Patent Number: 047946296
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention is directed to a contour collimator for radiation therapy having a prescribed plurality of diaphragm plates displaceably arranged relative to one another. It is particularly directed to a multi-leaf collimator that is utilized for limiting the radiation field of an ionizing radiation, preferably for limiting the radiation field of gamma radiation in a linear accelerator. 2. Description of the Prior Art Radiation apparatus employed in oncological radiation therapy are equipped with radiation field collimators which only allow the adjustment of rectangularly limited radiation fields. It is currently known, however, that better therapy results could be achieved in many oncological investigations if the radiation dose distribution could be adapted to the usually irregular shape of the target volumes (tumors are usually not spherical). To this end, irregularly shaped auxiliary collimators are individually fabricated for use in radiation therapy. The equipment required for the manufacture of such collimators is commercially available. Such equipment enables irregular radiation field shapes to be cut out of rigid plates of expanded plastic on the basis of X-ray picture models and to cast the part with metal alloys having a low melting point. This manufacturing procedure can only be carried out given individual irradiation angles and is rather involved. The use of arbitrarily adjustable collimators based on the multi-leaf principle (what are referred to as "multi-leaf collimators") was proposed by Takahashi as early as 1965 ("Confirmation Radio Therapy", Acta Radiologica, Suppl. 242 (1965), 1-142). Such manually adjustable collimators were then likewise utilized world-wide in various radio therapy centers in the further course of radiation therapy. The advantage over collimators manufactured in accord with the casting principle, however, is slight. Cutting out and casting is replaced by the likewise time-consuming manual adjustment of the individual "collimator leafs" or "diaphragm plates". The development of motor-adjustable multi-leaf collimators is currently being pursued at many radio therapy centers in view of the availability of inexpensive microelectronic control components. These collimators are provided for employment at neutron irradiation systems, at photon radiation sources and, in particular, at linear accelerators. These unit share the principle of the single-leaf drive. Every leaf (diaphragm plate) of the multi-leaf collimator is driven by its own stepping motor. The number of required stepping motors is identical to the number of individual leaves. The outlay for complicated electronics which is susceptible to malfunction and the space requirement for the integration of such a collimator into an iradiation installation is extremely high since a total of at least forty leaves and, as a result thereof, forty stepping motors, is required. German patent No. 192 300 discloses a contour collimator wherein two oppositely arranged groups of mutually displaceable, small rods impermeable to X-rays are provided for admitting only a prescribed profile from the radiation field of an X-ray source. This collimator is not suitable for radiation therapy wherein, in particular, high-energy photons (gamma radiation) are employed, since no actual "diaphragm plates" are employed. Moreover, only a manual adjustment of the small rods is provided. Such a manual adjustment, however, is usually too slow for radiation therapy, wherein a plurality of radiation fields having different profiles are successively applied. German AS No. 1 010 659 discloses a collimator for shaping a useful radiation beam from the radiation of a high-energy radiator, for example a cobalt-60-preparation, comprising diaphragm plates which are adjustable perpendicularly relative to the central ray of the beam to be shaped. In this collimator, a separate adjustment element is provided for every individual diaphragm plate. A drive element, for example, a drive shaft shared by all adjustment elements is connected to each of the adjustment elements only via friction clutches. The limitation of the desired radiation field is prescribed by a perforated plate into which pins are plugged. Given such a collimator, it is difficult to set a new radiation field within a short time. Moreover, the collimator is not suitable for oscillations in a vertical plane. In a certain position, the sliding clutch responds under the influence of the weight of the collimator plates; diaphragm plates would thus fall out, and a change in the contour results. Further, a sliding clutch does not guarantee the patient safety in what is referred to as a one-time irradiation in which the total dose required is applied in fractions as the radiation source and collimator are moved around the patient. German patent No. 30 30 332 discloses a primary radiation diaphragm for an X-ray examination installation wherein a plurality of gating elements limiting the radiation cone from various sides are employed, these being