Patent Number: 051608472
Section: description

DETAILED DESCRIPTION OF INVENTION FIG. 1 depicts a linear accelerator 1 in a radiation therapy or treatment room. The linear accelerator is a source of electrons for electron arc therapy. In this example, the linear accelerator is a Varian Clinac 2100C linear accelerator manufactured by the Varian Radiation Division of Varian Associates, Inc. of Palo Alto, Calif. Other linear accelerators can be used in the practice of this invention. Other manufacturers of linear accelerators include Siemens and Phillips. Linear accelerator 1 includes gantry 2 and head 3. Linear accelerator head 3 includes accesory mount assembly 4. Dynamic multivane electron arc beam collimator 5 is mounted within the accessory mount assembly 4. Linear accelerator gantry 2 is rotatable about a fixed axis of rotation defined by point 7. This gantry rotation provides for rotational movement of the head 3, mount 4 and collimator 5 about patient table 8 as shown by arrows 9. Arrows 9 indicate what is referred to herein as rotation of head 3, mount assembly 4 and/or collimator 5 through or along the arc of rotation or treatment arc. Such movement is also referred to as linear accelerator rotation. Patient table 8 is used to support the electron arc therapy patient. Patient table 8 is adjustable as indicated by arrows 10 to position the target area (i.e. treatment area) of the patient to receive the electron beam from linear accelerator 1 through head 3 and collimator 5. Collimator 5 includes an electron aperture which dynamically defines the electron field of the beam emitted by linear accelerator 1. FIG. 1 further depicts computer 11 and computer 12 in a control room. Computer 11 is used for control, operation and monitoring of the linear accelerator and represents conventional hardware and software typically used by users of the Varian Clinic 2100C linear accelerator. Computer 12 is an IBM PS/2 Model 80 system (or other computer system) that runs the Collimator Treatment Conversion Program and Remote Collimator Control Program described below. Monitor 13 is a local display panel used to display actual collimator vane positions through nine numerical displays indicating percentage of opening for each of the nine dosage planes defined by the collimator vanes. Linear accelerator 1 and patient table 8 are separated from computers 11 and 12 and monitor 13 by wall 14 (i.e. they are in separate rooms). This separation provides protection (i.e. shielding from electron radiation) to the operators in the computer control room. FIG. 2 depicts collimator 5 which is attachable to the linear accelerator head 3 by attachment to accessory mount assembly 4. The housing of Collimator 5 includes a bottom plate 20 which includes an extension which is insertable into accessory tray groove 30. The standard accessory connector 31 is not used because the collimator 5 operates independently of any electrical connection to linear accelerator 1. Accessory connector 31 could optionally be used to provide power and communication to collimator 5 thus eliminating the need for local power (e.g. local battery) and/or communications (e.g. infra-red transceiver). After insertion of collimator 5 into accessory mount assembly 4, latch 32 is closed to secure collimator 5 in place. In this manner collimator 5 is inserted into accessory mount assembly 4 and attached to the linear accelerator head 3. Accessory mount assembly 4 is standard equipment with the Varian Clinac 2100C and accepts a wide variety of accessory trays, including, for example, fixed collimators. Through the technology of this invention, a dynamic electron beam collimator (i.e. collimator with a dynamically configurable electron aperture) can be sized and adapted to utilize the standard accessory mount assembly of the linear accelerator without any electrical connection to the linear accelerator. Collimator 5 as shown in FIG. 2 includes brass collimation vanes 40 and 41. The numeral 40 represents nine vanes positioned in a row. The numeral 41 represents nine other vanes positioned in a second row parallel to the first row of vanes 40. Only part of one vane 41 is shown in FIG. 2. Vanes 41 are positioned opposite vanes 40. Each vane 40 has a corresponding opposite vane 41. Each such vane pair (i.e. pair of opposite vanes) defines a dosage plane, i.e., rotation of the vane pair through the arc of rotation (the selected treatment arc) defines a plane. The vanes 40 and 41 and the opening 43 in bottom plate 20 (a clinical use bottom plate) define the electron aperture. The electron aperture can be dynamically defined or configured (i.e., the aperture shape can be changed) by movement of the vanes 40 and 41. A vane movement means causes each vane to move linearly (i.e. along a single axis). Each vane can be caused to move towards or away from its opposite vane counterpart. The Bottom View in FIG. 4 shows two rows of vanes through the opening in the bottom plate (maintenance plate) of the collimator housing. At one extreme, the two opposing vanes have been moved towards each other as close as the vane movement means will permit and define a minimum ("MIN") aperture or opening between the two vanes. In this embodiment of the invention the minimum opening is 0.450 inches. At the other extreme shown in the Bottom View of FIG. 4, the two opposing vanes have been moved as far away from each other as the vane movement means will permit and define a maximum ("MAX") aperture or opening between the two vanes. In this embodiment of the invention the maximum opening is 2.30 inches. In the Bottom View of FIG. 4, the remaining seven vane pairs have been caused by the vane movement means to assume various intermediate positions and to thereby define various intermediate apertures or openings. The electron aperture is defined and shaped by the apertures or openings defined by the vane pairs. A separate