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UBC Theses and Dissertations

An automatic dose plotter Hardy, William Lyle 1961

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AN AUTOMATIC DOSE PLOTTER by WILLIAM LYLE HARDY B. A. Sc., University of British Columbia, 1959 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE • r t in the Department of PHYSICS We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA October, 1961. In presenting t h i s thes is i n p a r t i a l fu l f i lment of the requirements for an advanced degree at the Univers i ty of B r i t i s h Columbia, I agree that the L ibrary s h a l l make i t f r e e l y ava i lab le for reference and study. I further agree that permission for extensive copying of th i s thes is for scho lar ly purposes may be granted by the Head of my Department or by h i s representat ives . It Is understood that copying or p u b l i c a t i o n of th i s thes is for f i n a n c i a l gain s h a l l not be allowed without my wri t ten permiss ion. Department of Physics  The Univers i ty of B r i t i s h Columbia, Vancouver 8 , Canada. Date October 2nd. 1961 ABSTRACT This thesis describes the design, operation, and performance of an automatic system which can be used to plot isodose curves such as are used in radiation therapy. The system consists of an apparatus to move an ionization chamber by remote control along rectangular axes so that the position of curves of equal per cent dose relative to any chosen reference point in the field are found and recorded automatically. The apparatus consists of a commerical servo system which operates from the output of an accurate dose comparator to control motion of the ionization chamber along one axis so that it hunts to locate the set per cent dose in the field to be plotted. A second manually-controlled drive system is used to provide a scan motion of the ionization chamber along the second axis. The drive systems are linked by two pairs of synchros to a plotting pen which records the path of the probe. The system is capable of producing an adequate set of isodose curves for a field in two hours or less with an accuracy of better than one-half per cent of the maximum dose in the field. - i l l -TABLE OF CONTENTS PAGE Abstract 1 1 Table of Contents iii List of illustrations iv Acknowledgements v Introduction 1 Functional description of the automatic plotter 5 The probe drive 7 The probe-drive control system <. 9 The pan drive 1 4 Operation » ki Discussion 20 Bibliography 23 - iv -LIST OF ILLUSTRATIONS Figure To follow page 1. Block diagram of automatic plotting system 5 2. Photograph of probe drive 7 3. Wiring diagram of probe-drive control system 9 4. Photograph of control-system housing 13 5. Photograph of pan drive 14 6. Photograph of probe drive and tank in position under a radiation source , 16 7. Photograph of comparator, control system and pen drive 16 8. Sample isodose curve as traced 17 9. Complete isodose chart as traced 18 10. Comparison of isodose curves produced by the automatic plotter and by point-by-point measurement 20 - v -ACKNOWLEDGEMENTS The apparatus which Is described in this thesis was constructed while the author was on a National Cancer Institute Fellowship at the British Columbia Cancer Institute. Financial support for the project was gratefully received from the Order of the Eastern Star of British Columbia. The writer would like to acknowledge the assistance of members of the Staff who have contributed. Much of the careful machine work was done by Mr. J. F . Brydle. The photographs were proficiently produced by Mr. P. T . Knowlden. The author is particularly grateful to Dr. H. F . Batho for supervising the research and assisting with the writing. INTRODUCTION In radiotherapy, It is necessary to be able to determine the dose at various points in the tissue being irradiated. Since It Is not possible to make dose measurements directly in the patient, the data required for dose estimation must be measured in advance in a tissue equivalent medium, preferably water. These data are presented in the form of isodose curves. From these, the doses at required points in the treated tissue can be estimated. The Isodose curves are expressed as percentages of the dose at a fixed reference point in the field, i.e., all points on a given isodose curve receive the same percentage of the dose at the reference point. To measure the relative doses used in preparing isodose curves, it is convenient to use a dose comparator. This Instrument permits the comparison of the dose rate at any point in the field to the dose rate at a fixed reference point. A comparator consists of two radiation measuring devices, a ratio bridge to compare the outputs from these measuring devices, and a,-null detector to indicate when the bridge is balanced. The comparison is independent of dose-rate fluctuations. Several comparators have been reported in the literature. 