Page images
PDF
EPUB

head has a sufficiently large area to shield the small angle high energy scatter from the opposite beam. The design is similar to one discussed with Dr. Smithers of the Royal Cancer Hospital early in 1952 and was included in our discussions with Grimmett early in 1950. Although the design will accomplish all kinds of motion, it is cumbersome to operate, it takes up much more floor space than is necessary, and both bed and machine must operate to get all kinds of motion. Little attention has been given to the problems of automaticity of controls and the repetition of pattern. The advantage of having the beam always enter at 180° opposing fields, thereby making wedge filters unnecessary, may be offset by the difficulties of stopping the radiation at selective points during their sweep around the body. Another advantage lies in the use of smaller sized sources and lower curiages. Treatment distances, however, may be closer than is usually contemplated with other rotational schemes. The cesium 137 source material is apparently being separated at Amersham.

At Oxford, Dr. Frank Ellis and his group have designed a large machine which departs completely from the usual concept of a teletherapy machine. It is an idea similar to the igloo that Dr. Newell of Stanford suggested to us last year. The idea has been under active design for 2 or 3 years and working drawings have been partially prepared for cobalt 60 sources. A large concrete cave is contemplated. Many small sources of cobalt 60 or any other isotope will be focused to one point within the cave. On a continuous, ellipsoid, single track in the ceiling, two beds are suspended directly opposite each other. The patient in the bed outside the cave is positioned under a series of lights duplicating the beams of cobalt 60 and is then pushed into the cave. One patient can then be irradiated while the other is being set up. The advantages of this design are the handling of two patients at once, the use of small-sized low specific activity sources, and fixed dosage, time, and field-shape relationhips. The disadvantages are that only one kind of 360° field is available with some theoretical possibilities of turning off individual elements of the source. With the great number of moving parts in the machine the maintenance and calibration costs may be higher than expected. The suspension-bed mechanism will have to be a precision machinery job and is not so simple a design as that suggested by Newell. The fixed pattern of the radiation field is adaptable to only certain tumors, but Dr. Ellis' large service has sufficient patients that he can afford a machine for limited

Neither Dr. Ellis nor Dr. Newell agree, but in my opinion an intense field field of scattered radiation within the cave will build up the integral dose to unacceptable limits. The cost of building and loading such a machine will be prohibitive unless most of the work is done by home talent not counted in the cost. In my opinion, if high specific activity sources were available at low cost, such a design would not be considered even by Dr. Ellis.

In Cambridge I saw the teleiridium unit described by Freundlich Haybittle and Quick (Acta Rad., 34: 115, July 1950). The advantages of this machine

the radium unit it displaced are obvious. No one considers more advantageous than the same machine would be if it could be supplied with a longerlived isotope. According to Dr. Mitchell, however, the use of the 70-day isotope is not so great a chore as would be expected. A routine procedure is now set up for monthly interchange of iridium sources. For an organization within cheap trucking distance from a reactor, the use of such a short-lived source has proved practical. Yet if many of these sources were put into operation logistic difficulties would suon arise. In spite of the availability of short-lived sources, no other groups have attempted to develop the idea in and around Harwell. The number of advantages appears to be insufficient to compensate for the continuous maintenance cost. The machine is adapted from an old Grimmett radium-pack design with the shielding of uranium. It is thus one of the smallest and most maneuverable devices I have seen. The use of nonfissionable uranium metal as a shield for housing therapy sources in the United States should be reexamined.

At the Royal Cancer Hospital in London, Dr. Lederman demonstrated several new uranium treatment applicators for the two radium packs he is using. Some of these are of irregular, rectangular, and triangular shapes, and have great advantages in reducing the transmitted penumbra at close distances, especially for radium and cobalt energies. The use of irregular fields is in line with Lederman's desire to shape the field of effective radiation to fit the tumor. While shaping of a field is best done with some form of rotation or moving fields, such complications are impossible with the low intensity of the old radium packs. Lederman appears to be far more interested in the development of the shaped fields which are possible with complete freedom of rotation.

use.