composed of thin metal strips pressing against one another. The elements are mutually displaceable in the longitudinal direction and are combined in packets. For remotely controllable adjustment, every metal strip carries a nose extending transversely relative to the displacement direction and perpendicularly relative to the gating plane. The nose is disposed at the side of the element facing away from the symmetry axis of the primary radiation diaphragm. Every metal strip packet has an adjustment element allocated to it, this adjustment element being adjustable by an x, y-drive and being engageable with the individual metal strips. This contour collimator is only suitable for low energies since relatively short diaphragm plates are employed. Given a 360.degree. rotation of the collimator around a patient, the individual diaphragm plates would fall out because no interlock is provided. An adjustment element for the individual diaphragm plate is provided, but this can only move low diaphragm plate weights. As a result of its design, moreover, this is limited only to the adjustment of softly shaped contours or profiles, i.e. contours or profiles without steps. SUMMARY OF THE INVENTION An object of the present invention is to fashion a contour collimator of the type initially cited such that a simple, finely stepped adjustability of the diaphragm plates is guaranteed with low outlay, whereby an adequate security against the maladjustment of a selected radiation contour is simultaneously established. This object is achieved in accordance with the principles of the present invention in a collimator having: (a) teeth at every diaphragm plate, PA0 (b) an adjustment element shared by the prescribed plurality of diaphragm plates for the adjustment of a first diaphragm plate relative to the remaining diaphragm plates, this adjustment element being in engagement with the teeth of the first diaphragm plate, PA0 (c) an interlock mechanism in engagement with the teeth of the remaining diaphragm plates, and PA0 (d) a mechanism for displacing the adjustment element from the teeth of the first diaphragm plate to the teeth of a neighboring, second diaphragm plate, the first diaphragm plate being locked during this displacement and the second diaphragm plate being unlocked. The second diaphragm plate need not be the plate placed immediately next to the first diaphragm plate; it can also be a further diaphragm plate. What is guaranteed in such a contour collimator is that the contour set for a prescribed irradiation direction does not automatically change. As a result thereof, the collimator is especially suited for application in combination with radiation sources which move during the irradiation. In particular, this collimator can be utilized when circling around a tumor to be irradiated. Tumors are normally irregularly shaped. In radiation therapy, they are usually approached from various irradiation directions. The through aperture or contour of the contour collimator in continuous or stepped revolution around the tumor can thus be quickly adapted to its respectively current contour, i.e. the contour seen from the irradiation direction. This enables short irradiation times, being particularly significant for high-energy gamma radiation which is generated by a linear accelerator. Given a known profile of the tumor which, for example, can be identified by a computer tomograph exposure and by three-dimensional calculation and irradiation planning following thereupon, the contour can be motor-adjusted when circling around without having to fear that individual diaphragm plates will change their established position or will even fall out. What is thereby achieved is that the tumor is irradiated tightly bounded and healthy tissue is optimally preserved. In one embodiment of the invention a further prescribed plurality of diaphragm plates is arranged next to the first-cited diaphragm plate, and an identically constructed interlock and displacement mechanism is allocated to these further diaphagm plates. In this way, a displacement of diaphragm plates for the purpose of adjustment of a new contour can be carried out from two sides, which shortens the access time and thus the irradiation duration. A further embodiment has a symmetrical structure with respect to a center line. Packets of diaphragm plates which are respectively displaceable relative to one another are thus arranged at both sides of this center line. The diaphragm plates of the two packets are thereby arranged so as to be moveable toward one another. The arrangement is preferably undertaken such that the diaphragm plates can be respectively swiveled beyond the center line into the region of the other packet of diaphragm plates. Asymmetrical radiation fields can be set in this way. Given a suitable selection of the tooth spacings in the teeth of every diaphragm plate, the individual diaphragm plates can be displaced to a greater or lesser degree in the direction toward the center line in fine steps. The desired irradiation profile can be set with great precision in this way.