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.

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.

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.

A PTW Riga diamond detector has been evaluated for use in electron beam dosimetry, by comparing results with those obtained using a diode (Scandihonix p s i ) 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 dioide, but for both detecton 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 m m 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 depthdose curves both agree well with corrected ionization chamber results at I5 MeV, while at 6 MeV the diamond is in better ageement. This indicates that the diode provides better spatial resolution than the diamond for measuring profiles, but the diamond is preferable for low-energy depth-iose measurements.

With the continuing improvement in computer speed, dose distributions can be calculated quickly with confidence. However, the resulting biological effect is known with much less certainty, despite its critical importance when assessing treatment plans. To assess plans accurately, biologically based methods of ranking plans are necessary. Many authors have suggested the use of dose volume histograms with reduction schemes and Niemierko has recently introduced another method based on the cell kill occurring in the tumour. This study presents an investigation into this value and suggests a use in prescribing dose. Equivalent uniform dose (EUD) can obviously be used for assessing treatment plans, although in its current form it is not adequate for assessing normal tissues; however, it can also be used to adjust the prescription dose ensuring all plans deliver the same EUD to the tumour. Once this is performed, plans can more easily be assessed on the effects to the normal tissues. In calculating the EUD another concept is introduced--the equivalent uniform biologically effective dose (EUBED). This value considers the distribution of dose and dose per fraction when comparing plans. Reduced dose per fraction at the edge of the target volume will exacerbate the effect of reduced dose on cell kill. Two methods are suggested for calculating the necessary prescription dose: one using an iterative method and one using the gradient of the EUBED function. A comparison was made for a series of stereotactic cases using different collimator sizes. Interestingly, using this method, although the maximum doses were different, the dose volume histograms (DVHs) for the brainstem were similar in all cases.

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.

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 for 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.

Some mathematical characteristics of tumour control probability (TCP) have been examined in the light of potential intra-tumour variations in clonogenic cell characteristics. In particular, an explicit expression for the relationship between the dose distribution and the probability of local clonogen eradication is obtained which maximizes the TCP using both general and specific TCP models, for the case of fixed energy deposition into the tumour volume. Characteristics of this expression are considered for the cases of varying clonogen cell density (uniform radiosensitivity) and uniform cell density (uniform radiosensitivity), yielding the 'uniform TCP' and 'uniform dose' results respectively for TCP maximization. These situations are examined graphically to highlight the possible consequences of neglecting intra-tumour heterogeneity in dose prescription.