At the Royal Cancer Hospital, Dr. Smithers and Dr. Wheatley showed me the projected designs for a rotational cobalt 60 or cesium 137 teletherapy machine. The principle is similar to that shown in the model of our preliminary device exhibited in August 1952 (but without the secondary extension for conical rotation which, in the Royal Cancer Hospital machine, would be accomplished by moving the bed). The engineering is farther advanced than we had gone before we discarded the idea, and consists of a single large casting that would be both counterweight and suspension arm. The casting appears to be extremely expensive and although it offers the opportunity for investigating most forms of moving field therapy, this process can be accomplished only by moving both bed and source simultaneously. Such simultaneous movement seems difficult to set up and to duplicate with precision. A preliminary model of the unit has been constructed.

At the symposium on teletherapy units at the annual meeting of the Faculty of Radiologists, Dr. Johns (Saskatchewan) discussed a modification of the diaphragm of the Saskatoon cobalt 60 unit for accomplishing a continuously variable rectangular field size without increasing the small size of the nose of the Saskatoon unit. Dr. Johns has placed a series of interleaves of steeljacketed lead dow the length of the collimating tube. These crossbar interleaves are thick toward the source and become thinner at the skin end so that the problem of nonuniform penumbra is effectively solved and the field size can be adjusted by two finger-action screws. Circular and irregular fields are impossible except with individual adaptors; thus, the unit is of value only on stationary or multiple field techniques. It is, however, a big advance over the clumsy arrangement of our first design. It has an advantage over our circular portal continuously variable diaphragm in that it takes up much less space close to the body when used for stationary fields. For those who are not interested in moving fields (a minority outside of the United States) the Johns' design should be seriously considered. This diaphragm arrangement has been adapted into the commercially available Picker unit.

Dr. Moffatt (Vancouver), who has used the early Eldorado design, presented preliminary clinical results. His impressions that cobalt 60 has an advantage especially in skin reaction and depth dose are similar to those of many others. Dr. J. R. Clarksor (Southhampton) presented the previously discussed design for his cesium 137 unit and some of its advantages in rotational fields. Mr. Eastwood summarized the problems of cesium 137 production and the characteristics of the cesium isotope. Much of the material he presented is similar to that found in the ORINS report on cesium 137. The problem of production from English fission products is essentially the same as that in America. A precipitation process, however, is being used rather than the ion exchange process of Oak Ridge. Following Mr. Eastwood's talk on cesium 137, I presented the preliminary designs for the ORINS teletherapy machine and some of the reasons for including cesium 137 in a consideration of teletherapy sources.

In Switzerland, Dr. Muller has given little attention to the problem of teletherapy primarily because the university has a cyclotron and has little hope of funds for purchasing a large isotope source. The story in Geneva was the

At Heidelberg there is an intense desire to obtain a large radioactive source, and Professor Becker has a room set aside for a teletherapy machine in his new laboratory. The present impossibility of obtaining funds for purchase of the isotope is holding up the consideration of design problems. Dr. Becker, however, would consider only a complete rotational design. He is at present using one of the Muller (North American Phillips) 360° rotational 250 k. v. p. machines and has found it superior to his other low-energy machines. He has experimented with a number of tumors (including stomach) which he would not have considered with multiple fields and is enthusiastic about the preliminary results. At the present time he is installing a 15 m. e. v. betatron with pendulum motion since even at betatron energies he considers rotation an ahsolute necessity.

In Paris, Dr. Simone Laborde is using an historic radium pack. The machine is of simple design consisting of a pot of lead with a large master openin. A through and through lead plug lined with steel rotates by an electric-powered motor and a manual control. The source is approximately 5 by 5 centimeters with radium in multiple capsules. There is little in the design that does not appear in Failla's early articles. A wood cover is used over the treatment applicators and undoubtedly causes a serious increase in low-energy electrons close to the skin. The treatment cones are of lead lined with steel and all contain these wooden electron filters. There is little positioning gadgetry. The switch

same.

is manual but with the long treatment times this seems justifiable. The machine is in constant routine operation. No outstanding research or observations were available for this discussion. Dr. Loborde's results have been reported periodically in the French literature.

