Modelling of output factors for conformal megavoltage X-ray beams.

A method is described for calculating the output from conformally shaped megavoltage X-ray beams. The model has been developed for Varian accelerators but is shown to work for accelerators from another manufacturer. The use of dynamic wedging and both static and dynamic multileaf collimated beams are included in the model. For any linear accelerator, the data required are a set of measured output factors for square beams, an in-air profile and a limited number of readily available parameters defining the geometry of the head of the accelerator. The three components of the output, namely primary, head scatter and phantom scatter are modelled and calculated individually for any point in a beam. An optimization procedure is developed that automatically determines the eight parameters required to model an accelerator in order for these calculations to be performed. The performance of the method is demonstrated for shaped beams using asymmetric and multileaf collimation, both with and without wedging, and for a range of beam energies. The model has been incorporated into a computer program that is used clinically.

A generic computer program for checking photon beam dose calculations.

A computer program for checking photon external beam dose calculations is described. It provides a check of the treatment planning calculation by the use of machine data that are independent of that used in the initial calculation. Data required to specify any radiation beam are easily and rapidly set up and the program works for all the major manufacturers' machines. The user interface uses an interactive screen for machine data input and also for dose checking, where dose calculation is performed in real time as data are entered. The dose resulting from the planned field parameters is calculated and compared with the prescribed dose with a warning provided if the agreement is outside a set range (+/-5%). Its purpose is to act as a final quality assurance check to ensure that no significant errors occur in the monitor unit calculation.

The use of compensators to optimise the three dimensional dose distribution in radiotherapy of the intact breast.

BACKGROUND AND PURPOSE: Dose heterogeneity in tangential breast irradiation has been shown to be as high as 20% and may lead to problems in local control and cosmesis. In this study, dose heterogeneity in three dimensions (3D) in the breast irradiated with wedged tangential beams is assessed and the improvement which can be made by the use of individualised two dimensional (2D) compensators is established. The compensation required is calculated in two ways: (I) by an iterative technique giving a uniform dose on a plane through the isocentre normal to the central axis of each beam, and (II) by inverse planning using an optimisation technique based on simulated annealing. MATERIALS AND METHODS: A total of 17 patients with histologically proven T0-3, N0, N1, M0 breast cancer undergoing breast irradiation following wide local excision, were CT scanned using contiguous 1 cm slices from approximately 2 cm superior to 2 cm inferior of the irradiated volume. The dose distributions are determined using a 3D algorithm that calculates primary and scatter dose separately using a differential scatter air ratio method and corrects both for the presence of heterogeneities. The iterative technique achieves a dose variation of better than 0.5% on the plane through the isocentre with compensation on both beams. Compensation for the lateral beam only is calculated using the optimisation technique in order to minimise the scatter dose to the contralateral breast. The optimisation algorithm minimises the dose variance over the target and sets upper dose limits for the lung and the remainder of the irradiated volume. RESULTS: For the group of patients the average dose heterogeneity in 3D using wedges is 12% (range 8-17%), which reduces to 8% (5-16%) using compensation on a plane and to 5% (4-7%) using the optimisation technique. CONCLUSIONS: Inverse planning is normally used for complex radiotherapy techniques but when applied to tangential breast irradiation, can reduce the dose heterogeneity through the breast as a whole to as little as 4%, with potential benefits in local control and cosmesis.

Planning of beam intensity modulation using an advanced 3D dose calculation algorithm and a simulated annealing method.

