Peter Hoban

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.

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.

Directional dependence in film dosimetry: radiographic and radiochromic film

The trend towards conformal, intensity modulated radiotherapy treatments has established the need for a true integrating dosimeter. In traditional radiotherapy, radiographic film dosimetry is commonly used. The accuracy and reproducibility of film optical density as an indicator of dose is influenced by several variables, including the chemical processing conditions. As a result radiochromic film, with all the advantages of radiographic film but without the need for chemical processing, has increased in popularity, although the low-dose sensitivity of radiochromic film does remain a disadvantage for some experiments. Several studies have investigated the reproducibility of radiochromic film results, but none have specifically addressed the well-known directional dependence seen with traditional radiographic film. In this study, the directional dependence of radiographic (Kodak X-omat V) and radiochromic (Gafchromic) films were measured. It was found that both films over responded when exposed parallel to the central axis of the beam as opposed to perpendicular exposure. An attempt is made to explain the reason for the responses of both films in terms of spectral effects and the air gap between the phantom segments. Although radiographic film exposed parallel rather than perpendicular to the central axis of the beam exhibits a measured difference in film response at depth, this over response does not occur when the extent of the film is restricted to a small region at the centre of the phantom (in this case an air gap is not introduced across the phantom). This suggests that it is the air gap rather than the orientation of the film that is the cause of the over response. Furthermore, when film occupies a slice through the entire phantom an over response occurs for both radiographic and radiochromic film, indicating that spectral effects are not the cause.

Superposition dose calculation incorporating Monte Carlo generated electron track kernels

The superposition/convolution method and the transport of pregenerated Monte Carlo electron track data have been combined into the Super-Monte Carlo (SMC) method, an accurate 3-D x-ray dose calculation algorithm. The primary dose (dose due to electrons ejected by primary photons) is calculated by transporting pregenerated (in water) Monte Carlo electron tracks from each primary photon interaction site, weighted by the terma for that site. The length of each electron step is scaled by the inverse of the density of the medium at the beginning of the step. Because the density scaling of the electron tracks is performed for each individual transport step, the limitations of the macroscopic scaling of kernels (in the superposition algorithm) are overcome. This time-consuming step-by-step transport is only performed for the primary dose calculation, where current superposition methods are most lacking. The scattered dose (dose due to electrons set in motion by scattered photons) is calculated by superposition. In both a water-lung-water phantom and a two lung-block phantom, SMC dose distributions are more consistent with Monte Carlo generated dose distributions than are superposition dose distributions, especially for small fields and high energies-for an 18-MV, 5 X 5-cm(2) beam, the central axis dose discrepancy from Monte Carlo is reduced from 4.5% using superposition to 1.5% using SMC. The computation time for this technique is approximately 2 h (depending on the simulation history), 20 times slower than superposition, but 15 times faster than a full Monte Carlo simulation (on our platform).

Dosimetry of 6-MV x-ray beam penumbra

The measurement of x-ray beam dose profiles in the penumbral region, using silicon diode, ionization chamber, TLD, and film dosimetry, has been investigated for a 6-MV beam defined by independent collimators. Penumbral width (80%-20%) at dmax, as measured by diode, film, and TLD was found to be 3.6, 3.6, and 3.4 mm, respectively. These results reflect the relative sensitive widths of each of the measurement systems (2.5, 2.0, and 1.0 mm, respectively). An empirical forming function was used to relate the penumbral shape measured with a finite-sized detector to that which would be measured with a point detector, the width of the point detector penumbra calculated from the diode penumbra is 3.4 mm, indicating that the TLD rods are a good approximation to a point detector. An alternative method of determining the width of a point detector penumbra is to extrapolate the penumbral widths obtained using two or more detectors of sensitive width. With this method, using Farmer and RK ionization chambers, a point detector penumbra width of 3.1 mm is obtained. An EGS4 Monte Carlo simulation, where a point source was assumed, gave a penumbral width of 2.8 mm. Negligible differences between the penumbra of beams defined by symmetric and asymmetric collimators was observed.

Evaluation of a PTW diamond detector for electron beam measurements

A PTW Riga diamond detector has been evaluated for use in electron beam dosimetry, by comparing results with those obtained using a diode (Scanditronix p-Si) and an ionization chamber (Scanditronix RK). The directional response of the diamond at 6 MeV and 15 MeV is more uniform than that of the diode, but for both detectors there is a dip in response when the beam axis is perpendicular to the detector stem. Spatial resolution of the diode detector, measured beneath a 2 mm wide slit, is slightly better than that of the diamond detector with detector stems both parallel and perpendicular to the beam axis. Diamond and diode depth-dose curves both agree well with corrected ionization chamber results at 15 MeV, while at 6 MeV the diamond is in better agreement. This indicates that the diode provides better spatial resolution than the diamond for measuring profiles, but the diamond is preferable for low-energy depth-dose measurements.

