P. N. Johnston

A temporal method of avoiding the Cerenkov radiation generated in organic scintillator dosimeters by pulsed mega-voltage electron and photon beams.

The output signal of an organic scintillator probe consists of a scintillation signal and Cerenkov and fluorescence radiation (CFR) signal when the probe is exposed to a mega-voltage photon or electron beam. The CFR signal is usually unwanted because it comes from both the scintillator and light guide and so it is not proportional to the absorbed dose in the scintillator. A new organic scintillator detector system has been constructed for absorbed dose measurement in pulsed mega-voltage electron and photon beams that are commonly used in radiotherapy treatment, eliminating most of the CFR signal. The new detector system uses a long decay constant BC-444G (Bicron, Newbury, OH, USA) scintillator which gives a signal that can be time resolved from the prompt CFR signal so that the measured contribution of prompt signal is negligible. The response of the new scintillator detector system was compared with the measurements from a plastic scintillator detector that were corrected for the signal contribution from the CFR, and to appropriately corrected ion chamber measurements showing agreement in the 16 MeV electron beam used.

The Effective Atomic Number of Dosimetric Gels

Radiological properties of gel dosimeters and phantom materials are often compared against each other and against water or tissue by consideration parameters including their effective atomic number, Zeff. Effective atomic numbers have been calculated for a range of ferrous-sulphate and polymeric gel dosimeters using mass attenuation coefficient data over the energy range 10 keV to 10 MeV. Data is presented relative to water to allow direct comparison over a range of energies. These data provide energy specific values of Zeff which improves on the practice of applying a power-law based formula to estimate an energy independent value. For applications that require a single value of Zeff, the data presented here allows the choice of a value appropriate to the energy of the photon source or a spectrum-weighted average. Studying the variation of Zeff, which is equivalent to taking into account the variation of mass attenuation coefficients with photon energy, it is found that gels typically match water better than water matches human tissues. As such, the subtle differences in effective atomic number between water and gels are small and may be considered negligible. Consideration of the mean disparity over a large energy range shows, broadly, BANG-1 to be the most water equivalent gel.

A comparison of mammography spectral measurements with spectra produced using several different mathematical models

Due to the relatively complex nature of spectral measurements from x-ray machines, many researchers use mathematical models to simulate the spectra they need. However, there is concern over their accuracy, and hence the impact that their accuracy may have, on subsequent calculations that rely upon the spectra modelled. With this in mind spectral measurements have been performed on a mammography machine and a comparison with spectra calculated using several different models is presented. Several different techniques have been investigated in the spectral measurements to allow for pulse pileup and other effects of high count rate. Comparison with half value layer (HVL) measurements shows that the use of a gating signal in conjunction with the air-free path provides accurate results without the need for a pinhole collimator. Comparison of the measured spectra with those calculated using different models proposed in the literature suggests that accurate results can be produced by all models, but only if the user attempts to match the calculated HVL of the modelled spectrum with the physically measured HVL. If this is not done the modelled spectra may be in error. The impact of such an error is demonstrated in calculations of mean glandular dose, which indicate a possible underestimate of the dose by up to 20%.

Systematic variations in polymer gel dosimeter calibration due to container influence and deviations from water equivalence.

There are a number of gel dosimeter calibration methods in contemporary usage. The present study is a detailed Monte Carlo investigation into the accuracy of several calibration techniques. Results show that for most arrangements the dose to gel accurately reflects the dose to water, with the most accurate method involving the use of a large diameter flask of gel into which multiple small fields of varying dose are directed. The least accurate method was found to be that of a long test tube in a water phantom, coaxial with the beam. The large flask method is also the most straightforward and least likely to introduce errors during the set-up, though, to its detriment, the volume of gel required is much more than other methods.