PURPOSE: The aim of this work was to develop a fast inverse planning algorithm that will calculate optimum beam intensity distributions and beam shapes, and to incorporate the algorithm into a three-dimensional CT planning system. METHOD: The algorithm is based on the technique of simulated annealing and produces beam intensity distributions that could in principle be implemented clinically, either by the use of compensators or dynamic multileaf collimation. Dose distributions are calculated using a voxel beam model based on a spherical co-ordinate system, and transformations are given allowing the dose to be determined at any point within the patient. The dose calculation algorithm calculates primary and scattered dose separately from a knowledge of tissue/air ratios and differential scatter/air ratios, and both are corrected for the presence of heterogeneities in three dimensions. Specific attention is given to the execution time of the algorithm, and the methods developed allow satisfactory results to be achieved in calculation times which are sufficiently fast to be used interactively in the planning system. Several objective functions have been developed and can be selected in a simple manner by the user. In general, these attempt to achieve a uniform dose within the target while limiting the dose to organs at risk, either by upper dose limits or by specifying constraints on their dose volume histograms. RESULTS: The beam intensity distributions produced from the optimization have been used automatically by the forward planning system to produce three-dimensional dose distributions, and the results obtained in a number of clinical situations are presented. CONCLUSIONS: The inverse planning algorithm developed has been successfully incorporated into a three-dimensional planning system and is capable of producing beam intensity modulated distributions for clinical implementation. The execution time of the algorithm is sufficiently fast to be used as an optimization tool in an interactive forward planning system.

A technique for the fast calculation of three-dimensional photon dose distributions using the superposition model.

Techniques for reducing computation time in 3D photon dose calculations are addressed with specific emphasis given to the convolution/superposition approach. A single polyenergetic superposition model calculating absorbed dose per incident photon fluence (Gy cm2) was developed in terms of TERMA and a total energy deposition kernel (a total point spread function). A novel approach was devised for reducing calculation time. The method, named the CF method, was based on the use of a conventional, fast model (here a modified power-law method was used) for the generation of 3D dose distributions on a fine dose matrix. Superposition calculations were carried out on a coarse matrix and calculation speed was increased simply by reducing the number of calculations. A set of correction factors was derived on the coarse grid from the ratio of the dose values from superposition to those from the conventional algorithm. These were interpolated onto the fine matrix and used to modify the dose calculation from the conventional algorithm. The method was tested in a worst-case example where large dose gradients were present and in a clinically relevant irradiation geometry. It is shown that the time required for the generation of a 3D matrix with superposition can be reduced by at least a factor of 100 with no significant loss in accuracy.

Measurement of CT lung density in patients with chronic asthma.

Evidence for the presence of emphysema in patients with asthma is controversial. We have previously shown that decreased lung density, measured by computed tomographic (CT) scanning, preoperatively, correlates with morphometric measurements of microscopic emphysema in subsequently resected lungs. The aim of this study was to compare CT lung density, in 17 patients with chronic asthma (forced expiratory volume in one second (FEV1) 1.98 (SD 0.77) L), 17 patients with chronic bronchitis and emphysema (FEV1 0.97 (0.56) L) and seven normal subjects (FEV1 3.5 (034) L). All subjects underwent CT scanning of the lungs and respiratory function testing within 2 days of each other. In five of the asthmatic patients a CT scan was performed on two occasions before and after treatment with nebulized bronchodilator. In a different group of five asthmatics these measurements were performed at the end of and 6 weeks after an exacerbation. The mean value of the lowest fifth percentile of the CT lung density in the patients with chronic obstructive pulmonary disease (COPD) was -942 (SD 36) Hounsfield units (HU), in the 17 asthmatic patients was -912 (34) HU, and in normal subjects was -880 (13). Despite a significant increase in peak expiratory flow rate from 266 (SD 102) to 406 (83) L x s(-1) (p<0.02) following nebulized beta2-agonist in five patients with chronic asthma, there was no significant change in CT lung density (p>0.05) Our study indicates that at least some patients with chronic, stable asthma develop a reduction in computed tomography lung density, similar to that in patients with emphysema.

A beam model for three-dimensional radiotherapy planning.

A comprehensive beam model for three-dimensional radiotherapy planning is described. Input of patient data is either from an arbitrary spaced set of computed tomography (CT) scans throughout the treatment volume or from outlines traced on a digitizer. Any gantry, head or table rotation is allowed as well as any beam portal shape including internal blocking. Account is taken of beam modification due to the presence of the beam flattening filter, wedges and compensators. Both portal image and beam's eye view displays are readily available. A dose calculation algorithm is presented which uses a differential scatter-air ratio method to calculate the scatter dose in the presence of inhomogeneities. The beam model and algorithm form the basis of an interactive three-dimensional planning system running on a UNIX workstation, and performance times for dose calculation are given.

Parenchymal emphysema measured by CT lung density correlates with lung function in patients with bullous disease.

