Biomedical subjects
Publications and source records attributed to A R Hounsell.
We report the case of a 61-year-old man with severe Parkinson's disease, who was successfully treated with postoperative radiation therapy for a T4N2cM0 transglottic squamous cell carcinoma of the larynx, with the aid of a deep brain stimulator.
The introduction of functional data into the radiotherapy treatment planning process is currently the focus of significant commercial, technical, scientific and clinical development. The potential of such data from positron emission tomography (PET) was recognized at an early stage and was integrated into the radiotherapy treatment planning process through the use of image fusion software. The combination of PET and CT in a single system (PET/CT) to form an inherently fused anatomical and functional dataset has provided an imaging modality which could be used as the prime tool in the delineation of tumour volumes and the preparation of patient treatment plans, especially when integrated with virtual simulation. PET imaging typically using 18F-Fluorodeoxyglucose (18F-FDG) can provide data on metabolically active tumour volumes. These functional data have the potential to modify treatment volumes and to guide treatment delivery to cells with particular metabolic characteristics. This paper reviews the current status of the integration of PET and PET/CT data into the radiotherapy treatment process. Consideration is given to the requirements of PET/CT data acquisition with reference to patient positioning aids and the limitations imposed by the PET/CT system. It also reviews the approaches being taken to the definition of functional/tumour volumes and the mechanisms available to measure and include physiological motion into the imaging process. The use of PET data must be based upon a clear understanding of the interpretation and limitations of the functional signal. Protocols for the implementation of this development remain to be defined, and outcomes data based upon clinical trials are still awaited.
BACKGROUND AND PURPOSE: To describe the clinical implementation of dynamic multileaf collimation (DMLC). Custom compensated four-field treatments of carcinoma of the bladder have been used as a simple test site for the introduction of intensity modulated radiotherapy. MATERIALS AND METHODS: Compensating intensity modulations are calculated from computed tomography (CT) data, accounting for scattered, as well as primary radiation. Modulations are converted to multileaf collimator (MLC) leaf and jaw settings for dynamic delivery on a linear accelerator. A full dose calculation is carried out, accounting for dynamic leaf and jaw motion and transmission through these components. Before treatment, a test run of the delivery is performed and an absolute dose measurement made in a water or solid water phantom. Treatments are verified by in vivo diode measurements and real-time electronic portal imaging. RESULTS: Seven patients have been treated using DMLC. The technique improves dose homogeneity within the target volume, reducing high dose areas and compensating for loss of scatter at the beam edge. A typical total treatment time is 20 min. CONCLUSIONS: Compensated bladder treatments have proven an effective test site for DMLC in an extremely busy clinic.
BACKGROUND AND PURPOSE: A phantom has been designed and built for a multi-institutional technique audit of the planning and delivery for radiotherapy to the prostate. The phantom was designed to test both the geometric and dosimetric accuracy of each aspect of the process. MATERIALS AND METHODS: The phantom consists of two curved water filled perspex tanks either side of a central block of solid water equivalent material. There are two options for the central section; a target defining block and a dose measurement block. The target defining block uses air holes to define a 3-D target volume for imaging via a CT scanner or a simulator. These holes can subsequently be filled with steel pins to allow megavoltage imaging. The dose measurement block allows thimble chamber measurements to be made at pre-selected points in a 5x5mm array. Five dose measurement points, typical for a prostate planning target volume (PTV) were selected. Initial evaluation of the phantom was performed by auditing the prostate radiotherapy planning and treatment chain at one institution. RESULTS: Agreement between the phantom and planned geometry confirmed that the stages of image acquisition, transfer and manipulation were accurately performed. Agreement within 0.5% was found between phantom and water tank measurements for dose calibration at a reference point. The measured dose delivered was within 2% of the dose calculated by the planning computer for all of the selected measurement points. The target volume marked by the steel pins was visible using electronic portal imaging. CONCLUSIONS: The phantom is a useful tool for the technique audit of prostate radiotherapy.