vane movement means is associated with each vane. Both asymmetric and symmetric motion of each vane of the vane pair is allowed. The vane movement means provides for simultaneous and independent movement of each vane, i.e., each vane can be moved independent of the movement of the other vanes and each vane can be moved simultaneously with the movement of other vanes. Such movement occurs during rotation of the linear accelerator head 3 and collimator 5 through a selected arc of rotation or treatment arc (see arrows 9) as shown in FIG. 1. As the collimator 5 is rotated through the treatment arc the vanes can be moved by their respective vane movement means to appropriate vane positions. In this manner the electron aperture is dynamically defined or configured (i.e. changed) as the head 3 and collimator 5 move through the treatment arc of rotation. Such dynamic configuration or shaping of the electron aperture (and, therefore, the electron field) allows for greater uniformity in electron radiation dosage of the patient's target area and reduces electron radiation to areas outside of the target area. The movement of a vane pair through the selected treatment arc of rotation defines a dosage plane. The nine vane pairs define nine dosage planes which are parallel to each other and to the plane defined by gantry rotation. In this embodiment of the invention 9 vane pairs (18 vanes) arc used. This number of vane pairs can be varied in the practice of this invention. Typically the number of vane pairs will be within the range of 3 to 71. The preferred number of vane pairs ranges from 5 to 31. An odd number of vane pairs is preferred because this provides a center vane pair which is desirable because the patient or target treatment area is easily located and centered with respect to the center vane pair. The vanes 40 and 41 can be constructed of brass or other material suitable to provide shielding against the electrons emitted by the linear accelerator. Dimensions and details concerning the vanes in this embodiment of the invention are shown in FIG. 4. Associated with each vane is a separate vane movement means. Each vane can be moved independently of the other vanes and simultaneously with the movement of any other vane(s). The vane movement means in this embodiment of the invention is a vane assembly 45 (see FIG. 2). A separate vane assembly is associated with each vane 40 or 41. FIG. 3 depicts a vane assembly and its relationship to vane 40 or 41. The vane assembly includes sliding assembly 50, sliding plate 52, floating nut 53, worm screw 54, gear motor 55, geared pulleys 56 and 57, toothed timing belt 58, bearing 59, potentiometer 60 and circuit board 61. These components are assembled together in the manner and arrangement suggested by FIG. 3. The sliding assembly 50 is comprised of aluminum plates 51a, 51b and 51c which are bolted together. Vane movement is accomplished through gear motor 55 which drives (rotates) worm 54 (an 8 pitch brass screw assembly) which in turn causes vane 40 or 41 to move linearly to the desired vane position. Gear motor 55 is a 12000 RPM motor gear reduced 58 to 1. Gear motor 55 and worm 54 can cause vane 40 or 41 to move toward or away from its opposite vane. Full scale single vane movement of 0.925 inches can be accomplished in 2.3 seconds. Each vane is simultaneously monitored and controlled by an absolute means to a resolution of 4 mils. Potentiometer 60 serves as a vane position monitoring means. In this embodiment of the invention the potentiometer is an absolute positioning potentiometer (hybriton element 7/8" dia. 10,000 ohms res.) Proper vane movement (i.e. movement to the correct vane position) is monitored by the potentiometer 60. The potentiometer 60 provides feedback to its associated local controller as well as vane physical limit information to the drive circuitry. The local controller (i.e. three-axis processor) cannot drive the vane beyond its physical stops which are metal pins used to define a minimum opening of 0.450 inches between vanes in a vane pair and a maximum opening of 2.30 inches between vanes in a vane pair. Minimum and maximum vane openings (i.e. the width of the opening between the vanes of a vane pair) can be assigned different values as desired by the practitioner of this invention. The worm 54 is physically attached by set screw to the potentiometer 60 through toothed timing belt 58 which operates in conjunction with geared pulleys 56 and 57. This minimizes the modes of possible failure. False vane movement can only be indicated to the controller by the failure of the floating nut 53 attached to the vane itself. This floating nut is made of Delrin 500 or another sufficiently strong material to reduce the possibility of failure. Motor current trends are also monitored by the potentiometer for possible failure. Circuit board 61 provides the drive electronics for the associated gear motor 55 and vane 40 or 41. The circuit board 61 provides H-drive motor circuitry for full four-quadrant drive and control capability. FIG. 14 depicts an electrical schematic of circuit board 61. FIG. 15 depicts the placement of components on circuit board 61. A list and description of these components are set forth below as Table 1: TABLE 1 ______________________________________ Component List - FIGS. 14 and 15 Manufacturer P.N. Location Manufacturer Description ______________________________________ BD1 PCB-0034B-01 Vane Assembly Motor Drive parvus pcb C1, C2 C320C104K5R5CA Ceramic dipped 100 v Kemet capacitor .1 ufd D1, D2 1N960A 1/2 watt zener diode Motorola D4-D7 1N914 small signal diode-fast SEM recovery J1 68016-1-5 sng row rt agl ml hdr .10" grid Dupont 5 .times. 