A programmable motion phantom for quality assurance of motion management in radiotherapy

A commercially available motion phantom (QUASAR, Modus Medical) was modified for programmable motion control with the aim of reproducing patient respiratory motion in one dimension in both the anterior–posterior and superior–inferior directions, as well as, providing controllable breath-hold and sinusoidal patterns for the testing of radiotherapy gating systems. In order to simulate realistic patient motion, the DC motor was replaced by a stepper motor. A separate ‘chest-wall’ motion platform was also designed to accommodate a variety of surrogate marker systems. The platform employs a second stepper motor that allows for the decoupling of the chest-wall and insert motion. The platform’s accuracy was tested by replicating patient traces recorded with the Varian real-time position management (RPM) system and comparing the motion platform’s recorded motion trace with the original patient data. Six lung cancer patient traces recorded with the RPM system were uploaded to the motion platform’s in-house control software and subsequently replicated through the phantom motion platform. The phantom’s motion profile was recorded with the RPM system and compared to the original patient data. Sinusoidal and breath-hold patterns were simulated with the motion platform and recorded with the RPM system to verify the systems potential for routine quality assurance of commercial radiotherapy gating systems. There was good correlation between replicated and actual patient data (P 0.003). Mean differences between the location of maxima in replicated and patient data-sets for six patients amounted to 0.034xa0cm with the corresponding minima mean equal to 0.010xa0cm. The upgraded motion phantom was found to replicate patient motion accurately as well as provide useful test patterns to aid in the quality assurance of motion management methods and technologies.

Stereotactic fields shaped with a micro-multileaf collimator: systematic characterization of peripheral dose

Despite the highly localized doses that may be delivered via stereotactic radiotherapy, a small dose is nonetheless delivered to out-of-field regions, which may cause detriment to the patient. In this work, a systematic set of dose measurements have been undertaken up to a distance of 45 cm from the isocentre, for stereotactic fields shaped by a BrainLAB mini-multileaf collimator (MMLC) mounted on a Varian 600C linear accelerator. A range of treatment parameters were varied so as to determine the factors of greatest influence and establish relationships with dose. The commercial treatment planning software (TPS) miscalculates the dose to out-of-field regions. Measured dose decreases consistently out to 45 cm, whereas the TPS decreases out to 10-15 cm, at which point the predicted dose is constant. At the 5-10 cm off-axis distance (OAD), measurements indicate doses of about 5-10% of the dose at the isocentre, 1% at 15 cm OAD and 0.1% at 45 cm OAD. There are several observed trends. Greater MMLC field sizes (with static jaw) result in higher out-of-field dose, as do shallower depths. The source-to-surface distance does not greatly influence peripheral dose. However, the results given in this work do indicate that simple treatment arrangements, such as preferable collimator rotation, would in certain cases reduce out-of-field dose by an order of magnitude. Peripheral dose raises questions of treatment optimization, particularly in cases where patients have a long life expectancy in which secondary effects may become manifest, such as in the treatment of paediatric patients or those with a non-malignant primary. For instance, for a 20 Gy hypo-fractionated treatment, dose to out-of-field regions is of the order of cGy-a substantial dose in radiation protection terms.

Comparison of organ dosimetry methods and effective dose calculation methods for paediatric CT

Computed tomography (CT) is the single biggest ionising radiation risk from anthropogenic exposure. Reducing unnecessary carcinogenic risks from this source requires the determination of organ and tissue absorbed doses to estimate detrimental stochastic effects. In addition, effective dose can be used to assess comparative risk between exposure situations and facilitate dose reduction through optimisation. Children are at the highest risk from radiation induced carcinogenesis and therefore dosimetry for paediatric CT recipients is essential in addressing the ionising radiation health risks of CT scanning. However, there is no well-defined method in the clinical environment for routinely and reliably performing paediatric CT organ dosimetry and there are numerous methods utilised for estimating paediatric CT effective dose. Therefore, in this study, eleven computational methods for organ dosimetry and/or effective dose calculation were investigated and compared with absorbed doses measured using thermoluminescent dosemeters placed in a physical anthropomorphic phantom representing a 10xa0year old child. Three common clinical paediatric CT protocols including brain, chest and abdomen/pelvis examinations were evaluated. Overall, computed absorbed doses to organs and tissues fully and directly irradiated demonstrated better agreement (within approximately 50xa0%) with the measured absorbed doses than absorbed doses to distributed organs or to those located on the periphery of the scan volume, which showed up to a 15-fold dose variation. The disparities predominantly arose from differences in the phantoms used. While the ability to estimate CT dose is essential for risk assessment and radiation protection, identifying a simple, practical dosimetry method remains challenging.