Radical abdominal radiotherapy in men runs the risk of impairing their fertility owing to scattered dose to the testes, outside of the treated volume. In patients for whom this is a concern it is important to be able to predict the dose to the testes before treatment in order to determine whether semen cryopreservation should be undertaken and testicular shielding performed during treatment. Measurements have been made on an anthropomorphic phantom to determine the magnitude of these doses for a four-field treatment consisting of an anterior-posterior parallel pair and a lateral parallel pair. A dataset is presented, which, together with a correction for patients size, allows an estimate of testicular dose to be made given only the photon energy, interfield distances and the distance from the testes to the nearest beam edge. Thermoluminescent dosimetry has been carried out in 17 patients to validate the use of the data tables. The results indicate that testicular doses may be estimated with a standard deviation corresponding to 1%-2% of the tumour dose, which is sufficient for the purpose of determining whether fertility is threatened by a planned treatment.
In 1996. the IPEMB issued a new code of practice detailing the procedures by which the output of therapeutic kilovoltage x-ray devices are to be determined. For x-ray beam qualities in the range 0.035-1.0 mm Al half-value thickness (HVL), the equation for converting the instrument reading into absorbed dose to water contained a factor named kch. This was included to account for the change in response of the parallel plate chamber from its calibration conditions free in air to those in the user's measurement phantom. As no data were available with which to quantify this factor at very low energies, the code advised that the user take a value of unity until sufficient data became available upon which to base an addendum. In this work, kch values have been determined for four chambers: two PTW 23344 large volume soft x-ray chambers, one PTW 23342 small volume soft x-ray chamber and a Markus electron chamber. Variations in the value of kch were investigated for changes in FSD, applicator size and beam quality. The water equivalence of the Mix-D phantom used for these measurements was also verified. A comparison of the results for the two PTW 23344 chambers showed no significant differences for any experimental situation, indicating that kch is a factor of chamber design rather than variations in construction. No variation in kch was identified with changes in FSD. A small dependence on applicator size was identified for larger applicators, and this was found to be dependent upon chamber design. The measured values of kch were found to increase with energy and again differences were seen between chamber designs. Overall, the values of kch recorded during these measurements ranged from 1.01 to 1.08. These results highlight a significant underestimation of doses calculated using the very low energy code of practice. This supports the need for further work to confirm these results. and the production of an addendum to the code in its present form.
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BACKGROUND AND PURPOSE: This paper describes the development of customised compensation by intensity modulated radiation therapy (IMRT), delivered by dynamic application of a multileaf collimator (MLC), in order to improve dose homogeneity in treatments of the pelvic region. The introduction of this simple IMRT procedure will help facilitate the clinical implementation of more complex 3D conformal therapy techniques. MATERIALS AND METHOD: Computer software is used to generate profiles of the intensity modulated beams which are required to deliver a uniform dose in a plane, passing through the isocentre and normal to the beam axis, under an irregular surface contour. These profiles are then operated on by interpreter software which determines the leaf trajectories that are necessary to deliver these beam profiles using a single, unidirectional sweep of the MLC leaves. A full dose calculation based on the calculated leaf positions is subsequently performed, allowing further fine adjustments to the modulation where required. RESULTS AND CONCLUSION: The compensation procedure has been successfully tested using films placed under a test phantom. The effect of the compensation procedure on dose distributions in the transverse plane has been investigated using an anthropomorphic phantom. Overall dose homogeneity has been improved through the use of customised compensation delivered by dynamic multileaf collimation.
The dose in the build-up region depends upon the primary photon beam, backscattered radiation from the patient and contamination radiation from outside the patient. In this paper, a model based on measured data is proposed which allows the build-up dose for arbitrarily shaped treatment fields to be determined. The dose in the build-up region is assumed to comprise a primary photon component and a contamination component that is a function of the field size and shape. This contamination component, for modelling purposes, is subdivided into contributions that correspond to elements of 1 cm by 1 cm cross-sectional area at the plane of the isocentre. The magnitude of these components has been obtained by fitting measured data to an exponential function. The exponent was found to vary linearly with depth for energies between 4 MV and 20 MV. The coefficient decreased linearly with depth at 4, 6 and 8 MV, but exhibited a broad build-up region at 20 MV. The primary component, in the build-up region, could be approximated by a 100 - (100 - PSD) e(-mu d) function, where PSD is the primary surface dose. The values obtained during the fitting procedure were used to calculate dose in the build-up region for arbitrarily shaped fields. Good agreement was found in each case.