In subjects with chronic obstructive pulmonary disease (COPD) computed tomographic (CT) lung density correlates with direct pathological measurements of the size of the distal airspaces, as well as with measurements of airflow limitation and impairment of the diffusing capacity. Thus, CT lung density can be used to quantify emphysema in life. We wanted to assess the use of CT scanning to detect and measure the extent of bullous lung, and to quantify the severity of emphysema in the non-bullous areas of the lungs. In patients with bullous emphysema (21 males and 2 females; aged 31-69 yrs; forced expiratory volume in one second (FEV1) 14-84% predicted; volume corrected diffusing capacity of the lungs for carbon monoxide (DLCO/VA) 17-114% predicted). CT lung density was measured in electromagnetic imaging (EMI) units. The extent of bullous emphysema correlated poorly with all respiratory function measurements. In contrast, the severity of emphysema in the non-bullous parts of the lungs, expressed as either the mean EMI number, or the EMI number of the lowest 5th percentile of the CT lung density histogram, correlated well with measurements of airflow limitation and diffusing capacity. Our findings, thus, suggest that in patients with bullae the major determinant of respiratory function is the severity of the emphysema in non-bullous lung, and that the extent of the bullae has less functional importance.(ABSTRACT TRUNCATED AT 250 WORDS)

Stage IB cervical carcinoma: a clinical audit.

All patients with FIGO Stage IB cervical cancer registered with the Department of Clinical Oncology at the Western General Hospital, Edinburgh, during the 6 years from 1979 to 1984 have been reviewed, as part of a continuing programme of clinical audit. Of the 140 patients with Stage IB disease, 68 (49%) were treated by primary surgery of whom 44 (31%) also received adjuvant radiotherapy. Radical radiotherapy was the definitive treatment for 69 patients (49%). Three patients (2%) were not treated with curative intent. The crude 5-year survival rate for all cases was 72% and the cause-specific 5-year survival rate was 78%. Local tumour control at 5 years was 72%. There was no significant difference in outcome between the surgically treated and irradiated groups of patients. Age, histology and nodal status did not influence outcome. Irradiated patients with bulky tumours fared significantly worse than the other patients who received radical radiotherapy. Multivariate analysis of all patients revealed no significant independent prognostic variables. Primary surgery appears to confer no benefit over radical radiotherapy in terms of either survival or local control. Treatment-related late bladder and bowel morbidity was, however, significantly worse in irradiated patients.

A multidisciplinary approach to improving the quality of tangential chest wall and breast irradiation for carcinoma of the breast.

Physicists, radiotherapists and radiographers have worked together to enhance the quality and accuracy of radiotherapy for tangential irradiation of the chest wall and breast. Each stage of the process has been reassessed and improved. A technique has been developed on the treatment simulator which determines the required beam directions and sizes in a straightforward manner. A computerised tomography facility has also been developed on the simulator and this provides one or more slices for planning through the treatment volume, thus allowing accurate determination of external contour and lung position with the patient in the treatment position. The beam edge entry points and the isocentre position can be seen from radiation opaque markers placed on the skin surface, allowing accurate reproduction of the treatment beam positions as set on the simulator. A photon beam algorithm that corrects for the changes in scatter dose in a 3-dimensional (3-D) inhomogeneous situation has been developed and applied to tangential chest wall irradiation. This has shown large differences (up to 10%) in dose compared to a conventional 2-dimensional algorithm. The changes in dose distribution due to the accurate determination of lung position have also been investigated. A method has been developed of measuring the volume of ipsilateral lung irradiated. Dose-area histograms are used to evaluate the fraction of the lung area irradiated in the central slice, and an estimate of the volume irradiated can be made using a beam's eye radiograph and the simulator CT image.

Lung CT density correlates with measurements of airflow limitation and the diffusing capacity.

