Patent Number: 048516945
Section: summary

FIELD OF THE INVENTION The present invention relates to the field of radiotherapy, and more particularly to a device with which a source holder can be introduced into and positioned in an applicator tube. BACKGROUND OF THE INVENTION In treating certain diseases such as breast cancers, it is known to introduce one or more tubes of small diameter, known as "applicator tubes" or "applicators", into the diseased organ, one or more radioactive sources being introduced into these applicators and allowed to act for a variable duration, generally on the order of one to two days. Most often, there are about 20 applicators involved, the number typically being between 5 and 30. The elements most recently used as the sources are iridium, cobalt, cesium and californium. These sources can be contained inside a tube called a "source holder", and it is this source holder that is introduced into the applicator. The sources may be in the form of grains, each of them several millimeters in length, or in the form of filaments of greater length. Hence the source holder may contain either a single filament or a plurality of grains disposed one after another, and separated from one another by spacers. The active length, that is, the length occupied by the sources, is equal to 150 mm at most. The source holder is mounted in the anterior portion of a movable cable along a guide tube that terminates in the applicator. The problems that arise are not only that of driving the cable between an initial or storage position, in which the source holder is for instance inside a block of lead, to a final position or treatment position, in which the source holder is located inside the applicator, but also the problem of positioning the source holder, because it must be located at a very precise region inside the applicator, as well as the problem of detection of obstacles. It is essential that any obstacle encountered by the cable be detected, before it reaches its final position, and that the site of this obstacle be determined so that rapid intervention is possible. Several solutions have been proposed so far. French Pat. No. 2 033 653 describes a device used especially in plesioradiotherapy, in which the source holder tube is driven by a metal cable. Controlling the position of the posterior end of this cable, that is, the end opposite that carrying the sources, is done with the aid of two end-of-course detectors, which detect the passage of this end. There is no provision for detection of obstacles in this device, and hence the cable is in danger of being blocked before its posterior end has passed the end-of-course detector corresponding to the final position, and thus it can happen that the sources are not located inside the applicator. French Pat. No. 2 536 531 relates to a method of controlling the positioning of a source holder in a device of this generic type. Control is accomplished by analysis of the induced current of the motor driving the cable; the presence of an obstacle over the course causes the induced current of the motor to change, which enables the detection of the presence of this obstacle. A variation in the intensity of the current represents an increase in the load moment, and detecting this variation makes it possible to define a displacement time of the cable on which the source holder is mounted. This displacement time is compared with a reference time, which enables determining whether the cable is occupying an extreme position along its course, or is in an intermediate position. Although the presence of an obstacle can be detected with such a method, the exact position of the obstacle cannot be determined, because a certain amount of slippage, or sliding, can take place between the tube and the roller of the driving motor, and the exact length of the cable that has unwound is not known. For the same reason, it is no longer known with certainty that when the cable is in its final position, the source holder is indeed in its position inside the applicator. European Pat. No. 0 152 124 also describes a device for driving and positioning a source holder in an applicator used in radiotherapy. In this device, the source holder is actuated via a metal cable, and the end of the cable opposite the sources is wound in a spiral over a groove provided on one face of a wheel driven by a rack. Positioning of the source holder in the applicator is done with the aid of a pneumatic system. Compressed air is sent through an opening located at one end of the guide tube, in which the cable moves, and the air exits by an opening located at the end of the guide tube closest to the applicator. The cable has a head that comes into contact with a stop when the source holder is in its place in the applicator, which prevents the air from leaving. A monitoring apparatus detects the resultant increase in pressure inside the tube sheath and stops the driving of the cable. There is no provision for an obstacle detection system in this device. As a result, if the cable is stopped by an obstacle prior to reaching its final position, the air will continue to exit via the second opening in the guide tube, and the cable will continue to unwind. It is therefore impossible to know whether there is an obstacle, or to detect the position of such an obstacle. SUMMARY OF THE INVENTION It is the object of the invention to overcome these disadvantages by providing a device for driving and positioning a source holder in an applicator used in radiotherapy with which the source holder cannot only be easily displaced but also positioned very precisely in the applicator, and with which an obstacle can be rapidly detected and the site of the obstacle can be determined easily and precisely. More precisely, the subject of the present invention is a device for driving and positioning at least one source holder in an applicator tube used in radiotherapy, the applicator tube having a first end and a second end, the second end being open, the device comprising the following: a guide tube having one end capable of being connected to the second end of the applicator, a cable adapted to be displaced longitudinally along the guide tube and the applicator between a first position and a second position, the cable having a first end in the vicinity of which the source holder can be mounted, and a second end, and means for driving the cable along the guide tube and the applicator, characterized in that according to the invention, these drive means include: a motor-driven roller capable of being put into tangential contact, with sliding, with the cable, means for causing the rotation of this motor-driven roller, and an encoder equipped with a roller capable of being put into tangential contact, without sliding, with the cable, this encoder being adapted for detecting an interruption in the rotation of this roller. The words "device for driving and positioning at least one source holder in an applicator tube" are intended to mean that the invention applies to any number of applicator tubes. It can be used for placing a single source holder into an applicator, but typically, a plurality of applicators (on the order of 20) is used in radiotherapy. There is only a single source holder per applicator, and each source holder contains one or more sources. Preferably the encoder used is an incremental encoder; that is, it emits a signal each time the roller is displaced by rotation about a given angle, that is, practically each time that the cable is displaced by a predetermined length. Advantageously, a detector past which the source holder moves is mounted on the cable, in the course of the movement of the cable, can be provided, in order to count the sources. With this detector, it is possible not only to count the sources mounted on a given cable, but also to verify that the number of sources mounted on this cable is correct. In another feature of the invention, the device further includes a first end-of-course detector in front of which the second end of the cable is located when the cable is in its first position. Advantageously, the device may include a second end-of-course detector as well, located in such a manner as to detect the passage of the second end of the cable if the cable moves beyond the second position. Finally, a second guide tube in which at least a portion of the cable can be displaced, can also be provided. This second guide tube has one end closed by a stop, and the second end of the cable comes into contact with this end, if the cable arrives on the near side of the first position. The invention will become more apparent from the ensuing detailed description, which is given solely by way of illustrative example and in no way limits the scope of the invention, taken in conjunction with the accompanying drawings.