1 »2,3,4 In the usual comparator, two small ionization chambers with associated DC amplifiers are used to measure the radiation. * * If the ionization chambers consist of small air cavities surrounded by air-equivalent walls, they measure dose in roentgens. The roentgen is that exposure dose of X-or gamma radiation such that the associated corpuscular emission per 0.001293 grams of air produces, in air, ions carrying one electrostatic unit of electricity of either sign. -2 -The first or "measure" ionization chamber (probe) is located at the point at which the measurement is required. The second or "monitor" chamber is placed at a convenient fixed point in the field. The DC amplifiers are designed to have linear responses to the ionization current inputs. The voltage outputs from the amplifiers are compared on the ratio bridge which can be scaled to read directly the dose at the point of measurement as a percentage of the dose at any chosen reference point in the field. When the ratio bridge is balanced, the percentage dose on the measure probe is read from the dials of the bridge. To represent a radiation field with sufficient detail, it is necessary to measure the dose at some two hundred to three hundred points (usually ten or more Isodose curves, each requiring twenty to twenty-five points to determine the curve). Since as many as thlrty^to forty different field sizes may be used with a single therapy unit, it is obvious that the preparation of the necessary isodose charts involves considerable work. The time and labour required can be reduced greatly by the use of a well-designed automatic plotter. The dose comparator described above provides an obvious basis for automatic plotting. The dials of the ratio bridge can be set to a required per-centage depth dose and a signal from the null detector can be used as the input to a servo system to move the probe along one axis to a point at which its output produces a balance in the ratio bridge. Scanning motion at right angles to the automatic hunt is provided by an independent drive system. The probe is coupled to a pen which traces on a plotting table the path of the probe. / - 3 -Several automatic plotters have been reported. The earliest, which was built by Kemp,^ compared integrated doses by comparing the charges accumulated on condensers. His servo system was controlled by a light beam reflected from a galvanometer which served as null detector. The light beam fell on either of two photo-cells depending on the direction of deflection of the galyanometer't Each photo-cell was connected to a relay which controlled the power to a motor to move the probe in.the radiation field. The direction of motion of the probe depended on which relay closed. Kemp plotted on a rectangular coordinate system using the automatic control to move the probe along one axis and a cycled control to move along the second axis. The direction of automatic hunt could be along either axis as was most suitable. All subsequent plotters which have been reported have used dose-rate comparators, rather than lntegrated-dose comparators, as a source of input signal to the servo system, and all have used servo motors to drive the probe. The plotters described by Laughlin and Davies,**. and by Berman, Laughlin, Yonemitsue, and Vacirica used an orthogonal plotting system, but the direction of automatic hunt could not be changed from one axis to the other. The shortcoming of this design will be discussed in a later section. 7 The plotter described by Mauchel and Johns differs from all others reported in that it employs a polar coordinate system with radial hunt instead of a rectangular coordinate system. The superiority of a rectangular plotting system . will be pointed out in the body of the thesis. 9 10 Cole ' has recently described a fully automatic plotter which operates - 4 -on a rectangular coordinate system. The direction of automatic hunt may be along either axis; the equipment is cycled to select the axis along which the sensitivity will be the greater. This is the most elaborate plotter yet reported. Its description appeared after the design of the plotter which forms the subject of this thesis was completed. The automatic plotter to be described here was designed to operate on a rectangular coordinate system using available servo-system components and a previously constructed comparator as the basis of automatic control. The 4 comparator has already been described by Mibus . It is capable of measuring percentage dose at points in a radiation field with an accuracy of one-quarter per cent. The automatic plotting system was designed to retain this accuracy in plotting isodose curves. This plotter differs from previous plotters, in particular, in that the servo motor does not drive the probe directly but is used to control power to a probe-drive motor. The advantage of this method will be discussed later. - 5 -FUNCTIONAL DESCRIPTION OF THE AUTOMATIC PLOTTER Figure 1 is a block diagram showing the