The increasing use of irregularly shaped, off-centre fields in advanced treatment techniques, particularly intensity modulated radiation therapy, has strained the limits of conventional, broad-beam dose calculation algorithms. More recent models, such as kernel-based pencil beams and Monte Carlo methods, are accurate but suffer from the time needed for calculations and from the lack of clearly established methods for determining the parameters needed to match calculations with the particular dosimetric characteristics of an individual machine. This paper presents the implementation of a model that uses an extended source model to calculate the variation of fluence at the patient surface for any arbitrarily shaped field. It uses a macropencil beam model to calculate phantom scatter. Both head scatter and phantom scatter models use exponential functions fit to a series of measurements to determine the model's parameters. The means by which the model can be implemented in a clinical setting using standard dosimetric equipment is presented. Results for two separate machines and three energies are presented. Comparisons with measurements for a set of regular and irregular fields demonstrate the accuracy of the model for conventional, conformal and intensity modulated treatments. For rectangular and irregular fields at depths up to 20 cm, the accuracy was better than < or =1.5%, compared with errors of up to 7.5% with a standard algorithm. For a 20-step intensity modulated field, the accuracy was 3.4% compared with 18% with the conventional algorithm. The advantages of this model for IMRT are discussed.
Backscattered radiation into the machine monitor chamber can affect the machine output variation, with changes in field size and shape. For intensity modulated radiation therapy (IMRT) where many field, which may have small dimensions, are summed to give an intensity modulated field, the magnitude of backscatter will be different due to both the backscattering surface area changing, and the delivered monitor units being larger than for the equivalent static field. The effect of backscatter variation with field size for a Philips SL15 accelerator has been investigated at 8 MV for static and IMRT fields both in the standard clinical operating condition where an anti-backscatter plate is fitted, and also for a case where the anti-backscatter plate has been removed. The results show that in the absence of the anti-backscatter plate the variation in output between a 4 cm by 4 cm field and a 40 cm by 40 cm field size due to backscattered radiation was 5% for static fields. The anti-backscatter plate reduced this variation to less than 1%. When the accelerator operated in IMRT mode, with the backscatter plate in place, changes in the output due to additional backscattered radiation were less than 0.3%. With the backscatter plate removed, the outputs were lower, indicating the presence of additional backscattered radiation. It can be concluded that for the Philips MLC and SL accelerator with its anti-backscatter plate, the effects of backscattered radiation can be ignored for both static and IMRT fields.
BACKGROUND AND PURPOSE: Linear accelerators equipped with multileaf collimators (MLCs) are becoming more common and are widely available from most commercial manufacturers. There is a need to ensure they retain their commissioning specification using a preventative maintenance and quality control (QC) programme. This paper considers the design criteria of the Philips MLC which are important to the production of a comprehensive quality control programme. MATERIALS AND METHODS: The specific QC problems related to MLCs are identified as the positional accuracy of the leaves and their relationship to the back-up collimators, leakage considerations, the relationship of X-ray to light field and the influence of gravity on the positioning and leakage characteristics of the leaves. These problems are considered in relation to the general design considerations of the MLC, and methods of performing routine quality control checks are discussed. RESULTS AND CONCLUSIONS: The introduction of MLCs into clinical use results in new QC procedures being developed but it can be concluded that for the Philips MLC only an extra 30 min of QC time is needed per month and that its use has added little to the general down-time of this department.
Scattered radiation from within the treatment head can contribute significant dose to all parts of a radiotherapy treatment field. A multileaf collimator may be used to create an arbitrarily shaped field, and may also be used, under dynamic control, to modulate the beam intensity over the field. This method of intensity modulation is effectively a superposition of a large number of fields which have the same beam direction, but different shapes, and some of these shapes may have unusually small dimensions, particularly in the direction of the leaf movement. Two models for predicting the head scatter under these conditions have been investigated. These are a first-order Compton scatter approximation from the flattening filter, and an empirical fit to measured data using an exponential function. The first model only considers scatter from the flattening filter and has been applied to field sizes between 2 cm by 2 cm and 10 cm by 10 cm, where agreements are all within 1%. However it is not satisfactory at larger field sizes where small scatter contributions, from scattering sources other than the flattening filter, are integrated over large areas. The second model uses measured data between 4 cm by 4 cm and 30 cm by 30 cm to optimize the exponential function and is used to calculate the head scatter contribution for all field sizes. In this case good agreement is achieved over the full field size range, and hence this is a more generally applicable model. Results are presented for static irregularly shaped fields and intensity modulated beams created using a Philips multileaf collimator.