1 pos Q2, Q3 MPS6727 lw TO-92 genl purpose PNP hi Motorola current transistor Q4, Q1 MPS6715 lw TO-92 genl purpose NPN Motorola high current transistor R1-R4 RC05GF103J 10K 1/8 watt resistor Resistors U1 LM324N quad 5 v compensated op amp Motorola U2 MC74HC10N triple input NAND gate Motorola VR1, VR2 3292X-1-102 1K 3/8" sq. potentiometer, Bourns multiturn, trimmer ______________________________________ FIG. 3 also depicts local controller 70 and local controller interface 71. A local controller bracket 72 connects local controller 70 and local controller interface 71 by bolts to the lower collimator bracket (see FIG. 12). The collimator includes six local controllers (and six local controller interfaces). Each local controller is a three-axis processor. In this embodiment of the invention each local controller is a four-axis processor manufactured by the parvus Corporation. However, because the fourth axis is inactive in this application the parvus four-axis processor functions as a three-axis processor. For this reason, the local controller of this embodiment of the invention may be described herein as a three-axis processor. Each local controller (i.e. three-axis processor) controls and monitors three vanes. Each axis of the three-axis processor is dedicated to the control of a single vane. Thus the 18 vanes are controlled by six three-axis processors. As shown in FIG. 3, a vane assembly is connected to local controller 70 through local controller interface 71. The vane assembly connects to local controller interface 71 by connecting ribbon cable 76 and ribbon cable plug 75 to ribbon cable receptacle 77. In a similar manner, two other vane assemblies can be connected to ribbon cable receptacles 78 and 79 for control by local controller 70. The other five local controllers are connected to the other fifteen vane assemblies through local controller interfaces in a similar manner. FIG. 8 depicts a functional block diagram of the connections between a local controller and the circuit board 61 of each of the three vane movement means (i.e. vane assemblies) controlled by the local controller. The local controller is based on a standard parvus Node product the multi-purpose ParvNET Node. It is used unmodified except for the Local Controller Software embodied in the onboard 64K bytes of EPROM. The Local Controller has a local processor with a full module bus interface including analog input capability, an internal watchdog system, 32K RAM, 64K serial port with a (serial hardware protocol driver) port, and memory expansion to 192K bytes. The node can handle exception stacking with time tagging on up to 1000 packets. All features of the parvNET PROTOCOL STANDARD (STD-003.times.-01) are implemented in the on-board software. An 8 Mhz 68HCll processor is used to control all functions and execute the enhanced 6801 code set 68CHll. The application of distributed processing to the local controllers in an innovation of this invention. Such distribution processing is described below for this embodiment of the invention. A plurality of local controllers are used to distribute the collimator processing load for simultaneous independent motion of all eighteen vanes. Each of the local controllers is uniquely assigned to a separate grouping of three vanes (i.e. a separate grouping of three vane movement means). One local controller handles all required processing and communications for its assigned three vanes. That local controller processes information independent of all other controllers. By distributing the processing over multiple local controllers, efficiency of operation is increased and overall controller complexity and size are reduced. Multiple local controllers require less space than one local controller of equivalent processing power for controlling the same number of vanes. Distributed processing also provides a completely independent means of error detection, critical to clinical use of dynamic collimators. Once a system for distributed processing inside a collimator has been established, future enhancements and future additions are easily accomplished. One future addition can be a secondary level of collimator vane verification. FIG. 16 depicts an electrical schematic of the local controller circuitry. FIG. 18 depicts an electrical schematic of the local controller interface circuitry. FIG. 17 shows the placement of components on the local controller circuit board and on the local controller interface circuit board. A list and description of the local controller and local controller interface components are set forth in Table 2 below: TABLE 2 ______________________________________ Component List - FIGS. 16, 17 and 18 Local Controller and Local Controller Interface Manufacturer P.N. Location Manufacturer Description ______________________________________ BD1 PCB-0032B-01 Multi-purpose parvNET Node parvus pcb C1 AS303A20 20 pin flat capacitor, .03 ufd Rogers C11 C320C104K5R5CA Ceramic dipped 100 v Kemet capacitor .1 ufd C2 PDT6.8/50K 6.8 ufd 50 v capacitor Panasonic C3 513D686M016AA4 68 ufd 16 v tantilum cap, .1 Sprague lead spacing C7 TB47/6.3V 47 ufd, 6.3 v tantalum capacitor C8 C320C104K5R5CA Ceramic dipped 100 v Kemet capacitor .1 ufd C9 TB47/6.3V 47 ufd, 6.3 v tantalum capacitor D1 1N4005 1 amp rectifier, 100 piv Motorola J1 929647-01-20-10 dual 10th in grid male Dupont connector - 20 pos J2 929647-01-11-10 sgl row .10" ml header, .235 3M above plastic, gold, 11 pos J3 KRE3 3 position Lumberg terminal Lumberg R1 R25J103 10k resistor, 1/4 watt, 5% film Rohm U1 MC68HC11A1FN CMOS processor Motorola U1 821551-1 52 pin quad pack socket Amp U2 MC74HC573N tri-state octal d-type latch Motorola U3 XC68HC24PN Port replacement unit Motorola U3 821575-1 44 pin quad pack socket Amp U4 NMC27C256Q20 32k .times. 8 CMOS EPROM National 200NS U4 1CT-286-S-TG 28-pin machined low profile Robinson/Nug socket U5 1CT-286-S-TG 28-pin machined low profile Robinson/Nug socket U5A M5M5256P-12L 32K .times. 