Of far greater personal interest was the work of Professor Tubiana, Dr. Dutreix, and their associated physicists. They have just installed a new AllisChalmers 24 m. e. v. betatron which seems to be the primary interest of the department. No patients have yet been treated; all work has been on the physical measurements of the beam. Since this recent purchase they have measured only the central dose lines for a very few fields. They contemplate the first use with patients sometime in October. Considerable work is being done particularly on problems of scattered radiation, which is of great interest to Dr. Tubiana.

At the Seventh International Congress there was a surprisingly large turnout for the cobalt 60 reports. Dr. Braestrup (New York) presented the dose distributions in water phantoms using a small mockup of his rotational design. Ionization chamber and photograhic measurements were presented for stationary, rotating, and oscillating patterns. Dr. Miller (Sheffield, England) described a modification of their old 5-gram radium pack to accommodate a 50-curie cobalt 60 source. His data on the dose distributions at a 15 c. m. source-skin distance showed the physical superiority of cobalt 60 over radium, but to my thinking pointed up even more strongly the fallacy of using 50 curies when larger sources could so easily be made available. Dr. H. R. Marcuse (Hague, Holland) described a low-cost, homemade cobalt 60 unit which is, for all practical purposes, the same as the Failla-Grimmett idea. The machine was designed to hold 1 kilocurie of cobalt 60 which they eventually expect to get. They now have it loaded with a 250 curie source. The criteria they used in setting design requirements were interesting: (1) Conventional larynx therapy must be possible for which they demand a minimum of 25 centimeter source-skin distance. (2) Conventional abdominal therapy must be possible for which they demand a minimum 40 c. m. source-skin distance. (3) The overall irradiation during the treatment by leakage radiation must not exceed the scattering from the treated volume of tissue. This last demand is actually far less stringent than the criteria we used in our design. Our reasons for being more strict concerned not so much the patient but the MPE of the operators during setup time. Dr. Marcuse's demonstration served primarily to point out the tremendous economic advantages of the isotope sources from the point of view of all phases of treatment once the source itself is made available.

During the therapy sessions Dr. R. W. Johnson (Los Angeles) presented the preliminary results with a series of inoperable bronchogenic carcinomas and carcinomas of the bladder. Dr. Johnson described the machine now in operation at the Los Angeles Tumor Institute. He indicated a feeling of improvement of the immediate palliative results over those of 250 k. v. p. therapy. Some dramatic regressions of pulmonary and mediastinal lesions were demonstrated. Dr. Burkell (Saskatoon) presented some of their work on applicator design and on bollus materials in the path of the beam. They have investigated the skin effects of cobalt 60 and have noticed a "thickening hardness" under the skin not previously noted with other forms of radiation therapy. The Saskatoon group is beyond doubt making the most significant investigations of conventional techniques with cobalt 60 of any of the groups now publishing papers. Dr. Ivan Smith presented a general statement of his work with the Eldorado unit at London, Canada.

1112. RUMOR AND SCUTTLEBUTT During the discussions with many of the therapists and physicists of Great Britain, it was brought out that a report had been prepared by the Medical Research Council of Great Britain on the direction of major emphasis in the official policy toward various forms of radiation therapy. Apparently all major allocation of funds for new and experimental equipment is controlled by the Medical Research Council. Since the early papers on isotope machines, especially since the report on cesium 137, there has been a considerable pushing by various radiotherapy and physics groups to be allowed to build, design, or purchase a teletherapy machine. At the same time considerable emphasis has been placed on the development of high-energy accelerators. To determine which of these is the most logical and effective, and which could best be pushed with the available funds, they formed a committee which consisted of an equal number of therapists and physicists. The report of this committee compared the calculations of all the essential data on low- and high-energy X-ray generators, cesium, and cobalt isotope machines, and very high energy accelerators. The report brought out the fact that the physical data showed all machines to be considerably superior to the usual X-ray generators and that each had various advantages and disadvantages so that no single machine could legitimately be either discarded or accepted to the exclusion of all others. On the basis of the estimates of total economic cost, in spite of a considerable bias against cesium 137, it turned out to be most economical of all types of units. Although this should have invited an immediate emphasis on cesium 137 production, such did not occur since two members of the committee refused to recommend cesium. I have talked to all of the members of the committee and many others who were acquainted with the committee's functions. There are apparently a number of specific objections to cesium 137.

