Stereotactic radiotherapy of irregular targets: a comparison between static conformal beams and non-coplanar arcs.

Stereotactic radiotherapy using a linear accelerator is usually equated with the technique of delivery using multiple non-coplanar arcs, which achieves a spherical dose distribution. As the majority of intracranial lesions are not spherical, a range of schematized tumour shapes were planned to assess the role of static conformal beams in the treatment of irregular lesions. A sphere and 2 ellipsoids, ranging from 20 to 50 mm maximum diameter located intracranially were planned using 3, 4, and 6 non-coplanar static beams with conformal blocks and were compared with four 120 degree non-coplanar arcs. Comparison of the plans was made by the relative sparing of normal tissue outside the target volume using three-dimensional dose-volume distributions. Non-coplanar arcs spared more normal tissue at low isodoses and achieved the best high dose sparing for spherical targets. For the majority of irregular targets, 3 and 4 static beams spared more tissue at doses > or = 50% and > or = 80% than the arc technique. For all irregular volumes, maximum sparing of normal tissue to isodoses > or = 50% and > or = 80% of the treatment isodose was obtained with 6 static conformal beams. We conclude that irregularly shaped tumours suitable for stereotactic radiotherapy with a linear accelerator are better treated with conformal static non-coplanar beams rather than with the multiple arc technique.

A rotate-translate CT scanner providing cross-sectional data suitable for planning the dosimetry of radiotherapy treatment.

An isotope scanner (J & P Engineering Multipoise scanner, Mk. III) has been converted to provide a simple transmission Ct scanning device, using a 137Cs gamma-ray source. The performance of the scanner has been evaluated by imaging a variety of phantoms and an anaesthetised rabbit. The relationship between the measured linear attenuation coefficient and the known electron density was found to be linear, for a range of materials with densities between those of bone and air. For reconstructed pixels of size 5 X 5 mm2 the percentage standard deviation of linear attenuation coefficient is typically 2.4% for bone and 3.6% for muscle. The spatial resolution of the device was measured and the performance of the scanner with the maximum design source strength was predicted. In the reconstruction of a cross section of the rabbit and of a human thorax phantom, air-muscle, lung-muscle, and muscle-bone interfaces are clearly visible. From experience gained with this scanner the role of simple CT scanning devices in radiotherapy treatment planning is considered. The resolution in linear attenuation coefficient and the spatial resolution available from the scanner are considered sufficient for radiotherapy treatment planning.

The performance characteristics of a simulator-based CT scanner.

Computed tomographic (CT) images have been obtained from a modified radiotherapy simulator (the prototype Royal Marsden Hospital ;CT simulator'). The images are suitable for planning radiotherapy treatment in which the treatment volume is a whole organ, such as in the postoperative conservative management of breast cancer. The performance characteristics of the prototype equipment are reported for scanning conditions typical of those encountered in clinical practice.

An x-ray detector system and modified simulator providing CT images for radiotherapy dosimetry planning.

A high count rate position-sensitive detector was developed in order to obtain CT information using a modified radiotherapy simulator. The detector is attached to one arm of the simulator gantry and acquires projection data from a fan beam of X-rays derived from the simulator X-ray tube. The scanner is designed principally for planning the dosimetry of breast cancer radiotherapy. During the collection of transmission projection data, suitable for the digital reconstruction of patient cross sections, the simulator gantry rotates continuously through about 360 degrees. A film cassette holder can be positioned above the detector to provide conventional simulator check films. Cross sections through patients receiving post-operative radiotherapy for breast cancer were reconstructed from projections acquired with the patient lying in the treatment position. These scans are an average over the breathing cycle. The outlines of internal anatomy and the patient external contour were used to produce treatment plans which include tissue inhomogeneity corrections. The data produced by this wide aperture CT scanner are suitable for planning those conditions in which radiotherapy is delivered to an organ such as the breast or the whole body, rather than to a localised lesion.

Clinical dosimetry for radiotherapy to the breast based on imaging with the prototype Royal Marsden Hospital CT simulator.

The result of conservative management of breast cancer is strongly dependent on the homogeneity of the dose delivered a schedule of post-surgical radiotherapy. In addition to improvements in local control, late normal-tissue effects should be minimised by achieving a good dose homogeneity. The Royal Marsden Hospital prototype CT simulator has been used to image patients in the treatment position. With the CT data incorporated into the planning process a quantitative measure of the dose homogeneity was made. There are strong indications that unless tissue compensators are used and/or conformation therapy is performed, the dose inhomogeneity in a widely used treatment geometry is too large to be clinically acceptable.

Verification and recording of radiotherapy treatments using a small computer.

A small computer has been interfaced to a linear accelerator used for routine radiotherapy. The objectives are to assess the rate and magnitude of some of the mistakes which may occur, to prevent these where possible and to assist radiographers with record keeping. In this paper we describe our approach to the problem, the apparatus used, the computer program, and the results of several years of operating experience while the system was continually being developed. On one linear accelerator, we estimate that an error has been prevented roughly once in every 500 field parameters set. The most common error is in setting the diaphragms and a detailed analysis is given of possible reasons. Doses in monitor units are recorded automatically and a running total of cumulative dose is maintained; plans for the printing of the dose in rads at selected points are discussed.

Environmental monitoring programs vs Good Laboratory Practice (GLP) programs: differences and similarities.

Environmental monitoring and Good Laboratory Practice programs are similar when looked at empirically. Both address quality issues, human or environmental safety, and have set procedures to assure the concomitant results. However, when compared at the operational level, they can be best described as very different. Good Laboratory Practice programs deal basically with two governmental agencies and their divisions- the Environmental Protection Agency and Food and Drug Administration. These are administered from the federal level involving no state resources. These programs are objective driven with the procedures being defined in study plans, protocols, and standard operating procedures. The environmental monitoring testing programs deal with a profusion of federal legislation including CERCLA (also known as CLP), RCRA, CWA, CAA, SDWA, NPDES and others. These acts require analysis by specific procedures mandated by the statutes. States operate many of these programs and have been given the authority by the federal government. Many of the states require separate certifications to conduct these analyses. Environmental monitoring testing laboratories often must acquire multiple state certifications to participate in multiple state programs. This is not cost effective and often leads to conflicting requirements. Much of the direction for having a national certification program comes from problems associated with these state-operated programs.