8 static RAM Mitsubishi U6 D5C031-40 20 pin EPLD Intel U7 MTOT1537.3728MHZ 7.3728 MHz Crystal M-Tron U8 7805CT 5 vdc regulator, TO-220 case Motorola U9 S8054HN low voltage indicator Seiko BD1 PCB-0033A-01 Three-axis interface printed parvus circuit borad C1 TB47/6.3V 47 ufd, 6.3 v tantalum capacitor C2-C4 C320C104K5R5CA Ceramic dipped 100 v Kemet capacitor .1 ufd J1-J3 68021172 Berg Stic 11 header -rt angle Dupont double row J4 68016-1-11 sng row rt agl male header .10" Dupont grid 11 .times. 1 pos J5 929852-01-20-10 dbl row fem header, .10" grid Dupont 20 .times. 2 pos J6 929850-01-11-10 single row fem header, .10" pos 3M grid, gold plate, 11 U1 7805CT 5 vdc regulator, TO-220 case Motorola ______________________________________ The local controllers, the infra-red transceiver and the vane movement means (e.g. gear motor) are powered by a local power source positioned within the collimator housing. In this embodiment of the invention, the local power source is a 12 v 1.9 amp battery (GS Portalac). View of Battery Power Location and Infra-red Transceiver Location is depicted in FIG. 6. Battery power system attachment is depicted in FIG. 7. The concept of a local power source (e.g. battery) in a portable electron beam collimator is one of the innovations of our invention. The dynamic multivane electron arc beam collimator is used on an intermittent basis for electron arc therapy. Its entire powered use cycle will typically be short (less than 15 minutes in many cases). The collimator can be removed from the linear accelerator and from the treatment room when not in use. During the periods of nonuse, the battery in the collimator can be charged by a battery charger through the battery charger port 39 (see FIG. 2). The local power source (e.g. battery) need only sustain the collimator for short periods of time such as one to two hours. High current usage occurs during transient vane movement. The concept of a local power source within the portable collimator eliminates the need for extra wiring and connections to the linear accelerator. The collimator is powered independent of the linear accelerator or any connection to the linear accelerator. FIG. 1 depicts infra-red transceivers 6a and 6b. Transceiver 6a is positioned within the housing of collimator 5. Transceiver 6b is positioned on wall 14 within the radiation treatment room. The infra-red transceivers provide for noncontact communication between collimator 5 and host controller 12. Noncontact communication means the communications from one device to another without physical electrical connections. FIG. 19 depicts an electrical schematic of the circuitry of infra-red transceivers 6a and 6b. FIG. 20 shows the placement of components on the infra-red transceiver circuit boards. A list and description of infra-red transceiver components are set forth in Table 3 below: ______________________________________ Component List - FIGS. 19 and 20 Infra-red Transceiver Manufacturer P.N. Location Manufacturer Description ______________________________________ BD1 PCB-0054B-01 Infra-red printed circuit board parvus C1 T350E685M025AS 6.8 ufd 25 v capacitor Kemet C10 C320C104K5R5CA Ceramic dipped 100 v capacitor Kemet .1 ufd C11 C320C104K5R5CA Ceramic dipped 100 v capacitor Kemet .1 ufd C12 C320C104K5R5CA Ceramic dipped 100 v capacitor Kemet .1 ufd C13 C320C104K5R5CA Ceramic dipped 100 v capacitor Kemet .1 ufd C14 C320C104K5R5CA Ceramic dipped 100 v capacitor Kemet .1 ufd C2 C320C104K5R5CA Ceramic dipped 100 v capacitor Kemet .1 ufd C3 CW15C103M Ceramic dipped 100 v capacitor Centralab .01 ufd, 20% tolerance C4 C320C104K5R5CA Ceramic dipped 100 v capacitor Kemet .1 ufd C5 C315C221K2G5CA 220 pfd 200 v ceramic Kemet capacitor, +/-10% C6 CY15C222M .002 ufd ceramic dipped capacitor CentraLab C7 C320C102K5R5CA .001 ufd ceramic capacitor Kemet C8 T350E685M025AS 6.8 ufd 25 v capacitor Kemet C9 C320C104K5R5CA Ceramic dipped 100 v capacitor Kemet .1 ufd D2 SDP8403-301 Infra-red phototransistor Honeywell D3 1N914 small signal diode-fast recovery SEM D4 1N914 small signal diode-fast recovery SEM D5 1N914 small signal diode-fast recovery SEM D6 1N914 small signal diode-fast recovery SEM D6 1N914 small signal diode-fast recovery SEM D8 1N914 small signal diode-fast recovery SEM J1 929850-01-11-10 shs row fom 1 dr, .10" grid 3M 11 .times. 1 pos LD2 SLR34UR# LED, Rad 1/8" dia. Rohm LD3 SEP8703-001 high output infra-red LED, clear Honeywell LD4 SEP8703-001 high output infra-red LED, clear Honeywell Q1 2N4400 general purpose NPN Motorola Q2 2N4400 general purpose NPN Motorola R1 R25J102 1k ohm, 1/4 watt, 5%, film Rohm R10 R25J103 18k resistor, 1/4 watt resistor, Rohm 5%, film R11 RN55DS103F 510K 1/4 watt resistor, Dale 5%, film R12 R25J103 10k resistor, 1/4 watt, Rohm 5%, film R13 2.2k ohm, 5%, film Rohm R14 R25J333 33K 1/4 watt resistor, Rohm 5%, film R15 R25J470 470 ohm resistor, 1/4 watt, Rohm 5%, film R16 R25J102 1k ohm, 1/4 watt, 5%, film Rohm R17 R25J102 1k ohm, 1/4 watt, 5%, film Rohm R18 R25J104 100k, 1/4 watt, 5%, film Rohm R19 R25J511 510 ohm, 1/4 watt resistor Rohm R2 R25J273 27K 1/4 watt resistor, 5%, film Rohm R20 R32GF100J 10 ohm 1 watt resistor Resistors R21 R25J102 1k ohm, 1/4 watt, 5%, film Rohm R22 R25J102 1k ohm, 1/4 watt, 5%, film Rohm R23 R25J102 1k ohm, 1/4 watt, 5%, film Rohm R24 R25J102 1k ohm, 1/4 watt, 5%, film R25 R25J103 10k resistor, 1/4 watt, 5%, film Rohm R26 R25J101 100 ohm, 1/4 watt, 5%, film Rohm R3 2.2k ohm, 5%, film Rohm R4 R25J104 100k, 1/4 watt, 5%, film Rohm R5 R25J221 220 ohm 1/4 watt resistor, 5%, film Rohm R6 CRB14FX2002 28k, 1/4 watt resistor, metal oxide Rohm fixed R7 R25J221 220 ohm 1/4 watt resistor, 5%, film Rohm R8 R25J105 1 meg ohm, 1/4 watt, 5%, film Rohm U1 TDA2320 Infra-red pro-amp S6S U2 LM358N LIN - dual op amp Motorola U3 MC74HC00N quad 2-1 NAND gate Motorola U4 M74HC14B1 hex schmitt trigger S6S-ATES U5 LM311N General purpose comparator Motorola VR1 VRN182-20K 20k potentiometer Vernitron ______________________________________ The vanes, the vane movement means, the local controllers (including interfaces), the