g-1 for 90Y (maximum energy 2.27 MeV) and approximately zero to 0.62 mGy h-1 per MBq g-1 for 131I (maximum energy 0.61 MeV). These results emphasize the importance of adequate finger protection when using high energy beta emitters and especially for clinicians who typically inject specific activities of the order of 100 MBq g-1 of 32P; in such cases annual permissible dose rates are exceeded in a matter of minutes. It is recommended that a minimum of 5 mm perspex finger protection be used for 90Y and 32P.