Accounting for primary electron scatter in x‐ray beam convolution calculations

Fermi-Eyges electron-scattering theory has been incorporated into the primary dose calculation for external x-ray beam radiotherapy using the convolution method. Incorporating scattering theory into the convolution technique accounts for the density distribution between the interaction and deposition sites, whereas conventional convolution methods only consider the average density between these two points. As the lateral spread of electrons ejected from an interaction site depends on the density distribution, the energy deposition (and hence dose distribution) is predicted more accurately if scattering is accounted for. This new method gives depth dose curves which show better agreement with Monte Carlo calculations in a (slab inhomogeneity) lung phantom than a conventional convolution method, especially at high energies and small field sizes where lateral electronic disequilibrium exists at the central axis. For a 5 x 5-cm2 18-MV beam incident on the lung phantom, a reduction in the maximum error between the convolution and Monte Carlo depth dose curves from 5% to 2.5% is obtained when scattering theory is used in the primary dose calculation. Incorporating scattering theory into the convolution calculation increases the computation time of the primary dose by a factor of 3.

A model for electron‐beam applicator scatter

Applicators (or cones), used in conjunction with patient specific cutouts in electron-beam radiotherapy, may interact with the primary electron beam to produce a secondary beam component (applicator scatter). This component affects machine output as well as the shape of resulting dose distributions. A model has been developed to simulate this scatter component for applicators consisting of trimming plates of arbitrary shape. This model involves sampling established kernels of scatter from edge elements of appropriate materials, obtained through Monte Carlo simulations. The result of the model is a phase space (position, direction, energy, charge, weighting) of applicator scattered particles which can be incorporated into a further Monte Carlo simulation, or as input into another advanced treatment planning algorithm. This model is evaluated by comparison of measured profiles and applicator scatter component depth dose curves with Monte Carlo simulations using simulated phase-space data as input. Results are very consistent and reveal information on the angular and spatial variation characteristics of this beam component. The results obtained verify the developed model as an accurate predictor of the characteristics of applicator scattered particles.

The use of the linear quadratic model in radiotherapy: a review

To be able to predict the impact of any radiotherapy treatment the physics of radiation interactions and the expected biological effect for any radiotherapy treatment situation (dose, fractionation, modality) must be both understood and modelled. This review considers the current use and accuracy of the linear quadratic model which can be used to consider the variation in tissue response with fraction size. Cell kill following radiation damage results from damage to the DNA which can take a variety of forms. In many cases the linear quadratic model is used to estimate the relative impact for different situations especially clinical studies relating to fraction size. This is mainly undertaken using parameters derived from the linear quadratic model such as biological effective dose and standard effective dose. The model has also been adapted to consider the effect of overall treatment time, repair during treatment (as occurs for brachytherapy treatments) and other situations. There are some concerns over its use, mainly in the small dose ranges (both total low doses and low doses per fraction) where studies have shown its inaccuracy. In other situations however it does appear to provide a reasonable estimate of relative clinical effect. As with all models, however results should never be considered out of clinical context.

On the impact of longitudinal breathing motion randomness for tomotherapy delivery.

The purpose of this study is to explain the unplanned longitudinal dose modulations that appear in helical tomotherapy (HT) dose distributions in the presence of irregular patient breathing. This explanation is developed by the use of longitudinal (1D) simulations of mock and surrogate data and tested with a fully 4D HT delivered plan. The 1D simulations use a typical mock breathing function which allows more flexibility to adjust various parameters. These simplified simulations are then made more realistic by using 100 surrogate waveforms all similarly scaled to produce longitudinal breathing displacements. The results include the observation that, with many waveforms used simultaneously, a voxel-by-voxel probability of a dose error from breathing is found to be proportional to the realistically random breathing amplitude relative to the beam width if the PTV is larger than the beam width and the breathing displacement amplitude. The 4D experimental test confirms that regular breathing will not result in these modulations because of the insensitivity to leaf motion for low-frequency dynamics such as breathing. These modulations mostly result from a varying average of the breathing displacements along the beam edge gradients. Regular breathing has no displacement variation over many breathing cycles. Some low-frequency interference is also possible in real situations. In the absence of more sophisticated motion management, methods that reduce the breathing amplitude or make the breathing very regular are indicated. However, for typical breathing patterns and magnitudes, motion management techniques may not be required with HT because typical breathing occurs mostly between fundamental HT treatment temporal and spatial scales. A movement beyond only discussing margins is encouraged for intensity modulated radiotherapy such that patient and machine motion interference will be minimized and beneficial averaging maximized. These results are found for homogeneous and longitudinal on-axis delivery for unplanned longitudinal dose modulations.