units comprising the auto-matic plotter. The comparator consists of those blocks enclosed in the dotted outline, that Is, measure and monitor ionziation chambers connected through DC am-plifiers to a ratio bridge and null detector. To trace a specified isodose curve in the radiation field, the ratio bridge is set to a required percentage dose. If the radiation on the measure probe does not yield a voltage output which balances the bridge, there is an output signal from the null detector. The polarity of this signal depends on the direction of the imbalance,, i.e., whether the measure probe is at a position of higher or lower percentage dose than the value set on the bridge. The signal from the null detector is amplified in the servo amplifier of the automatic drive-control system, and the output is used to control the servo motor. The direction of rotation of the motor, for a given polarity of input signal to the amplifier, can be selected by the "direction-to-hunt" switch which precedes the servo motor in the block diagram. Since the servo system has insufficient torque to drive the probe directly (this will be discussed in a later section), it is used to operate a specially designed single-pole double-throw rotary switch. When one side or the other of this switch is closed by rotation of the motor, one of a pair of doutte-pole single-throw relays is actuated to close the power supply CCM*RATOR| MEASURE ^""1 PROBE T - i . . i i i L . 1 M O N I T O R P R O B E M O N I T O R U M P L F I E R 3= R A T I O BRIDGE =3= • DETECTOR M E A S U R E A M P L I F I E R A U T O M A T I C C O N T R O L S E R V O A M P L F E R D I R E C T I O N • * | T O - H U N T S W I T C H S E R V O I M O T O R RELAY HI S P O T R O T A R Y S W I T C H S P E E D C O N T R O L J H - V DRIVE INTER-C H A N G E SWITCH M A N U A L C O N T R O L DIRECTION C O N T R O L S W I T C H SPEED CONTROL PLOTTING SSJv'n SYSTEM V M O T O R PLOT TEA *|9EICRAT0<I M W S M M l I i Ul FIGURE I BLOCK DIAGRAM OF AUTOMATIC PLOTTER - 6 -circuit of a probe-drive motor. The direction of drive of this motor depends on which relay is closed. Also, there are in the power supply circuit, a speed control, which is a manually operated auto-transformer to control voltage to the motor, and a "horizontal-vertical drive interchange" switch, which is manually operated to place either the horizontal or the vertical probe-drive motor under the control of the automatic system. Thus an imbalance signal from the null detector causes the automatic drive-control system to move the measure probe parallel to the selected coordinate axis to the position of the percentage dose set on the ratio bridge. The second drive motor (i.e.,the one not operated by the automatic system) is controlled manually to provide scanning motion of the probe, that is, to move it perpendicular to the direction of the automatic hunt. The two probe-drive motors are coupled by mechanical linkages to the probe-drive mechanism, and drive synchro receivers which move a pen on a plotting table. - 7 -THE PROBE DRIVE The probe drive Is the mechanical system by means of which the horizontal and vertical drive motors move the measure probe in a vertical plane in a tank of water in which the isodose curves are to be measured.* The essential features of the probe drive can be seen in the photograph of Figure 2 in which the system is shown in position <tt top of the water tank. The mechanism consists of a double-carriage system with the horizontal carriage (A) carrying guides on which the vertical carriage (B) travels. A very straight round rod (C) and a parallel flat bar (D) are fastened between end plates (E and E 1) across the top of the tank. The horizontal carriage is supported on two V-wheels (F and F 1) which ride on the rod, and on a third wheel (G) (a bearing;.race), which rides on the bar. The drive for the horizontal travel is furnished by a long lead-screw (H) which rotates in a floating nut (I) fastened to the carriage. The screw is held in bearing races pressed into the end plates. The floating action of the nut is designed to eliminate the influence of possible bends in the screw on the motion of the probe. The tracks for the vertical carriage are fastened between the lower surface of the horizontal carriage and an end plate near the bottom of the tank. One of the tracks is a straight round rod (J) and the other three are bars (K). The vertical carriage * A remotely-controlled probe drive is essential for measuring isodose curves either manually or automatically since the operator must be physically separated and protected from the source of radiation by concrete or lead-lined walls. Auto-matic plotting imposes no special conditions on the design of the probe drive. TO FOLLOW PAGE 7 - 8 -is guided in essentially the same manner as the horizontal carriage, that is, by two V-wheels running on the rod and two ball races which ride on one of the flat bars. Spring-loaded ball races bearing