Patent Number: 049873091
Section: claims

1. Radiation therapy unit with a beam of rays propagating from a focal point along a beam axis and a radiator head arranged on the beam axis, having the following features: (a) the radiator head comprises a double-focus multi-leaf collimator;  (b) the multi-leaf collimator exhibits a plurality of adjacently arranged diaphragm plates which in each case have two side faces, two front faces and an inside and an outside face;  (c) means are provided for displacing each individual diaphragm plate; wherein  (d) each side face of each diaphragm plate forms a part of a surface area of a cone, all such cones having both a common cone axis which extends perpendicularly to the beam axis through the focal point and a common cone point which coincides with the focal point, and  (e) means are provided for guiding the diaphragm plates so that each diaphragm plate performs a pure rotation about the cone axis during its displacement.  (a) the outside faces of all diaphragm plates arranged laterally adjacently have overall as an enveloping surface a part of a surface area of an outer cylinder the axis of which is the cone axis, and  (b) the means for displacing the diaphragm plates engage the outside faces.  (a) the inside faces of all diaphragm plates arranged laterally adjacently have overall as an enveloping surface a part of a surface area of an inner cylinder the axis of which is the cone axis, and  (b) the means for guiding adjacently arranged diaphragm plates (23) comprise at least two holding yokes (24b, 24c) so that each diaphragm plate is supported at at least four points.  (a) an adjusting aid attached to the outside face of the diaphragm plate (23),  (b) a gear which engages this adjusting aid and  (c) a drive actuating this gear.  (a) the adjusting aid is a toothed rail mounted on the outside face,  (b) the gear is a self-inhibiting worm-rack drive and  (c) the drive is a stepping motor.  (a) multi-leaf collimators have a reduced height, the reduced height of a single multi-leaf collimator not being sufficient but the sum of the heights of the multi-leaf collimators arranged one above the other being sufficient for attenuating the beam of rays to a required level and  (b) a control circuit is provided which positions the individual diaphragm plates of the multi-leaf collimators in such a manner that a field point (34) to be shielded in the beam of rays is always covered by two diaphragm plates (23d, 23e). 2. Radiation therapy unit as claimed in claim 1, wherein 3. Radiation therapy unit as claimed in claim 1, wherein 4. Radiation therapy unit as claimed in claim 2, wherein means for displacing comprise for each diaphragm plate (23) 5. Radiation therapy unit as claimed in claim 4, wherein 6. Radiation therapy unit as claimed in claim 1, wherein the radiator head (3) exhibits two multi-leaf collimators which are arranged one above the other and are aligned perpendicularly with respect to one another. 7. Radiation therapy unit as claimed in claim 6, wherein the multi-leaf collimators have a reduced height, the reduced height of a single multi-leaf collimator not being sufficient but the sum of the heights of the multi-leaf collimators arranged above one another being sufficient for attenuating the beam of rays to a required level. 8. Radiation therapy unit as claimed in claim 1, wherein a matrix ionization chamber, by means of which the multi-leaf collimators are monitored, is arranged on the beam axis (S) opposite to the radiator head (3) and behind an isocenter. 9. Radiation therapy unit as claimed in claim 5, wherein the toothed rails of adjacent diaphragm plates of a multi-leaf collimator exhibit a different height and a resultant different pitch, the pitch being designed in such a manner that a given linear relationship which is valid for all diaphragm plates always exists between motor speed and plate advance. 10. Radiation therapy unit as claimed in claim 6, wherein the 11. Radiation therapy unit as claimed in claim 6 or 10, wherein the reduced height is about two thirds of the height needed for the required attenuation of the beam of rays.