infra-red transceiver 6a and the local power source (e.g. battery) are contained within a housing. The housing of this invention can be a housing, chassis or support for three other collimator elements. The housing is constructed of plates as shown in FIG. 10. The plates are bolted together to form the housing which can also be seen in FIG. 2. In FIG. 10 the Bottom View shows the bottom plate as a maintenance plate. The maintenance plate has a large opening to facilitate access to the vanes, vane movement means and controllers. In actual electron arc therapy use a clinical use bottom plate having a smaller opening should be used. A clinical use bottom plate is depicted in FIG. 11. Referring again to FIG. 10, the Top View depicts the top plate of the collimator housing. The Side View depicts the side plates of the collimator housing. FIG. 10 also includes a Front View and a Back View where a front plate (not shown) and a back plate (not shown) should be positioned and bolted to the side brackets described below. The Front View shows use of two side brackets to secure the side plates to the top plate. The Back View shows nine vane assemblies. The concept of including the vanes, vane movement means, local controllers, noncontact communications means (e.g. infra-red transceiver) and local power source within a portable collimator housing represents one of the innovations of our invention. This housing innovation facilitates the use and installation of the collimator. Computer 12 (see FIG. 1) functions as the host controller of the collimator 5. The host controller can be an IBM PS/2 Model 80 microcomputer, a microVAX II minicomputer or some other kind of computer. The host controller 12 executes a computer program which we have named the "Collimator Treatment Conversion Program" or "Conversion Program." The Conversion Program was programmed in the Fortran programming language. A source code listing of the Conversion Program is filed with, and as part of, this patent application and is incorporated by reference into this specification. The Conversion Program converts a graphical image of the target area (or treatment area) of the patient into nine dosage planes (which correspond to the nine vane pairs in this embodiment of the invention). These dosage planes are parallel to the gantry rotational plane of the linear accelerator. For purposes of rotational reference, the arc of rotation is defined by 360 degrees of rotation with the point directly below the isocenter designated as zero degrees. Because the linear accelerator head does not typically rotate the full 360.degree. around the patient, only a selected treatment arc (which is a subset of a full 360 degree rotation) is used and only the arc segments (described below) within the selected treatment arc are used. Typically, the selected treatment arc will represent a rotational arc of about 90 to 270 degrees. Our invention is not, however, limited to this typical range of operation. Only the vane pair openings and vane position data calculated for arc segments within the selected arc (or treatment arc) are needed and retained for use. The following process is conducted sequentially for each vane pair (i.e. for each dosage plane): 1. The selected treatment arc is divided by the user into a plurality of arc segments. Arc segments are defined by the user by defining or selecting reference angles. In this embodiment the reference angles are selected at every five degrees of rotation. Other reference angles can be used. PA1 2. The user selects reference points which are points located within the patient, preferably equally spaced across the treatment arc, at a depth below the patient surface corresponding to the depth of dose maximum of the electron beam. PA1 3. For the first arc segment, the Conversion Program calculates the dosage quantity for each reference point within the dosage plane for each possible width of the opening defined by the vane pair. Possible widths are defined incrementally (e.g. in 1 millimeter increments) and range from minimum opening to maximum opening. PA1 4. Step 3 is repeated for each of the remaining arc segments. PA1 5. The Conversion Program uses a least squares optimization algorithm to determine a single preferred vane pair opening for each arc segment which provides the most uniform dose distribution across the entire treatment arc. For example, if the treatment arc is divided into ten arc segments then the vane pair will have ten openings assigned to it, one for each of the ten arc segments. Steps 1 to 5 are repeated for each of the remaining vane pairs (i.e. dosage planes). For nine vane pairs the process is conducted nine times, once for each vane pair. For nine vane pairs and ten arc segments the end result is a data file of 90 preferred vane openings. Each vane pair opening is independent of the other vane pair openings. The preferred vane pair openings are defined by vane position and are represented by vane position data. This data file (i.e. the vane position data) is transported to the host controller 12. This data file is referred to as the vane position data file. The vane position data file is indexed by reference angle. The vane pair openings calculated by the Conversion Program are indexed by reference angle for each dosage plane. The electron arc dose calculations used in the Conversion Program explicitly account for changes in patient shape across the arc, changes in patient density within the treatment volume, and changes in dose rate and energy across the arc. The preferred vane pair openings are converted to data representative of vane positions which will define the electron aperture needed to provide the desired uniform electron dosage. The vane position data are stored sequentially in a data file for later transmission to the collimator at the appropriate times (i.e. at appropriate reference angles) during the rotation