[Three-dimensional conformal radiotherapy with multi-leaf collimator]

Abstract

In radiation therapy, it is very important to concentrate the high dose to the clinical target volume. For this purpose, S. Takahashi developed the dynamic conformal radiotherapy in 1960. In the first half of 1990' this conformal technique with multi-leaf collimator (MLC) was gradually becoming popular, because the three dimensional extent of the clinical target volume and the surrounding normal tissues was accurately recognized by the use of the serial CT-images, and the 3D-treatment planning was considerably easily obtained by the development of computer-technique. The 3D-CRT (conformal radiation therapy) based on the 3D-treatment planning in a broad sense means the coplanar or non-coplanar irradiation with multi-leaf collimator, and at present it is one of the most reasonable treatment techniques. In order to correspond the high dose region to the clinical target volume as possible, IM (intensity modulation) technique was introduced to the 3D-CRT using MLC in the second half of 1990'. Therefore, the static CRT is routinely used not only in Europe/ USA but also in Japan. But, it is often very difficult to obtain the concave high dose region in static CRT with MLC, even though the IM-technique is applied. At this occasion, the concave high dose region is easily obtained by the use of dynamic CRT such as the hollowed -out technique developed by S. Takahashi in Japan. The dynamic CRT technique is also useful with the static IM-CRT technique. There are two main aims of conformal radiotherapy: First, the protection of the surrounding normal tissues, and second, improvement of the local control rate. Usually the second aim can't be easily achieved, because the tumor control probability curves are often flat. The DVH concept is usually valuable to choose the optimal treatment planning. But, it is difficult to find the optimal dose-distribution, when two DVH-curves cross each other on the middle range of irradiated dose. On that occasion, the NTCP-concept is useful. From the standpoint of the tolerable dose, normal organs are classified into two types: parallel and serial. Therefore, two types of the NTCP-formula should be prepared to apply the NTCP-concept clinically.