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Unclassified Health and Biology University of Cslifornia School of Medicine, San Francisco Department of Radiology far period ending December 31, 1955 Robert S. Stone, LE., Director Atanic Energy Ccmmirrion Contract
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TABU OF CONTENTS I. Beam Shape and Isodose Plotter 111. Preparation for Therapy IV. Activities by Person I. Relative Biological Efficiency of 1 0 0 0 - k ~and 5A. Introduction. Plan of Exposure and Dosimetry B. Determinations Based on the Dose-Survival Curves Determinations Eased on the LD,,,,, and the Determinations Based on the Dose-Response Curves 15of Saccharomyces cerevisiae, Haploid and Diploid 25Killing Time of the Mouse 33for Testicular Weight in the Mouse 11. Effect of Cortisone on the Radiation Reaction of the Rat Lung 42 49111. Regression Analysis Based on Poisson Counts It should be noted that the semiannual progress reports of the Radiological Laboratory do not attempt to describe progress in all of the research progPamG, but only those in which some significant results have been achieved.
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A l l professional personnel of this section except the chief resigned during the previous report period. slow and is not complete a t the time of this writing. s t i l l stands ready for radiobiological experiments and for therapy as soon as the physical calibration is completed. started during the previous report period but could not be completed for lack of adequate instrumentation and personnel. pied much of the pkriod, and a few highlights irre reported here. Replacement has been very This calibration was I. Beam Shape and Isodose Plotter The use of industrial x-ray film f o r point-to-point analysis ofi beam shapes and absorption phenomena by t h i s laboratory has been abandoned because of striations in the emulsion or backing which cause density fluctuations and thus exposure uncertainties of atleast s.the obvious advantages of film are the production of a permanent record and the availability of great resolution. thata replacement for film should produce a permanent record and reasIt w a s considered desirable A scintillation technique was investigated briefly and abandoned because of a large background reading apparently due t o direct irradlation of the photocathode. 5 mm') was connected by light-pipes of various sizes t o the multiplier phototube. Although the light contributed by the crystal could be detected, the response was s m a l l compared to the background. A small piece of anthracene crystal (about A t the same time, a small, guarded ionization chamber vas made with effective volume not in excess of 3 m3. drilling 0.060 in. i n t o a i/a-in. Lucite rod, coating internally with Aquadag, and inserting, coaxial with the drilled hole, the collector Vire and guard electrode in the form of Microdot f 5 O - w "Mini-noise" dim. and 2.0 mm length. large measure due t o the availability of the Microdot coaxial cable (4 rents of the order of 30 esu/cc x min a t the peak of the depth-dose curve i n unit density material, 2 meters from the target. For the 3 mm3chamber, 5 x loe1' amp would be collected, which develops 50 MV across a This chamber was made by Thus, the air volume is approximately a cylinder of 1.5 ma The bunediate ~ u c c e s sof this chamber is in The synchrotron currently produces cavity ionization cur-
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urable, in our case with a vibrating reed electrometer. This potential difference is readily measIt is convenient to connect the chamber to the electrometer with a cable approximately 10 feet long. foot, the cable would add 300 ppf to the input ca acitance, and an input time constant of at least 10l1 ohms x 3 x could be anticipated. As noted by the Applied Physics Corporation in their instruction manual for the vibrating reed electrometer, a large loop gain which is highly fed back can be used to reduce very considerably the apparent value of the input time constant. meter, not so constructed, was considered unusable by reason of excessive time constant. At a typical value of 30 wf per farads or 30 sec In order to produce a permanent record, a system was built around a strip-chart potentiometer which had been converted into a function plotter mainly by replacing the synchronous motor chart-drive with a selsyn. Figure 1 illustrates the use of this unit in two ways. per diagram shows the connections used for recording response of the ionization chamber probe 2 relative to a trans.iission monitor !. is thus useful for plotting beam shapes and depth-dose curves. lover diagram shows connections used for recording the position of a preset relative response. The unit is thus useful for plotting isodose of its uses, the probe is mounted in a large water phantom in such a way that its position in the tank can be adjusted remotely in all three velocity along one coijrdinatc. Motor drives selsyns to move 2 in the direction arbitrarily called 5 here, and also drives the chartdrive selsyn, s. The currents developed in 2 and! are amplified by vibrating reed electrometers z, the outputs of which are impressed on multiturn potentiometers El and 5 . A similar potentiometer, s9is mechanically connected to %, and this tandem unit can be rotated by the two-phase motor 2. If the signal taken from that from %, the chopper amplifier causes 2 to rotate in such a direction as to move the potentiometer slider on R motion stops when the difference signal