of the linear accelerator head and collimator through the selected treatment arc. The data file for the complete treatment is preferably calculated before rotation begins. The invention is not limited to the specific Conversion Program or settings described herein. Other computer programs can be developed to serve as the Conversion Program or described settings. Changes within the scope of our invention can be made to the Conversion Program. For example, the reference angles can be different than every five degrees of rotation. The Conversion Program in combination with the necessary computer hardware and user input represents a means for creating vane position data (the "vane position data creation means"). This also represents a means for determining preferred vane pair openings for each arc segment and for representing the preferred vane pair openings as vane position data for each arc segment. The data file containing the vane position data is indexed by reference angles. The data file contains a record of vane position data for each reference angle. [Note: Each reference angle identifies the arc segment which begins with the reference angle. (Two reference angles, a beginning reference angle and an ending reference angle, define an arc segment. The beginning angle, as determined by rotational direction, is the identifying reference angle. The ending reference angle will be the beginning or identifying reference angle for the next arc segment (if there is a next arc segment)]. Each such record contains data representing the desired position of each of the eighteen vanes for the reference angle and the arc segment identified by the reference angle. As the linear accelerator head and collimator rotate through the selected treatment arc, the vanes will be moved to their appropriate positions for each record, i.e. as each new reference angle is encountered the vanes will be moved to the positions that will define the electron aperture needed to provide the desired electron dosage corresponding to the arc segment. When the next reference angle is encountered the vanes will be moved to new positions to define the next electron aperture needed to provide the desired electron dosage corresponding to the next arc segment. Computer 11, the controller of the linear accelerator 1, monitors the treatment angle of the linear accelerator as the head 3 and collimator 5 rotate through the selected treatment arc. Computer 11 provides this treatment angle data to host controller 12 (i.e. computer 12) by transmitting an RS-232 asynchronous serial signal through cable 16 to a standard asynchronous serial port of host controller 12 (see FIG. 1). Host controller 12 executes a computer program which we have named the "Remote Collimator Control Program" or "Control Program". The Control Program was programmed in the Basic programming language (QuickBasic 4.0). A source code listing of the Control Program is filed with, and as part of, this patent application and is incorporated by reference into this specification. The Control Program monitors the treatment angle data received from computer 11 via cable 16. In this manner the host controller 12 is kept informed of the current treatment angle and can detect reference angles which are encountered by linear accelerator rotation. For each treatment angle that is a reference angle, the Control Program converts the vane position data corresponding to the arc segment identified by the reference angle into error checked communications packets (one packet for each local controller) and sends the communications packets (i.e. the vane position data) to collimator 5 via cable 17 and infra-red transceiver 6b. The Control Program appends a CRC value to the packet based upon the data values of the packet. The invention is not limited to the specific Control Program. Other computer programs can be developed to serve as the Control Program. Changes within the scope of our invention can be made to the Control Program. The Control Program in combination with the necessary hardware represents a means for monitoring the treatment angle of the linear accelerator rotation (i.e. during rotation of head 3 and collimator 5 along the arc of rotation or treatment arc) to detect reference angles when encountered by such rotation and for transmitting to the collimator vane position data for each reference angles encountered during linear accelerator rotation (the "vane position data transmission means"). The vane position data for a reference angle is the vane position data for the arc segment identified by the reference angle. The communications packets (i.e. vane position data) for the reference angle (and corresponding arc segment) are transmitted by infra-red transceiver 6b to infra-red transceiver 6a in collimator 5. Each communications packet is sent by host controller 12 to transceiver 6b in the form of an RS-232 asynchronous serial signal through cable 17 (see FIG. 1). The infra-red transceiver 6b includes an asynchronous converter/monitor and an infra-red converter (see FIG. 5). The asynchronous converter/monitor converts the RS-232 signal into a 0-5 v TTL signal (transistor/transistor logic signal). The infra-red converter converts the TTL signal into an infra-red signal for noncontact transmission to the infra-red transceiver 6a in collimator 5. Infra-red transceivers 6a and 6b are capable of transmitting infra-red signals to each other and receiving infra-red signals from each other as indicated by arrows 18 in FIG. 1. Transceiver 6a also includes an asynchronous converter/monitor and an infra-red converter as depicted in FIG. 7. In this manner noncontact transmission and reception can be conducted. Each infra-red transceiver includes distributed sensors to receive infra red transmissions from the other transceiver. Although only one infra-red transceiver 6b is shown in FIG. 1, a plurality of infra-red