against the other two flat bars maintain the contact. The drive for the vertical carriage is provided by a lead screw (L) and floating nut similar to that previously described. The nut is attached to the vertical carriage and the screw, which extends from the lower plate up into the horizontal carriage, is held at each end in races. One mitre gear (M)is fixed to the upper end of the drive screw and a second mitre gear, meshing with it, is carried from the horizontal carriage. The square bar(N), which passes freely through a square hole in the centre of the second gear, can be used to transmit torque through the gear pair to drive the vertical lead-screw. This bar is supported in ball races pressed into the horizontal end plates. This arrangement permits vertical drive and is preferable to the alternative of carrying a vertical-drive motor on the horizontal carriage. The square rod (N) and horizontal lead screw (H) are rotated by positive drive systems. A sprocket is attached to each, one at either horizontal end plate. These are driven by light ladder-chain belts from drive shafts located on a platform beneath the tank. Additional sprockets on adjustable arms are used as chain tighteners. The drive shafts are belt-driven by sewing-machine motors. Pulleys have been selected to reduce the shaft speed to about one-sixth the motor speed. Field and armature windings of the motors are brought out separately so that the motors can be reversed. The measure probe (P) is carried from a plate attached to the vertical carriage. Provision is made to bring the electrical leads out from the probe without interfering with merlon of the carriages. The drive system permits motion of the probe either horizontally or vertically, independently and simultaneously, so it can be moved to any position within bounds In the chosen coordinate plane. - 9 -THE PROBE-DRIVE CONTROL SYSTEM As pointed out in the functional description, the probe-drive control system consists of an automatic control to regulate one of the probe-drive motors, either horizontal or vertical, and a manual control to regulate the other motor. The functional components of the system are shown in the block diagram (Figure 1), and a wiring diagram is given in Figure 3. A Brown amplifier, model no. 356413-1, manufactured by the Minneapolis Honeywell Regulator Co., Brown Instrument DVislon, which was already available,, is used as the servo amplifier of the automatic control system. This unit consists of a DC-to-AC converter followed by an AC amplifier, The amplitude of the useful AC output is dependent on the magnitude of the DC input and the phase of the output changes by 180° when the polarity of the DC input is reversed. The sensitivity of the Brown amplifier was measured to determine whether it was sufficient to utilize the full sensitivity of the comparator with which it Is used. The latter has a sensitivity of 0.1 per cent, ije., it is capable of detecting a difference of 0.1 per cent between the percentage dose received by the measure probe and the percentage set on the ratio bridge dials. With the usual operating conditions ,0.1 per cent imbalance yields an output current from the null detector of the comparator of about 1.5 microamperes. The Input resistance of the the Brown amplifier Is about 30 ohms so that 0.1 per cent imbalance corresponds to an input signal of 45 microvolts. It was found by measurement that an input signal of less than VARIAC A L i — HORIZONTAL DRIVE M O T O R L 2 FIGURE 3 PROBE-DRIVE CONTROL SYSTEM WIRING DIAGRAM - 10 -30 microvolts gives sufficient output signal to drive the servo motor which follows the amplifier. Therefore, the servo amplifier has the required sen-sitivity. The servo motor used in the automatic control is Brown motor no. 364949-1 designed to be used with the above amplifier. It is a two-phase induction motor; one phase is supplied from the AC power line, the other phase from the servo amplifier. Therefore, reversal of polarity of the DC input to the amplifier reverses the direction of rotation of the motor. The maximum speed of rotation of the motor, however, is independent of the input to the amplifier. A "direction-to-hunt" switch (S^ in Figure 3) is located between the servo amplifier and motor. It is a manually operated double-pole double-throw switch which is used to interchange the two line-phase leads to the servo motor and, therefore, to select the direction of rotation of the motor for a given polarity of input to the amplifier. The need for this switch will be explained in the section on operation. It had been planned that the probe and plotter would be driven directly by the servo motor. However, the minimum starting torque to drive the system is about 2200 centimetre-grams and the maximum torque developed by the motor with an input of 45 microvolts to the servo amplifier (i.e., 0.1 per cent- imbalance of the comparator) is about 300 centimetre-grams. It would be possible to gear the