We studied 80 subjects (63 M, 17 F; 23-82 yrs) and related lung computerized tomography (CT) density with age, height, spirometry, lung volumes, diffusing capacity and arterial blood gas tensions. These subjects demonstrated a wide range of physiological impairment (forced expiratory volume in one second (FEV1) 8-116% predicted; diffusing capacity (Kco) 15-139% predicted; arterial oxygen tension (Pao2) 38-91 mmHg). They ranged from normal subjects to patients with chronic respiratory failure. Lung density was derived from CT density histograms measured as both mean Emergency Medical Information (EMI) number (EMI scale: 0 = water, -500 = air, EMI number of normal lung tissue range approximately -200 to -450) and the lowest 5th percentile EMI number, the latter value being more likely to represent the density of lung parenchyma. Lung CT density correlated most strongly with airflow obstruction (EMI 5th percentile versus FEV1/forced vital capacity (FVC) % predicted, r = 0.73, p less than 0.001) and diffusing capacity (EMI 5th percentile versus Kco, r = 0.77, p less than 0.001). This suggests that reduction in lung density, which reflects loss of the surface area of the distal airspaces, is a major index of respiratory function in patients with smoking related chronic obstructive pulmonary disease (COPD). These data provide no indication of other factors such as small and large airways disease, and loss of elastic recoil, which may contribute to airflow limitation, or disruption of the pulmonary vascular bed which may also affect CT lung density.

The use of an image processing system in radiotherapy simulation.

Image quality produced in radiotherapy simulation is often so poor in certain anatomical sites (e.g. the pelvis) that it can adversely affect clinical judgement with regard to treatment set-up. In treatment planning both the field arrangement and the availability of accurate anatomical information are important, and should ideally be provided at the time of simulation by methods based on the principle of computerised tomography. In this work an image processing system has been used as a link between the video output from the simulator and the treatment planning computer in order to implement a CT facility on the simulator, as well as allowing real-time image enhancement to be investigated as a method of improving picture quality. This approach also enables a new method of simulation to be considered which reduces patient dose and prolongs the useful working life of the X-ray tube.

Bladder cancer: the value of routine bone scintigraphy.

Two hundred and twenty-one patients with invasive bladder cancer had routine bone scintigraphy performed as part of tumour staging. The incidence of detectable metastases was 12%. Skeletal scintigraphy at diagnosis has a sensitivity of only 38%. The predictive value of a negative examination was 92%. These results suggest that skeletal scintigraphy is not indicated as a routine staging procedure in carcinoma of the bladder.

The reconstruction and dose-rate calculation of radium needle implants.

A comprehensive computer program has been developed for carrying out the various tasks associated with radium needle implants. Data input for the program are obtained by digitizing an orthogonal pair of radiographs with an analogue tracing system. Corresponding needles in the two radiographs are automatically located, and the implant may be rotated to any desired position in three dimensional space, at the same time presenting a view of the rotation on a storage oscilloscope. Automatic determination of the mean plane of a planar implant is possible, and the dose-rate distribution may be calculated in any specified plane. A Paterson--Parker exposure rate calculation may be performed with a biological dose and treatment time correction incorporated. The program is written in Fortran and runs on a small interactive computer system.

Radiation osteitis following irradiation for breast cancer.

Radiographs of the shoulder girdle were examined in 180 women who attended the breast follow-up clinic at the Department of Radiotherapy, Edinburgh. These comprised 52 controls who had been treated by radical mastectomy alone and 128 patients who had received post-operative X-ray therapy. The significance of osteoporosis as a sequel to irradiation is doubtful as it occurred in a substantial proportion of the control cases. Osteitis of severe degree occurred in 13.3% of patients who had been irradiated and was present in a mild form in a further 8.6%. The development of severe osteitis requires an NSD of 1650 rets or more. Estimates of the frequency of radiation osteitis are of no value unless detailed information is also provided about the techniques and quality of radiation.

A comprehensive radiotherapy planning system implemented in Fortran on a small interactive computer.

A suite of Fortran programs for carrying out the various dose computational aspects of radiotherapy has been developed on an enhanced RAD8 computing system. The hardward configuration of the computer is described and the main features of the programs in the suite are discussed. The include: (a) beam data input systems for use with linear accelerators and cobalt units; (b) static and rotational teletherapy planning, with computer optimization in the static planning; (c) irregular field calculations with isodose visualization; (d) interstitial calculations including routines which will reconstruct a radium needle implant in three dimensions in addition to presenting the isodose distribution in any desired plane. The problems of implementing the programs on another computer system are discussed.