vanizes. Apart from sensitivity considerations, the position of the balance point is independent of signal amplitude. recorded. The actual recording system employs the automatic balance feature of the strip-chart unit. sition of the slider on & is derived from the slider of & which, in Adjusting the slider of 5turn, must follow the changes on changes the scale factor of the de lection (motion on z4) and is particularly u s e m in adjusting to - scale (1cOjl). The following is a brief description of operation. For each The probe w i U normally be driven at a uniform is greater than away from ground. The This fact implies that relative signals are The signal being balanced by the poFor plotting isodose curves, a relatively minor reconnection is necessary. Motor 2 is replaced by a torque amplifier which operates a servomechanism motor E. For convenience, 2 i s disengaged from (and % I , and & i s replaced by another similar potentiometer which is driven by p also drives E in 8 co&Srdinate, y,through a gear reduction box.
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CONNECTED FOR RELATIVE RESPONSE. CONNECTED FOR POSITION OF RELATIVE RESPONSE FIG. 1. PLOTTER UNIT.
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other than the one of uniform motion, 2. point of m a x i m u m response from 2 and, by adjusting s,makes the balance point on 3fall at a convenient number. number (nearer ground) w i l l cause the difference signal seen by the chopper amplifier t o be converted i n t o motion of and, hence, of 2 along direction y until the signal is reduced t o the same fractional level as would be indicated by conpazing the numbers associated with the two positions of s. Similarly, if 2 i s moved i n t o a position where the balance i s upset, a motion may be provided ina Cirection such asto restore the curves ( s e t on S T can be traced within limits of resolution by overall sensitivity in response t o changes. It is noted that changing the full - scale output value of the vibrating reed electrometers does change the overall stability somewhat, but seems not t o change the sensiI n practice, one finds the Resetting & t o some smaller Moving P i n 2 direction, the 2 position of various isodose The servomechanisms used t o control the position of the compensating filter continue t o operate satisfactorily. (UCSF-111,the sensing element was a quadrRqt ionization chamber, ana the one then i n use was placed outside the field collimator. It was also mentioned that this position would not be satisfactory as a permanent solution because ( a ) other f i e l d collimators would be required, thus providing variable response, and ( k ) t h a t space would be needed t o provide been designed, constructed, and installed. The chamber was installed so as to retain the compensating filter in its support plate. The chamber slides onto four smooth studs and i s locked i n position together with the compensating filter by means of a snap-lock ring. The operation of an ionization chamber i n an extensive high-intenAs resorted earlier A new quadrant chamber has sity radiation field is best achieved by providing an electrode inter - posed between high-voltage and collector electrodes t o guard against be grounded or be connected t o any other potential t h a t may be found usefulto minimize spurious non-ionizing but radiation-induced currents. In this case, a guard electrode i s provided which may Figure 2 shows two views of t h i s chamber. Although the assembly contains well over 200 parts, the assembly procedure i s straightforward. Jig-punching and template-shearing of many of the parts contributed enormously t o the successful assembly. Connections from quadrants, highvoltage stack, and guard electrode (silver print paint) t o the Microdot #3102 connectors are made by m e a n s of Microdot #50-3&4 coaxial cable with braid connected t o guard potential. The use of 2-mm plate eeparation, even with 400 v applied, cannot be expected t o yield gg$ collection of the ions produced. be nearly identical between the quadrants and thus appears only as a slight reduction insensitivity. However, such inefficiency as does exist should
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A s e t of rectangular field-defining collimators, suitable for use When projected t o 200 cm from i n therapy, have been made with the following ratios of long t o short the target, the rectangles are just inscribed in a circle of 15 cm dim. We also have available the circular field (15 cm at2 meters). A method f o r rapid changing of the collimator end associated compensating filter has been incorporated. These compensating filters have been shaped except for the correction corresponding t o each specific collimator. This correction in shape i s necessary since the intensity at the edges of each field must be increased moderately i n order t o provide a substantially uniform absorbed dose r a t e in the neighborhood of the peak of the depthIV. Activities by Pereon Nicholas McIlvraith, B.S.,hss been a meuber of the section throughout this report period and is becoming ew oss e ? i n various problems Fonnected with the donut and radiofrequency power system f o r the synchroti'on. Will is m R. BaUneU., B.S., who joined the section recently, is concerned entirely with dosimetry, for which he i s using both induced activity and chemical agents. Edmund Wong, the new accelerator engineer, is becoming familiar with the operation of the synchrotron. t o produce from the shop the many unusual items which are needed.
