Radiation therapist Armand van Leeuwen operates the Siemens Somatom CT scanner.
A treatment plan can only be made if the anatomy of the patient has been accurately delineated. For this purpose, before 1970, the physicians in the NKI used palpation together with X-ray pictures taken from several directions. In 1973, a tranversal tomograph came into use. This device provides images of the anatomy in a transversal cross section. However the images are blurred and transformed, and have a limited field of view. In 1979, the development of the computer tomograph (CT) is at such an advanced level that the NKI-AVL decides to acquire a Whole Body CT scanner from Siemens.
A computer tomograph provides images of organs and tissues in a detailed way. In addition distances and relative positions in the reconstructed cross sections can be measured with high accuracy. Although the CT scan of a head as shown in this picture has a lower resolution and more artefacts compared to today's modern scans, in 1979 it meant an important step forward in the treatment planning of radiotherapy in the NKI-AVL.
By moving the patient through the scanner (translation) during the imaging process, part of the body is depicted. After scanning, the physician may reconstruct cross sections of the patient in each relevant direction. In these accurate reconstructions of the anatomy in several planes, the projection of the radiation beams can be precisely simulated. By considering the pros and cons of the various beam combinations an optimal plan can be obtained.
Anterior view of the skull by a conventional X-ray tomograph.
Computer assisted X-ray tomograph (CT) scan of the head.
CT scan of the thoracic region with delineated target volumes of a breast irradiation.
Professor Klaas Breur particularly insisted that the NKI acquires a CT scanner. In this way the Radiotherapy Department will obtain accurate transversal X-ray images, which will be the foundation for geometric planning and dose planning of radiation treatments. The device is installed in the Radiodiagnostic Department of the NKI.
A graphic workstation, the Evaluskop, is installed by the company Siemens in the Radiotherapy Department.
Radiation therapists are trained in operating the scanner and the Evaluskop. They develop, in cooperation with radiation oncologists and physicists, the scan parameters for their use in radiotherapy.
Hounsfield units. In addition to image formation, CT images are also used for the calculation of the dose distribution in the patient. The absorption of radiation in the body can be calculated if the position and density of tissues and organs in the projected path of the radiation beam are known. For each image point (pixel) in the cross section, the software of the CT scanner calculates the tissue density expressed in Hounsfield units. After calibration of the Hounsfield units, they can be used as input for the computer dose calculations in radiotherapy.
Before the start of the clinical use of the CT scanner, NKI physicist Harm Meertens performed a study of the calibration of Hounsfield units and of artefacts in the CT images which could cause deviations in the dose calculation. For these tests, he designed special tools that were then produced by the instrument makers of the Radiotherapy Department.
Test phantom in which various objects with different dimensions and densities could be inserted, to test the resolution of the CT scanner and the calibration of the Hounsfield units. The phantom is also provided with a motor driven mobile test object, which could be moved during CT scanning for the purpose of investigating the effect of organ motion on the CT reconstruction, for instance of the lungs during breathing.
Result of a CT scan of a test object made by Harm Meertens. It clearly shows the sensitivity of the scanning process for small density differences.
Cupping and capping effects. Another effect which is particularly disturbing during radiotherapy treatment planning is called cupping and capping.
A cylindrical object will attenuate X-rays more in the middle than at the edge, where the material thickness approaches zero. The attenuation is selective: the softer part of the X-ray spectrum is preferentially attenuated, resulting in a harder beam coming through the centre compared to the edge. This harder beam is less absorbed at larger depth in the body, yielding a measurement of too high a radiation intensity resulting in a calculation error in the reconstructed Hounsfield units.
By a smart calibration procedure this effect can in many cases be kept so small that it is acceptable. The effect is called cupping, while over-compensation is called capping.