motor down to provide the required torque (about a 7.5-to-l ratio would be -11 -necessary), but, with the lead-screws selected for the probe drive, the maximum probe speed would be limited to about two centimetres per minute. Since this is inadequate, the design of the system was modified so that the servo motor controls the power supply to an AC motor of adequate torque and power. Initially it was throught that the power control operated by the servo motor should be designed to control both direction'and speed of the probe-drive motor. However, a control device which would perform such a function and which would be completely reliable appeared to be difficult to design. Therefore, it was decided to use the servo motor to control the direction of rotation only of the probe-drive motor, and to use manual control of speed. The control operated by the servo motor is a specially designed single-pole double-throw rotary switch mounted on the motor. It is shbwn following the motor in the block diagram and on top of the motor in Figure 3 . A brass disc is mounted on the servo-motor shaft, and a bakelite disc which carries the pole of the switch slips over the shaft. The two discs are held in contact by an adjustable coil spring. Two flat springs, rigidly mounted on the bakelite disc, restrict its rotation. One spring is located between two contacts (A and B in the diagram) and the other slides in a slot in a third contact (C). The two springs, which fDrm the pole of the switch, are wired together across the bakelite disc. The contact pressure between the bakelite and brass discs is adjusted by com-pressing the coil spring so that when the servo motor rotates the bakelite disc rotates with it until the first flat spring makes contact at either A or B. With the - 12 -bakelite disc then stalled, slippage occurs permitting the servo motor to continue rotating while holding the switch closed. The second flat spring,'restrained in the slot at C, was designed to return the switch to the open position when the servo motor stops rotating. Since the rotary switch contacts were not designed to carry appreciable current, two relays- (A and B of Figure 3) were introduced between the switch and the probe-drive motors. The relays chosen require coil currents of only ten milliamperes. When the rotary switch closes to either A or B, the corresponding relay is actuated. The relays are wired to control the direction of rotation of the probe-drive motor operated by the automatic system. The leads from the armature winding of the motor are connected to the two poles of each relay. When both relays open (i.e., no signal to the servo amplifier) the motor receives no power; when either relay closes the armature is connected in series with the field of the motor. The direction of the connection and, therefore, the direction of rotation of the mctor depends on which relay closes. The speed control shown in the block diagram is an auto-transformer (Variac A of Figure 3) which is manually operated to determine the voltage to the motor. With this design of the automatic control system the direction of rotation of the servo motor and, therefore, of the probe-drive motor depends on the polarity of the signal from the comparator. The direction of rotation for a given polarity of signal can be selected by the "direction-to-hunt" switch. The speed of rotation of the drive motor depends only on the setting of the auto-transformer. - 13 -The second probe-drive motor is controlled manually. The double-pole double-throw switch,(S^ of Figure 3) serves the same function as the two relays in the automatic system, i.e., it reverses the armature winding with respect to the field winding to reverse the direction of rotation of the motor. The speed is controlled by an auto-transforrner (Variac M) as in the automatic system, but, in addition, a variable rheostat placed in series with the auto-transformer provides finer control. An eight-pole switch ( S £ in Figure 3) serves as the "H-V drive inter-change" switch of the block diagram (Figure 1) to connect the automatic control system to either probe-drive motor, and the manual control system to the Pther motor. The need for this switch will be explahed in the section on operation. The units comprising the probe-drive control system are housed in the two cabinets shown in Figure 4. The control panel of the system can be seen in this figure. FIGURE k PHOTOGRAPH OF CONTROL SYSTEM HOUSING - 14 -THE PEN DRIVE The pen-drive system is designed to trace on a plotting table the path of the measure probe in the water tank. A photograph of the system is shown in Figure 5. (This figure also shows the panel of the control system). It is essentially the same in construction as the probe drive and consists of a pair of carriages, one of which carries the pen and rides on tracks which are attached to the other carriage. The carriages are A and B in Figure 5. The penu, however, moves in a horizontal plane