US3090551A - Automatic dose computer - Google Patents

Description

May 21, 1963 R. G. BAKER ETAL AUTOMATIC DOSE COMPUTER Filed Feb. 26, 1960 3 Sheets-Sheet 1 w ST@ May 21, 1963 R. G. BAKER ETAL AUTOMATIC DOSE COMPUTER 3 Sheets-Sheet 2 Filed Feb. 26. 1960 FIG2C1.

In v ew [5v-5 May 21, 1963 R. G. BAKER ETAL AUTOMATIC DOSE COMPUTER I5 Sheets-Sheet 5 Filed Feb. 26, 1960 FIGA.

FIGB.

United States Patent @ddee Patented May 2l, 1963 Radiotherapy involves the exposure of a patient to a source of radiation in such manner that the radiation falls upon a selected locality of the patients body. The dose of radiation that the patient should absorb at this selected locality is prescribed by the radiotherapist according to the nature of the condition being treated.

1n most radiotherapy centres the required exposure dose for a prescribed absorbed dose at the site of interest is first determined and then, `from the exposure dose rate, the treatment time is calculated. Thus, the technician is required to calculate the treatment time for each given dose, set this time on a timer and switch on the radiation machine lwhich is automatically switched of at the expiry of the set time by switching means associated with the timer.

rIhe calculation of the treatment time is carried out in accordance with the characteristics of the radiation machine and with the technique being employed. Radiotherapy techniques include xed field techniques, in which the field oi radiation remains stationary, and rotation techniques in which the source of radiation rotates around the patient. In fixed field therapy the calculation of treatment time takes into account the intensity of the source, the tield size at the patients skin and the distance between the source and the patients skin. In rotation therapy, the field size at the skin and the distance between source and skin are not constant so that the calculation is fundamentally more difiicult; however it may be simplified in practice -by using the concept of tumor-air ratio and so Afar as the technician is concerned the calculation required for determining the treatment time in rotation therapy is similar to that for fixed iield therapy except that he is concerned with tumor-air ratio instead of field size.

Whether a fixed eld or a rotation technique is used the task of the technician in making the necessary calculation can be made fairly simple by the compilation of suitable tables correlating the variables involved in such manner that no great mathematical skill is demanded of the technician. However human error cannot be eliminated. On the other hand it cannot be tolerated, for obvious reasons. Therefore it is the practice in well-run hospitals to require that before treatment is commenced all such calculations are checked by someone other than the technician who makes the original calculation. This is often a great nuisance but the alternative is to risk a mistake which might have serious consequences and which would in any event result in a wrong treatment being administered.

Apart from the difficulty arising from the need to avoid human errors the calculation tables with which the technician is provided must be periodically revised to take into account the decay of the radioactive source or the change in exposure rate from the X-ray machine.

It is an object of the present invention to provide a device for computing the time that a patient should be exposed to radiation from a given source of radiation, under a specified radiotherapie technique, in order that he will receive the required exposure dose. For the sake of brevity such a device is referred to hereafter as a dose computer.

It is a further object of the invention to provide a dose computer which can be adjusted t-o take into account changes in the intensity of the source of radiation.

The invention provides a dose computer comprising a first rotatable tapered roller, a second rotatable tapered roller, arranged in axially parallel side-by-side relationship with said first roller and tapering inopposite direction to said first roller, and coupling means for connecting said rollers, said coupling means being linearly displaceable between said rollers, said rollers being tapered substantially in accordance with the equation D feo-1 +6-.,

wherein c is a constant, D is the distance between the axes of the rollers less the separation between the rollers and j'fx) is the radius of one of the rollers at an axial distance x from an origin located at the center of that roller.

One embodiment of the invention is illustrated in the accompanying drawings in which FlG. l shows schematically a dose computer;

FIGS. 2 and 2a show the manner in which the tapered rolls of the dose computer of FIG. l are mounted;

FIGS. 3 and 3a show means for coupling to each other the tapered rolls of the dose computer of FIG. 1;

FG. 4 is a diagram showing the parameters used in calculating an ideal shape for the rollers of the dose computer of FlG. l, and

HG. 5 is a graph showing the maximum percentage deviation from logarithmic scale produced by the use of linearly tapered rollers, as a function of speed range ratio R.