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A . Introduction. Plan of Exposure DosimetqFor x-rays of equivalent energy greater than 500 kev, reported values of the relative biological efficiency q based on tissue-dose measurements have ranged from 0.4 to l c 4 5 (1-8). beams of reference were of 1-2 Cu half-value layer. This considerable range of q is of practical concern as weii as theoretical interest. Before it may be discussed with respect to clinical implications, houever, it is important to h o w that the accuracy of the determinations was adequate, e.g. to within approximately +lo$, and that determinations were confirmed by other investigators. A critical review of the literature indicated that many results do not meet these requirements. The present series of ex-periments to determine q for 1000-kvp and 250-kv constant potential x-rays was planned with the foregoing considerations in mind. Four biological tests to give results suitable f o r both intra- and interlaboratory comparisons were selected. More than one test must be wed, since the value of q may depend on the nature of the biological systems Involved. In choosing the tests, due consideration was given to their w,iety, simplicity, proved accuracy, availability to other radiological and biological laboratories currently working in the field, previous use by others, and suitability for the basic plan of exposure and dosimetry adopted. tests does not imply that others may not be as good, but represents OUT attempt to meet the requirements listed above. The stan- tests are as follows: The selection of four particular for a haploid strain of the yeast Saccharomyces cerevisiae. I9 of the adult mouse, based on the number sun rl Ving four weeks after ular weight in the mouse 28 days after whole-body and partial-body exber surviving 6 hours after Irradiation of the egg at 37.6 C and subsequent incubation at that temperature. Standard biological procedures for these tests based on special studies (e121 have been adopted and will be described when the results are presented. series has been extended to other related tests. In addition, the
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The basic plan of exposure and dosimetry for the entire series was adopted in an attempt to meet three principal requirements. First, the tissue-dose distribution obtained with both beams should be practically homogeneous and identical. Second, similar techniques for both exposure and dosimetry should be used in all tests. ments for containing and exposing the organisms should be simple and On the basis of standard depth-dose data (13, 14) it was concluded that the first requirement would be satisfied by employing the following conditions of irradiation: large field, maximum scatter, and limited quirements to be readily fulfilled. The use of these conditions permitted the second and third reIn all experiments, except in some with the testis, the area exposed to the beam was to be greater than 400 cm2. M a x i m u m scatter was to be obtained by making the container ho1dir.i; the organisms part of a large mass in the irradiated field. organisms were to be exposed was limited to 0.2-1.0 cm at target distances of 50 to 65 cm, and to 0.w.5cm at target distances of ~ o Gto 115 cm. It was anticipated, and subsequently confirmed, that these conditions would limlt variation of dose with depth to tl$ for 0.2-1.0 a, The range of depth within which the To avoid repetition, methods and details of technique for exposure and dosimetry that are common to all tests are presented here. The x-ray beam of reference was from a Westinghouse Quadrocondex unit, operated at 250 kv constant potential and 15 m a . A transmission ionization chamber in the-path of the beam near the target added 1.5 mm Al to the fixed inherent filtration. Without further additional filtration, the hvl was about 0.6mm Cu. With filters of 0.5 mm Cu + 1 m Al, The x-ray tube was fixed with the port …
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