over the plotting table whereas the probe moves in a vertical plane in the tank. The "pen," which may be either a ball-point pen or a soft draughting lead, is gravity loaded. The loading used is the minimum required to produce a readable trace. The pen carriages are driven by synchro receivers which are controlled by synchro generators directly coupled to the drive shafts of the probe-drive system. (The generators are located with the drive-motors under the water tank, and the receivers are beneath the plotting table). With the gearing used, each synchro generator rotates at the same speed as the lead screw of the probe drive to which it is coupled. The receivers also are coupled to the pen drive so that each lead screw of this system rotates with the same velocity as the synchro receiver by which it is driven. (The sprockets driving the lead screws are shown at C and C in Figure 5.) Therefore, provided,there is no slippage between the synchro TO FOLLOW PAGE lk - 15 -generators and receivers,the pen must reproduce the motion of the probe exactly. Errors in the position of the pern due to slippage of the synchro system can result either from insufficient torque developed by the receiver to drive the plotting system or from insufficient torque to overcome inertial loading during periods of acceleration. By direct measurement, it was found that the plotter requires a starting torque of about 125 gram-centimetres and that the synchros will transmit a torque of about 1200 gram-centimetres. Therefore, the synchros are more than adequate to overcome static friction. Inertial loading was kept to a minimum by making the plotting system as light as possible consistent with rigidity. Where practical,aluminum and acrylic plastic were used and tubing was substituted for rod. With this design, most of the inertia is due to the synchro receivers themselves. The inertia of a receiver is large enough to cause slippage when a probe-drive motor is reversed at full power, but in plotting isodose curves there is no reason toXuse high accelerations, and with reasonable care no slippage does occur. -16 -OPERATION Figures 6 and 7 show the complete equipment set up for plotting isodose curves. Figure 6 shows the water tank with the probe and the probe drive in position under the radiation source. * This equipment is in a heavily shielded room to protect the opsrator. The cables leading from the probe and probe drive to the control area outside the room can be seen in the photograph. The arrangement of the equipment in the control area is shown in Figure 7. In this photograph, A is the control panel of the radiation unit, B is the comparator, C and D are the housings for the control equipment, and E is the plotting table with the pen drive in position over a partially plotted isodose curve. To prepare isodose curves with the automatic plotter, the plane of the probe drive is first set to correspond with the plane in the radiation field in which the isodose curves are required. Power is switched on at the plotter control panel, -and the probe is driven under manual control to the position chosen as the reference point. With the shutter of the radiation source closed, the measure, monitor, and detector amplifier circuits are individually adjusted to zero. This is done using the galvanometer built into the comparator. The shutter is then opened to expose the measure probe (and the monitor probe where one is used ). ** * The radiation unit shown in the photograph is a "TheratronF" cobalt 60 teletherapy unit manufactured by Atomic Energy of Canada Linited. It is designed for rotational therapy. ** The monitor probe is required only when the output of the radiation source varies with time. Under this circumstance the monitor probe provides a reference standard which permits the measurement of percentage dose independent of dose rate. For measurements on therapy units with constant output? primarily those using radioactive sources, the output of the measure amplifier may be compared with any constant reference voltage. In this case the monitor ionization chamber can be eliminated. The monitor amplifier , however, may still be used, since a battery in series with the input resistance of the amplifier produces across the output an ideal reference voltage with almost zero impedance. (In the set-up of Figure 6, the monitor probe Is not being used.) IN POSITION UNDER RADIATION SOURCE FIGURE 7 PHOTOGRAPH OF COMPARATOR, CONTROL SYSTEMS, AND PEN DRIVE - 17 -With the ratio idials set to 100 per cent, the reference voltage with which the output from the measure circuit is compared is varied until the ratio bridge is balanced. Set-ting the zero and the 100 per cent establishes two points on the ratio-bridge scale, and since the scale Is linear any other percentage dose can then be read or set. A sheet <of paper on which the isodose curves are to be plotted is placed on the plotting table under the pen, and a set of coordinate