The dose computer' comprises two hardened steel, tapered rollers 1 and 2 rotatably mounted in bearings 3 and lon the sides 5' of a U-shaped frame 6. The frame 6 has been omitted from FIG. il tor the sake of clarity and is shown in FlG. 2 which omits most of the details of FiG. l. The roller 1 is driven at constant speed by a synchronous electric motor 7 through a gear box 8 and gear wheels Si and 10. The roller Z drives a digital counter' lil through gear wheels 12 and 13, the gear wheel 13 being secured to a rotatable shaft 14 to which is also secured a disc i5 carrying a pin 16. The pin 16 engages a slotted arm 17 connected to the counter 1:1 and rotation of the disc 15 causes a reciprocating movement of the arm 17 which serves to operate the counter 1l.

The rollers 1 and 2 are coupled to each other by two hardened steel balls 18 and .12 which are in contact with the rollers 1 and 2 respectively and with each other. For the sake of clarity the manner in which the balls 18 and 19 are mounted between the rollers 1 and 2 has not been shown in FIG. 1 but appears from FIG. 3 which v is a partial sectional view of the dose computer of FIG.

1 along the line III-Hl of FIG. l. The balls 18 and 19 are accommodated in a block 2th which is provided with a cylindrical bore 21 having an annular insert 22 of graphite-loaded nylon. The diameter of the insert 22 is only slightly greater than that of the balls 18 and 19 so as to give a snug tit while penmitting free rotation of the balls. The length of the bore 12d. is somewhat less than the sum of the diameters of the balls y18 and 19 so that each of these balls projects slightly above the face of the block Zit; thus the rollers 1 and 2 contact the balls 1S and 19 respectively without contacting the block 20. It may ybe mentioned at this point that the balls 18 and 19 could v'be replaced by a single ball but that this is not desirable because movement of the block 20 relative to the rollers 1 and 2 would then involve sliding of one face of the ball on the surface of one of the rollers instead of the rolling action at both rollers which is obtained by using two balls. Since sliding gives rise to greater frictional acarrear uw forces than rolling the useful life of the rollers and balls is diminished if only one ball is used. Experience has shown that with two balls the extent of wearing is unappreciable over a period equivalent to two years normal use of the dose computer.

Fl YS. 2 and 2a illustrate the way in which the rollers l and 2 are mounted so as to be in resilient contact with the balls i8 and lil. rThe frame 5 is divided into a movable upper `frame member 5l and a fixed lower trame member 52 hingedly connected by hinge members 53. Coil springs 54 are secured to the sides 5 of the trame 6 and urge the upper frame member 5l towards the lower `frame member 52. The bearings 3 of the roller l are mounted in the upper frame member 51 and the bearings 4 of the roller 2 in the lower frame member 52 so the action of the springs Se urges the rollers l and 2 towards one another. Thereby the rollers l and 2 are kept in rolling contact with the balls l and 19.

The block has a threaded bore 23 at right angles to the bore 2l. A coarse screw 24 passes in threaded engagement through the bore 23 and is mounted in bearings 25 in the sides 5 of the frame 6. The screw 24 is rotatable by means or" a knurled knob 216` fixed thereto and upon rotation serves to move the block 20', with the balls 1S and li?, linearly between the rollers 1 and 2. The block 20 is prevented from rotating with the screw 23 by means of a lixed rod 27 extending between the sides 5 of the frame 6 and fitting into a recess 27a located on the back -face or the block so that the block 20 slides along the rod 27.