Investigation of photon beam models in heterogeneous media of modern radiotherapy.

This study investigates the performance of photon beam models in dose calculations involving heterogeneous media in modern radiotherapy. Three dose calculation algorithms implemented in the CMS FOCUS treatment planning system have been assessed and validated using ionization chambers, thermoluminescent dosimeters (TLDs) and film. The algorithms include the multigrid superposition (MGS) algorithm, fast Fourier Transform Convolution (FFTC) algorithm and Clarkson algorithm. Heterogeneous phantoms used in the study consist of air cavities, lung analogue and an anthropomorphic phantom. Depth dose distributions along the central beam axis for 6 MV and 10 MV photon beams with field sizes of 5 cm x 5 cm and 10 cm x 10 cm were measured in the air cavity phantoms and lung analogue phantom. Point dose measurements were performed in the anthropomorphic phantom. Calculated results with three dose calculation algorithms were compared with measured results. In the air cavity phantoms, the maximum dose differences between the algorithms and the measurements were found at the distal surface of the air cavity with a 10 MV photon beam and a 5 cm x 5 cm field size. The differences were 3.8%, 24.9% and 27.7% for the MGS, FFTC and Clarkson algorithms, respectively. Experimental measurements of secondary electron build-up range beyond the air cavity showed an increase with decreasing field size, increasing energy and increasing air cavity thickness. The maximum dose differences in the lung analogue with 5 cm x 5 cm field size were found to be 0.3%, 4.9% and 6.9% for the MGS, FFTC and Clarkson algorithms with a 6 MV photon beam and 0.4%, 6.3% and 9.1% with a 10 MV photon beam, respectively. In the anthropomorphic phantom, the dose differences between calculations using the MGS algorithm and measurements with TLD rods were less than ±4.5% for 6 MV and 10 MV photon beams with 10 cm x 10 cm field size and 6 MV photon beam with 5 cm x 5 cm field size, and within ±7.5% for 10 MV with 5 cm x 5 cm field size, respectively. The FFTC and Clarkson algorithms overestimate doses at all dose points in the lung of the anthropomorphic phantom. In conclusion, the MGS is the most accurate dose calculation algorithm of investigated photon beam models. It is strongly recommended for implementation in modern radiotherapy with multiple small fields when heterogeneous media are in the treatment fields.

Efficiency enhancements to Monte Carlo simulation of heavy ion elastic recoil detection analysis spectra

Monte Carlo (MC) simulation can be used to simulate heavy ion elastic recoil detection analysis spectra, including the broadening and tailing effects of multiple and plural scattering, although it is very costly in terms of computer time. In this work, kinematic relationships and experimental parameters are exploited to implement efficiency improvements in the MC modeling process. For thin films, incident ions that pass through the sample without undergoing a significant scattering event need not be tracked. Ions that might generate a detectable scattered or recoiled ion are predicted by generating, in advance, the impact parameters which will define its path. Light recoiled target atoms may be dealt with in the same way. For heavy atoms, however, the probability of large angle scattering events is so high that the paths of most recoil atoms are dominated by several scattering events with large angular deflections.

A Comparative Study of the Effect of Calibration Conditions on the Water Equivalence of a Range of Gel Dosimeters

Gel dosimeters are of increasing interest in the field of radiation oncology as the only truly three-dimensional integrating radiation dosimeter. There are a range of ferrous-sulphate and polymer gel dosimeters. To be of use, they must be water-equivalent. On their own, this relates to their radiological properties as determined by their composition. In the context of calibration of gel dosimeters, there is the added complexity of the calibration geometry; the presence of containment vessels may influence the dose absorbed. Five such methods of calibration are modelled here using the Monte Carlo method. It is found that the Fricke gel best matches water for most of the calibration methods, and that the best calibration method involves the use of a large tub into which multiple fields of different dose are directed. The least accurate calibration method involves the use of a long test tube along which a depth dose curve yields multiple calibration points.