axis is drawn on it using the manual control of the probe drive to move the pen. The two lilies usually drawn are the mid-line of the field and a line corresponding to the surface of the water or at a known distance from it. The equipment is ready for the automatic plotting of an isodose curve when the percentage corresponding to the required isodose curve is set on the ratio-bridge dials, and a switch on the comparator is ..thrown to "automatic" position, thus replacing the galvanometer of the null indicator by the input stage of the servo amplifier. To describe the detailed operation of plotting an isodose curve, it is convenient to refer to the completed curve of Figure 8i Between points A and B of this figure the automatic system should control the horizontal drive motion of the probe, since the direction of automatic hunt should, for maximum sensitivity, be In the direction approximately at right angles toflie isodose curve. ., In addition, between points A and B the direction-to-hunt switch (Figure 1) mist be thrown so that the measure probe moves to the left to locate higher dose. The voltage supplied to the horizontal drive motor ( i .e . , the motor under servo control) is increased until the probe and pen move moderately rapidly. When the probe moves to the position TO FOLLOW PAGE 17 C O B A L T * 0 RADIAT ION SOURCE T O S K I N DISTANCE - 60 cn F I G U R E 3 ISODOSE C U R V E P L O T T E D B V AUTOMATIC P L O T T E R - 18 -of the required Isodose curve and begins to hunt across it, the voltage supplied to the drive motor is reduced until the probe barely hunta. Then, with the switch on the manual system thrown to the "down" position, the voltage from the manual speed-control auto-transformer is increased until the probe is driven downward at a suitable speed. The voltages supplied to the two motors are adjusted during the plot to ensure that the probe continues to hunt across the Isodose curve with small amplitude. At point, B, where the probe begins; to turn the lower corner of the isodose curve, the voltage supplied to the manual drive motor is reduced until the probe barely moves. At a point where the slope of the curve approaches 4 5 ° , the voltages to both drive motors are reduced to zero, and the H-V drive Interchange switch is thrown to transfer control of the vertical motor to the automatic system and the horizontal motor to the manual system. With the direction-to-drive switch thrown so that the probe Is driven upwards to find increasing percentage dose, the voltage supplied to the vertical drive is increased until the probe again begins to hunt. The horizontal drive motor is then given power and the bottom of the„ isodose curve plotted. At the next corner (point C , Figure 8), the control of the drive motors is again interchanged and the third leg of the curve is plotted. In this region the probe must hunt to the right for higher dose. On completing this plot, a second percentage dose may be set on the ratio-bridge dials and another curve plotted. A complete plot of isodose curves for one radiation field is shown in Figure 9. During plotting, the zero and 100 par cent settings of the comparator must be TO FOLLOW PAGE 18 COBALT 6 0 RADIATION SOURCE TO SKIN DISTANCE - 60 CM F I G U R E 9 I S O D O S E C U R V E S F O R 10 * I O c n F I E L D - 19 -checked frequently. The frequency of the checking depends on the stability of the comparator. In general, the detector and measure circuit zeroes are verified more often when small percentage doses are being run, since in this case errors in the zeroes correspond to larger percentages of the measured values. The number of curves shown in Figure 9 represents the radiation field adequately for practical use in radiotherapy. Because of the crowding of the curves on the sides of the field, not all curves are run to the surface. For practical application the curves are smoothed and inked. - 20 -DISCUSSION To check the overall accuracy of the completed automatic plotter, isodose curves plotted with this equipment were compared with points obtained with the same comparator using manual control. This comparison is shown in Figure 10 . The two curves of this figure were obtained by smoothing the traces produced by the automatic plotter. The points shown on the diagram were obtained with the same comparator and with the same setup of the probe in the radiation field, but the probe was driven under manual control to locate the points on the isodose curves. The galvanometer of the comparator circuit was used as the null indicator to show when the ratio bridge was balanced, i.e., to show when the probe was located on the isodose curve. The position of the probe was determined by means of revolution counters connected to the probe-drive system by the synchro generators and receivers. The comparator has an accuracy of about one-quarter per cent. It was the stated object in this project to design an automatic plotting system of comparable