Radiant energy – Radiation controlling means
Reexamination Certificate
2001-07-26
2004-05-04
Nguyen, Kiet T. (Department: 2881)
Radiant energy
Radiation controlling means
C378S150000, C378S152000
Reexamination Certificate
active
06730924
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention concerns a multiple leaf collimator for limiting a bundle of high-energy rays emitted by a substantially point-like radiation source and directed towards a treatment object and used in particular for the stereotactic conformation radiotherapy of tumors, wherein the collimator contains a plurality of opposing collimator leaves made of a radiation-absorbing material which can be moved by drive mechanisms into the optical path such that the contours of said optical path can be freely defined, wherein the front edges of the collimator leaves are always aligned in parallel to the optical path.
The treatment devices used today in oncological radiation therapy are provided with collimators which limit high-energy radiation, in most cases high-energy gamma radiation from a linear accelerator, in such a fashion that the rays assume exactly the same shape as the object to be treated. Since irradiation e.g. of a tumor, is implemented from different directions, a high irradiation intensity on the tumor can be effected with only limited exposure to the surrounding tissue. For absorbing high-energy radiation, the collimator must have a thickness of several centimeters, which produces a half shadow when the passage opening has straight walls in the passage direction. Since the rays diverge from the substantially point-like radiation source, the collimator opening is smaller than the actual shape of the tumor so that the collimated rays diverge to have exactly the size of the tumor upon impingement. When the walls of the collimator opening are straight, part of the radiation will not be shielded by the full material thickness due to the inclined path of the radiation. In consequence thereof, either healthy tissue surrounding the tumor is exposed to considerable radiation or the tumor tissue will receive too little dosage. This causes damage which should be prevented. For this reason, one of average skill in the art has tried to develop different collimators which reduce or prevent these half shadows.
One suggestion to prevent half shadows which has been described in the literature, consists in providing the collimator leaves (leaves) of a collimator (multi-leaf collimator) with an irregular trapezoidal shape such that their side surfaces and the side surfaces of the outer limits of the collimator opening have the angle of the optical path. It is, however, more difficult to achieve corresponding alignment of the front edges of the collimator leaves. Many suggestions have been made to solve this problem, none of which is satisfying.
In one suggestion made e.g. in EP 0 259 989 B1, EP 0 556 874 B1, EP 0 562 644 B1, U.S. Pat. No. 5,166,531 and DE 33 11 870 C2, the front edges of the collimator leaves have a rounded shape such that the outer rays of the bundle contact these front edges tangentially. Through this solution, the half shadow can be weakened but not prevented. The same is true for a further suggestion made in EP 0 259 989 B1, EP 0 556 874 B1 and EP 0 562 644 B1, wherein the radiation must pass two sequential collimator openings. In DE 195 04 054 A1, such graduation of the front edges of the collimator leaves was further refined by constructing each collimator leaf from a plurality of rods, disposed one on top of the other such that they can be displaced with respect to one another. This collimator is complicated due to the large number of parts and exhibits increased radiation leakage due to the bordering of many collimator leaf elements and the associated unavoidable tolerances. Moreover, no drive mechanism is provided. Adjustments must be made by hand and automatic computer-controlled adjustment of the collimator opening is not possible.
DE 33 11 870 C2, U.S. Pat. No. 3,151,245, U.S. Pat. No. 4,987,309, U.S. Pat. No. 5,144,647, EP 0 193 509, EP 0 245 768 B1, the substantially idendical EP 0 387 921, and EP 0 314 214 B1 proposed moving the collimator leaves along curved paths such that the front edges of the collimator leaves are always aligned parallel to the optical path. This requires complicated guidance of the collimator leaves. The arrangement of such complicated guidance means imposes limits on the goal of minimizing the thickness of the collimator leaves. Collimator leaves must be thin to exactly reproduce the shape of the tumor, since rough graduations result in healthy tissue also being irradiated and destroyed or badly damaged. Moreover, if the collimator leaves have the shape of irregular trapezoids and are guided on curved paths, jamming can occur in consequence of this geometrical shape. To prevent same, DE 37 11 245 A1 proposes tapering the collimator leaves towards their front end facing the optical path. Wide opening of a collimator of this type produces gaps which cause increased leakage of rays. Finally, the problems of EP 0 314 213 B1 and U.S. Pat. No. 4,987,309 were believed to be solved by disposing the trapezoidal collimator leaves and the collimator leaves which can be displaced on curved paths such that the bundle of rays must pass through both collimator openings. Although each of the two collimators has a half shadow which is reduced by the other respective collimator, half shadows can only be eliminated with twice the shielding, i.e. almost twice as much material thickness is required. The amount of effort needed for drive and control is also doubled.
In addition, collimators have been disclosed (FR A 2519 465 and EP A 0286 858) which are composed of two pairs of shielding blocks offset from each other by 90°. These shielding blocks have front and rear components for preventing half shadows, wherein the latter can be aligned parallel to the optical path. The front blocks have sidewardly projecting bearing pins for pivoting on a holding device which is also connected to the rear blocks and on which drive means act for adjustment. These collimators can, however, only define a rectangular beam and a shape in the form of an object which is to be treated within a living organism, such as a tumor, can not be generated. This requires a shaping multiple leaf collimator of the above mentioned kind. The technical solution cannot easily be transferred to a multiple leaf collimator since a sideward bearing of forward components of this type cannot be effected with leaves of a multiple leaf configuration at those locations at which an adjacent leaf must be disposed for forming the above mentioned shape. A strict requirement for multiple leaf collimators is the absence of any shielding gaps between the leaves forming the shape since the associated leakage would destroy healthy tissue.
It is therefore the underlying purpose of the invention to solve the above-mentioned problems and to produce a multiple leaf collimator which eliminates half shadows with as little effort as possible.
SUMMARY OF THE INVENTION
This object is achieved in accordance with the invention in that the collimator leaves consist of a rear part which can be displaced linearly, and a front part connected thereto, wherein the front part of each collimating leaf is adjusted in correspondence with the respective position of the associated rear part through drive means such that the front edges are always aligned parallel to the optical path and such that the connection between the front part and the rear part does not lead to any significant gaps in the volume of the radiation absorbing material.
The present invention omits complicated curved displacement of the collimator leaves to simplify mechanics and reduce leaking radiation, since the linear displacement permits closer tolerances. From the point of view of mechanics and drive technology, adjustment of the front part can be realized in a considerably better and easier fashion compared to the curved displacement of prior art. The complete surface of the full material thickness is used for shielding to completely eliminate half shadows without requiring either the increased effort or additional shielding of the above-mentioned prior art. The suggested technical solution is superior with respect to the previous
Echner Gernot
Ganter Walter
Pastyr Otto
Schlegel Wolfgang
Deutsches Krebsforschungszentrum Stiftung Des Oeffentlichen Rech
Nguyen Kiet T.
Vincent Paul
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