transceivers 6b should be secured to wall 14 within the treatment room (i.e. the radiation therapy room) at different locations in position to transmit to and receive from transceiver 6a as it rotates along the selected treatment arc. The transceivers 6b can, in an alternative embodiment of the invention, be positioned in the plane defined by the selected treatment arc. This alternative would require that transceiver 6a be located on a different side of collimator 5 and that the transceivers 6b be located on a different wall (i.e. on one or both of the walls perpendicular to wall 14). The infra-red signals (i.e. data packets) between transceiver 6a and 6b are amplitude modulated infra-red signals using a 100 khz carrier frequency. By using infra-red signals or some other noncontact form of communication (e.g., AM/FM radio frequency transmission) between host controller 12 and collimator 5, the need to add additional communication wiring to the rotating linear accelerator and collimator is eliminated. Infra-red transceiver 6a converts the communications packets (i.e. vane position data) received from infra-red transceiver 6b through the infra-red converter and asynchronous converter/monitor of transceiver 6a and passes this vane position data to the local controllers (i.e. the six three-axis processors) as shown in FIG. 7. As each communication packet reaches its destination local controller, the local controller (i.e. three-axis processor) checks the integrity of the communications packet by calculating an incoming CRC value (cyclic redundancy check value) based on the data of the received packet and compares this value to the transmitted CRC value appended to the received packet. If the CRC comparison is valid, an acknowledgment packet is sent to the host controller 12 and the process of moving the vanes controlled by the local controller to their new positions begins. After all three vanes of a local controller have reached their new position an additional acknowledgment packet is sent to the host controller 12 for vane position verification. Each local controller includes a computer program, the "Local Controller Program," embodied as firmware in the EPROM of the local controller. The Local Controller Program was programmed in Assembly language. A source code listing of the Local Controller Program is filed with, and as part of, this patent application and is incorporated by reference into this specification. The Local Controller Program provides negative feedback loop control for up to four simultaneous signals. FIG. 9 depicts a flow chart of a single axis loop. The Local Controller Program is a general purpose multiple axis control program. The parameters of the control loops of the Local Controller Program have been set to optimum values for the collimator hardware. These parameters are stored in permanent memory (EEPROM) of the local controller. The EEPROM embedded in the Local Controller is refreshed or rewritten on a periodic basis by the Local Controller Program to offset the gradual data retention degradation caused by exposure to elevated radiation levels. The Local Controller Program includes communication, monitoring and error detection systems. The Local Controller Program includes a time averaged 12-bit A/D (analog/digital) conversion routine to convert analog data from the position monitoring means (e.g. potentiometer) to 12 bit digital data. The output is a 12-bit PWM (pulse width modulated) signal derived from the system clock. Polarity is determined external to the PWM signal for additional resolution and control. Simple digital braking is provided for high inertial systems. Two inputs per channel allow for auto-calibration or position checking. The six local controllers (i.e. the six three-axis processors) are located on a network. The parvNET token passing network of the parvus Corporation can be used for this purpose. As previously described the local controllers (three-axis processors) control and monitor the eighteen vanes (and the eighteen vane movement means corresponding to the vanes). See FIG. 7. The local controllers are networked as nodes on a common network. The host controller and display panel are also nodes on the network. Commands are sent to each three-axis processor on the network at a speed 19200 baud with an average packet length of 13 bytes for a total 18 vane collimator access time of 40 ms per setting. Physical communications are sent using standard RS-232C signals and encoded infra-red pulses to allow minimum modification to Varian's 2100C. There are parvNET registers in the Local Controller Program that are specifically defined for this application. They are defined only for this application and have no bearing on other applications. All unmentioned parvNET registers follow the standard definitions found in the parvNET PROTOCOL STANDARD (STD-0003x-01) of the parvus Corporation of Salt Lake City, Utah. The definitions of the registers specific to this application are given in Table 4 below. TABLE 4 ______________________________________ Register Definition ______________________________________ Register: 10 Name: Desired Position Read: 8 total bytes Bytes 0-1 - Desired Position Axis 1 2-3 - Desired Position Axis 2 4-5 - Desired Position Axis 3 6-7 - Desired Position Axis 4 Write: 8 total bytes Bytes 0-1 - Desired Position Axis 1 2-3 - Desired Position Axis 2 4-5 - Desired Position Axis 3 6-7 - Desired Position Axis 4 Initialize: All bytes are set to zero as default. The on-board permanent memory is read, checksum verification is performed, and then if a proper checksum exists then all bytes are updated from the permanent memory values. The DESIRED POSITION register is the location in units that the negative feedback loop for the particular axis must try to achieve. The 12-bit right justified value stored for each axis can only be modified by the user. 2. Register: 20 Name: Actual position Read: 8 total bytes Bytes 0-1 - Actual Position Axis 1 2-3 - Actual Position Axis 2 4-5 - Actual Position Axis 3 6-7 - Actual Position Axis 4 Write 8 total bytes Bytes 0-1 - Actual Position Axis 1 2-3 - Actual Position Axis 2 4-5 - Actual Position Axis 3 6-7 - Actual Position Axis 4 Initialize: The A/D converters are read and entered into position calculations and stored in this register. The ACTUAL POSITION is the calculated 12-bit right justified value derived from the A/D converter connected to each axis. This value is compared to the DESIRED POSITION to determine the position error magnitude and polarity. 