Doses from CT examinations are difficult to estimate. However, they are requested more frequently due to the increase in CT examinations. In particular, fetal dose estimations are frequently required for patients who have discovered, subsequent to the examination, that they were pregnant when the examination was conducted. A computer model has been developed to facilitate such dose calculations. This model combines empirical beam data with anatomical information. The model has been verified using thermoluminescent dosemeter (TLD) readings of internal and surface dose from both phantoms and patients, including intrauterine doses for patients undergoing afterloading gynaecological intracavitary treatment. Although only limited experimental data were available, the results indicate that the model accurately predicts uterine doses within acceptable errors. This approach has been validated for fetal dose estimation. The model was also used in a comparison with the nationally available CT dose data from the National Radiological Protection Board (NRPB). The two models were found to be in agreement for fetal dose estimations.
A model for calculating the variation in output of symmetric, asymmetric and irregularly shaped wedged radiotherapy fields is presented. The variation in output from the treatment head when a wedge is used is calculated by dividing the output into a primary component and one due to scattered radiation. The scatter component is then further subdivided into contributions from elements which have a 1 cm x 1 cm cross-sectional area at the isocentre. The scatter from each element is determined as the contribution from the head scatter component modified by the presence of the wedge and a contribution due to additional scattered radiation from the wedge. The relative intensity of the scattered radiation from the wedge is modelled using a simple first scatter approximation. In this approximation the magnitude of the scatter is given by a t exp(-mu t) function where t is the thickness of the wedge for the selected element. The magnitude of the primary component and the relative intensity of scatter from each element are then obtained by an iterative fit to measured data. The technique has been applied to two different internally mounted wedge designs, for a standard treatment head, two asymmetric treatment heads and two similar multileaf collimators, over a range of energies between 4 and 20 MV. Calculations agree with measured values over a range of field sizes and shapes to within 1.5%.
Doses in the build-up region have been compared for regular fields with and without a perspex accessory tray and for two irregular fields defined by either a Philips multi-leaf collimator (MLC) or lead blocks. The results show an increase in doses within the build-up region, by up to a factor of two, primarily due to the presence of the accessory tray. Fields shaped with the MLC exhibited build-up characteristics similar to those in unblocked fields and hence there will be systematic changes to the build-up dose when treatments are transferred from lead blocking techniques to those using an MLC mounted within the treatment head.
Two techniques for the automatic selection of individual leaf positions of a Philips multi-leaf collimator are described. Target volumes are identified either on simulator images or on cross-sectional images from CT or MR scanners. The setting of each leaf is computed to position the beam edge to cover the target with an appropriate, user defined, margin. An important consideration in the development of the system was its robustness and so the applications initially implemented have been relatively simple, comprising single field, parallel opposed fields and coplanar 4 field box techniques. Attention has been paid to the overall integrity of the planning and treatment delivery process. Before treatment commences, the beam shapes, which have been generated by the computer and transferred to the MLC control computer over a local area network, are checked against a printed template representing each beam. All data used for planning is archived and is accessible for review or, if necessary, for treatment modification.
The dose received by anatomical structures which are apparently shielded by lead blocks during a course of megavoltage radiotherapy may not be negligible. The dose has three main components; there is that due to radiation transmitted through the shielding block, that due to radiation scattered within the patient and that due to radiation scattered prior to reaching the patient and so circumventing the shield. A calculation method which separates the three components has been investigated and the results have been compared with measurements in a wide range of test situations for both cobalt-60 and 6 MV photon beams. The method of calculation predicts the dose behind the shielding block with acceptable accuracy in all the conditions investigated.