The block 20 carries a pointer 28 that moves over a calibrated scale 29 which is one of tour scales carried by a drum 30 rotatably mounted in bearings 30a in the sides 5 of the trame 6. Each of these four scales corresponds to a given radiotherapy technique. The scale 29 is graduated in tield sizes ranging from 0 to 400` sq. cm. and is calibrated xed field therapy with a particular radiation machine using a source to patient distance of 80 cm. The knob 3l by means of which the drum 30 is rotated is marked at four points aligned with the four scales to indicate the nature of the technique to which each scale corresponds. In the illustrated embodiment the marking on the knob 3l adjacent the scale 29 indicates the source to patient distance of 80 cm. A dierent scale would be used for operation with a different source to patient distance. Also, one or more of the scales on the drum 30 may be calibrated in terms of tumor-air ratios -for use in rotation therapy. Selection of one of the four techniques corresponding to the four scales on the drum 30 simultaneously selects, through a linkage 32, an appropriate gear of the gear box S.

It will be seen that the rollers 1 and 2 and the balls 1S and 19 constitu-te the essential parts of a continuously variable speed drive between the gear box S and the counter ll, the ratio of which can be changed by rotation of the knob 26. When using the computer the radiographer merely has to select the appropriate technique by rotation of the knob 3l, bring the pointer 2S to the spot on the scale 29 corresponding to the field size or tumor-air ratio involved, set the prescribed dose on the counter 1l by means of the knob 33 and then switch on the motor 7, simultaneously switching on the radiation machine. When the counter has been driven tor such a length of the time that the prescribed dose has been administered, i.e., the reading on the counter has been reduced to Zero, a switch 34, such as is shown in U.S. Patent No. 1,992,841, to O. C. Roesen, associated with the counter is opened and breaks the circuit of the motor 7, simultaneously shutting off the radiation machine.

The dose computer of the invention is calibrated for use with a particular radiation machine but it is clear that provision has to be made to take into account the decay of the radioactive source or a change in output of the machine. For this purpose there is provided a scale 3S vfixed to the block 20. The pointer 23 is carried by slotted plate 36 which is secured to the scale 35 by means of two screws 37 passing through the slot in the plate 36 and engaging in threaded holes in the scale 35. By a simple screwdriver adjustment the plate 36 and therewith the pointer 28, may thus be displaced relative to the scale 35 so as to change the set distance between the pointer 23 and the balls 18 and 19.

The operation of the device depends upon the scales carried by the drum Sti being logarithmic. Suppose the output rate of rotation W0 is related to the input rate of rotation Wi by WozWieCX where x is the distance of the balls 18 and 19 from a reference point and c is a constant; then if one wishes to change the output speed by a factor eey, one merely -moves the pointer 28 a distance y relative to the balls i3 and 119 to yield Thus corrections `for decay or change in output may be made with one simple adjustment. Any other type of scale would require the construction of a new tield size scale or tumor-air ratio scale every time the exposure rate from the machine altered. Thus the rollers 1 and 2 of the dose computer should be designed so that Let the required shape of roller be represented by f(x) as indicated in FIG. 4 with the origin at the centre and the distance between the rollers axes Dl-A. Then the radii at distance x are f(x) and D-f(x) and V 0 fte) Wi D-f(x) lt is desired that this ratio be eCX. Solving there is obtained D ful-m (l) It would `be very dillicult to machine rollers with a taper given by Equation 1 so it was decided to investigate the possibility of a linear taper with f(x)=al-bx. The ratios WO/Wi lfor the two expressions are These may be made equal at x=0 and X=+x0 by the proper choice of a and b. When this is done it is easily seen that they match at x=-x0. The maximum deviation between the two expressions may be shown to occur where x is approximately itlrxo. At this value of x the differences between the two expressions for W0/ W1 of Equation 2 have been calculated and expressed as a percentage of the exponential expression. This deviation is plotted in FIG. 5 as a function of the overall range of speeds, R, from end of the roller to the other. For speed ratios less than 4:1 the maximum difference is less than 1 percent.

In the embodiment described it was found convenient to have a speed ratio of about 2:1 from one end of the roller to the other. The actual ratio is indicated on the auxiliary scale 3S which ranges from 1.0 to 0.60. When R=2 the maximum difference is 0.13 percent which is entirely satisfactory `for practical operations.

It will be appreciated that if the embodiment described were intended for use in connection with only one radiotherapy technique the gear box 8 and the drum 30 could be dispensed with.