3. Register: 30 Name: Raw A/D Values Read: 4 total bytes Bytes 0 - Desired Position Axis 1 1 - Desired Position Axis 2 2 - Desired Position Axis 3 3 - Desired Position Axis 4 Write: None Initialize: The A/D converters are read and stored in this register The 8-bit values stored at these register locations are the actual values converted from the on-board A/D. 4. Register: 40 Name: Deadband Read: 4 total bytes Bytes 0 - Deadband for Axis 1 1 - Deadband for Axis 2 2 - Deadband for Axis 3 3 - Deadband for Axis 4 Write: 4 total bytes Bytes 0 - Deadband for Axis 1 1 - Deadband for Axis 2 2 - Deadband for Axis 3 3 - Deadband for Axis 4 Initialize: All bytes are set to zero as default. The on-board permanent memory is read, checksum verification is performed, and then if a proper checksum exists then all bytes are updated from those permanent memory values. Any calculated error derived from the actual and desired positions that is greater the deadband value will cause an appropriate adjustment in the pwm output value. If the error is less than the DEADBAND then the output value is zeroed with the polarity set to positive. 5. Register: 50 Name: Maximum Velocity Read: 8 total bytes Bytes 0-1 - Desired Position Axis 1 2-3 - Desired Position Axis 2 4-5 - Desired Position Axis 3 6-7 - Desired Position Axis 4 Write: 8 total bytes Bytes 0-1 - Desired Position Axis 1 2-3 - Desired Position Axis 2 4-5 - Desired Position Axis 3 6-7 - Desired Position Axis 4 Initialize: All bytes are set to zero as default. The on-board permanent memory is read, checksum verification is performed, and then if a proper checksum exists then all bytes are updated from those permanent memory values. Each scan cycle the position error is calculated and an appropriate adjustment is made to the output value. The maximum rate at which that adjustment can be made is limited by the MAXIMUM VELOCITY value. A value of zero is considered as no-limit. 6. Register: 60 Name: Maximum Acceleration/Deceleration Read: 8 total bytes Bytes 0-1 - Maximum Acc/Decc for Axis 1 2-3 - Maximum Acc/Decc for Axis 2 4-5 - Maximum Acc/Decc for Axis 3 6-7 - Maximum Acc/Decc for Axis 4 Write: 8 total bytes Bytes 0-1 - Maximum Acc/Decc for Axis 1 2-3 - Maximum Acc/Decc for Axis 2 4-5 - Maximum Acc/Decc for Axis 3 6-7 - Maximum Acc/Decc for Axis 4 Initialize: All bytes are set to zero as default. The on-board permanent memory is read, checksum verification is performed, and then if a proper checksum exists then all bytes are updated from those permanent memory values. For each scan cycle the position is calculated and an appropriate adjustment is made to the output value. The rate is limited by the MAXIMUM VELOCITY value. Its rate of increase is controlled by the MAXIMUM ACC/DECC value. A value of zero is considered no limit. Register: 70 Name: Response Time Read: 4 total bytes Bytes 0 - Response Time for Axis 1 1 - Response Time for Axis 2 2 - Response Time for Axis 3 3 - Response Time for Axis 4 Write: 4 total bytes Bytes 0 - Response Time for Axis 1 1 - Response Time for Axis 2 2 - Response Time for Axis 3 3 - Response Time for Axis 4 Initialize: All bytes are set to zero as default. The on-board permanent memory is read, checksum verification is performed, and then if a proper checksum exists then all bytes are updated from those permanent memory values. The position error is calculated at a fixed speed or rate. That rate is set by the RESPONSE TIME In milliseconds. For each RESPONSE TIME value in milliseconds the system calculates the error and adjusts the output value. 8. Register: 80 Name: Actual Output Value Read: 8 total bytes Bytes 0-1 - Actual Output for Axis 1 2-3 - Actual Output for Axis 2 4-5 - Actual Output for Axis 3 6-7 - Actual Output for Axis 4 Write: None Initialize: All outputs are set to zero prior to the first scan cycle. The final output of the control loop is a PWM signal. That signal is a 12-bit right justified value that determines `ON` time of total 12-bit count. The system clock is used to drive the PWM. The ACTUAL OUTPUT is provided for reference purposes only. 9. Register: 90 Register: A0 Register: B0 Register: C0 Register: D0 Name: Undefined Register Read: 16 total bytes Bytes 0-F - Undefined Write: 16 total bytes Bytes 0-F - Undefined Initialize: All bytes are set to zero as default. The on-board permanent memory is read, checksum verification is performed, and then if a proper checksum exists then all bytes are updated from those permanent memory values. The above-identified registers are currently undefined in the Local Controller Program. As features and functions are added to the application these registers may be used. Unused registers are 16 bytes in length for both read and write. They can be used for temporary storage. ______________________________________ The foregoing description of invention so fully reveals the general nature of the invention (including apparatus and methods) that others can readily modify such invention and/or adapt it for various applications without departing from its generic concept, and, therefore such adaptations and modifications should be and are intended to be comprehended within the meaning and range of equivalents of the following claims, which claims define subject matter regarded by us to be our invention.