accuracy. If this has been accomplished, no points should differ from the smooth curves by as much as one-half per cent. The dose gradient (as determined from the complete isodose curves for the field) over the central portion of the thirty per cent isodose curve is about one-quarter per cent per millimetret Over this region, therefore, the points should not be more than two millimetres from the smooth curve. For corresponding accuracy, points should not be more than one millimetre from the central portion of the smoothed sixty per cent curve. It eo TO FOLLOW PAGE 20 C O B A L T R A D I A T I O N SOURCE TO SURFACE DISTANCE 60 cm FIELD SIZE - 10 X|0 cm 5 cm F I G U R E 10 C O M P A R I S O N O F A U T O M A T I C P L O T T I N G W I T H P O I N T - B Y - P O I N T M E A S U R E M E N T S Points obtained with comparator and revolution counters Curve obtained with automatic plotter (smoothed) -21 -Is seen that over these regions the plotter does meet the original specifications regarding accuracy. In fact, in Figure 10, the maximum departure of any point from the horizontal sections of the curves corresponds to less than one-quarter per cent. Over .the lateral portions of the curves the accuracy of the plotter cannot be checked adequately since one-half per cent error corresponds to about one-eighth millimetre. This is beyond the sensitivity of the revolution counters used to read the positions of the points. It should be emphasized that to obtain the required accuracy, the auto-matic system must "hunt" across the isodose curve. Otherwise, there is no proof that the measure probe has found the required curve. From this standpoint, the control system used in this plotter, in which the direction of hunt only is determined by the error signal from the comparator, and the voltage to the drive motor is independent of the error signal, is an advantage. With a voltage supply to the drive motor which is reduced as the probe approaches the curve, the driving power may be either excessive when the probe is remote from the curve or insuf-ficient to cause the probe to hunt in the vicinity of the curve. These difficulties are both avoided with the system used in this plotter. For maximum sensitivity, the direction of automatic hunt must be in the direction of thejdose gradient, l .e. , at right angles to the isodose curve. This condition can be most nearly satisfied by using a rectangular coordinate system with a choice of horizontal or vertical hunt. Only at the corners of the curve (regions B and C of Figure 8) is there appreciable departure from the optimum - 22 -direction. With radial hunting on a polar coordinate system, as used by Mauchel and Johns^, the direction of the hunt over a considerable part is almost parallel to the isodose curve. The same criticism applies to a rectangular system em-6 8 ployiug vertical hunt only as was used by Laughlin and co-workers ' . In addition, in their system, failure to provide for reversal of the "direction-to-hunt" along the vertical axis made it impossible to trace the upper lateral portions i d the isodose curves. The design and operation of the present plotter has proven to be com-pletely satisfactory. In operation the plotter has been remarkably trouble-free; no mechanical or electrical modifications of the original design have been found necessary. Some fifty isodose charts (i.e., about six hundred isodose curves) have been prepared with it to date. The tracings obtained are accurately repro-ducible. Isodose charts which formerly required about ten hours to prepare by point-by-point measurements can be traced with the automatic plotter in two or two and one-half hours . - 23 -BIBLIOGRAPHY Kemp, L.A. W., "Direct-reading x-ray intensity comparator; radiological and physical applications". Brit. J. Radiol., 19, 233-242 (1946a) Mauchel, G. A., Epp, E. R., and Johns, H.E., "A self-balancing device for the measurement of ionization current ratios". Brit. J.Radiol., 28, 50-53 (1955) Tout, E. D., Kelly, J.P., Lucas, A. C., and Furno, E. J., "A comparator chamber from commercially available components". Am. J. Roentgenol. Rad. Therapy, and Nuclear Med., 75, 573-580 (1956) Mibus, S. A., "An x-ray dose comparator". Unpublished Master's Thesis, the University of British Columbia (1956) Kemp, L.A. W., "The exploration of x-ray dose distributions; an automatic method". Brit. J. Radiol., 19, 488-501 (1946b) Laughlin, J. S., and Davies, W. D., "Procedure in dose distribution measurement of 25 Mev x-rays". Science 111, 514-516 (1950) Mauchel, G. A., and Johns, H. E., "Automatic isodose plotter". Nucleonics, 12, No. 12, 50-51 (1954) B.erman, M., Laughlin, J., Yonemitsu, M., and Vacirica, S., "Automatic Isodose recorder". Rev. Sc. Instr. 26, 328 (1955) Cole, A., "An improved automatic signal plotter". Radiology 74, 112 (Jan. 1960). Cole, A., "An automatic constant signal plotter". Rev. Sc. Instr. 31, 539 (May 1960) 


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