When the dose computer has been in use for a considerable length of time the whole of the decay adjustment provided by the scale 3S will eventually become used up. However it is then only necessary to change the ratio of the gear wheels 12 and 13 in order to make it possible to return the pointer 2S to its initial position with respect to the scale 35.

We claim:

1. A dose computer for a radiation machine comprising a first rotatable tapered roller, a constant speed motor for driving said iirst roller through a gear box at one of a plurality of constant speeds, a second rotatable tapered roller arranged in axially parallel side-byside relationship with said rst roller and complementary thereto, a counter driven by said second roller, coupling means for connecting said rollers, said coupling means being linearly displaceable between said rollers and com prising a pair of balls in rolling contact with each other and in rolling contact with said rollers, means for setting said coupling means at a desired distance from the ends ot said rollers, an indicator carried by said coupling means, a logarithmic scale calibrated in accordance with the characteristics of said radiation machine and a chosen radiotherapy technique, said indicator being movable over said logarithmic scale, the radii of each of said rollers being determined by the equation wherein c is a constant, D is the distance between the axes of the rollers less the separation between the peripheral faces of the rollers and f(x) is the radius of one of the rollers at an axial distance x from an origin located at the centre of that roller.

2. A dose computer according to claim 1 wherein said coupling means comprises a block secured against rotation and having a bore of such diameter as to accommodate said pair of balls with each of the balls projecting slightly beyond a face of the block to contact one 6 of said rollers, said block also having a threaded bore at right angles to said first-mentioned bore, and a rotatable coarse screw passing in threaded engagement through said threaded bore whereby rotation of said coarse screw effects linear displacement of said block between said rollers.

3. A dose computer according to claim 1 which further comprises a first frame member and a second frame member pivotally connected to said rst `frame member and wherein one of said rollers is mounted in said rst frame member and the other of said rollers is mounted in said second frame member and means are provided for resiliently urging said second frame member towards said rst frame member whereby said rollers are held in contact with said balls.

References Cited in the iile of this patent UNITED STATES PATENTS 558,315 Shaw Apr. 14, 1896 755,084 Vermehren Mar. 22, 1904 825,363 Vermehren July 10, 1906 1,992,841 Roesen Feb. 26, 1935 2,159,882 Borden May 23, 1939 2,206,252 Gould July 2, 1940 2,602,338 Opocensky July 8, 1952 2,731,831 Schaefer Jan. 24, 1956 2,801,795 Williams Aug. 6, 1957 2,872,814 Brown Feb. 10, 1959 2,979,256 Cushman Apr. 11, 1961

Claims (1)

1. A DOSE COMPUTER FOR A RADIATION MACHINE COMPRISING A FIRST ROTATABLE TAPERED ROLLER, A CONSTANT SPEED MOTOR FOR DRIVING SAID FIRST ROLLER THROUGH A GEAR BOX AT ONE OF A PLURALITY OF CONSTANT SPEEDS, A SECOND ROTATABLE TAPERED ROLLER ARRANGED IN AXIALLY PARALLEL SIDE-BYSIDE RELATIONSHIP WITH SAID FIRST ROLLER AND COMPLEMENTARY THERETO, A COUNTER DRIVEN BY SAID SECOND ROLLER, COUPLING MEANS FOR CONNECTING SAID ROLLERS, SAID COUPLING MEANS BEING LINEARLY DISPLACEABLE BETWEEN SAID ROLLERS AND COMPRISING A PAIR OF BALLS IN ROLLING CONTACT WITH EACH OTHER AND IN ROLLING CONTACT WITH SAID ROLLERS, MEANS FOR SETTING SAID COUPLING MEANS AT A DESIRED DISTANCE FROM THE ENDS OF SAID ROLLERS, AN INDICATOR CARRIED BY SAID COUPLING MEANS, A LOGARITHMIC SCALE CALIBRATED IN ACCORDANCE WITH THE CHARACTERISTICS OF SAID RADIATION MACHINE AND A CHOSEN RADIOTHERAPY TECHNIQUE, SAID INDICATOR BEING MOVABLE OVER SAID LOGARITHMIC SCALE, THE RADII OF EACH OF SAID ROLLERS BEING DETERMINED BY THE EQUATION

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