(1) In a discussion with Patterson, he stated that the penumbra of the cesium 137 source beneath the skin surface was inferior to that from very high energy machines. This is true but only to a minor degree, since most of the scatter at 660 k. e. v. is forward in low Z materials. If the scatter penumbra beneath the skin is compared to 10 to 20 m. e. v. radiation, then both cesium and cobalt are physically inferior. If compared to 250 to 1,000 k. v. p. X-ray generators, then both cesium and cobalt are physically superior. Published phantom measure ments have answered the practical question for cobalt 60. Calculations that have been made shown cesium 137 to be essentially similar to cobalt 60 but since, as yet, there is no large source there are no phantom measurements. All of the physicists in England that I talked to, with one exception, believe that Patterson's argument is in favor of rather than against cesium 137.

(2) The second argument against cesium 137 was given by one therapist who did not believe it to be a serious argument and was unsure of its validity. If an atom-bomb attack explodes near one of the cesium 137 machines, the addition of many thousands of curies of cesium 137 to the already present fission products from the bomb would increase the long-term hazards of atom-bomb attack. This argument can be answered only by comparing estimates of the amounts of longlived fission products produced in various sized atom bombs and the chances of shattering a large lead protective shield. If an explosion is close enough to vaporize a lead shield, all of the shield's radioactive contents will be carried up into the cloud. If the explosion is at such a distance that a 1-ton shield is only melted, then the Cs 137 or Co 60 will be trapped in a safe container. Based upon rate of metabolism, energy and body distribution studies, the international commissions seem to believe that there is a much higher allowable body burden for Cs 137 than Co 60. Hence, the Cs 137 machine is safer than a Co 60 machine with equal output. From the figures on allowable concentration in water, Cs 137 is either 10 times more dangerous or 100 times less dangerous than Co 60, depending upon which way the international commission jumps. The argument is an exceedingly difficult one to answer but I think it is a molehill-mountain argument.

(3) The third argument was brought up by one of the physicists who is working with a high-energy cyclotron. When the electron buildup immediately beneath the skin is studied for very low and very high energy electrons there is a steady increase in the depth of peak ionization with increasing energy. From the point of view of physical measurements this is true; however, the practical question is, At what energy is the peak sufficiently deep? If one wishes to treat deepseated tumors at the peak ionization, then the primary photons must be at very high energies. If one considers only the isotope range of energies, the peak ionization should be beneath the germinal layer of skin or a depth of at least 0.1 to 1 millimeter. In an article in press we show that, with cobalt 60, improperly designed treatment cones can bring the peak to the surface of the skin but that proper design lowers its depth to about 2 to 4 millimeters. Cs 137 has not yet been measured but should be about 30 percent less deep.

Some persons considered the percent of peak ionization at the surface of the skin important since it will determine the dose at the germinal epithelium. For cobalt 60, under best conditions, the percent of peak ionization directly at the germinal layer of skin is about 30-40 percent of the peak. It has not yet been measured for cesium 137. The practical point of this argument disappears, however, if multiple or moving fields are used.

The consensus of opinion among about 30 physicists and therapists in England is that cesium 137 is potentially a far more economical source than any other and should, for all practical purposes, be as good or better than any other isotope source for therapy. There is a fair degree of uncertainty on the economic phase of isotope versus very high energy accelerators. With some outstanding exceptions, the accelerators are generally considered expensive and are maintenance problems. There is no uniformity of opinion on the relative therapeutic value of various energy radiations.

III. ROTATION AND MOVING-FIELD THERAPY

If any one problem can be said to be foremost in the discussions on radiotherapy, it is the emphasis now being placed on moving-field therapy. The word "rotation” has been largely superseded since it does not include some types of motion that are not rotatory and, hence, the phrase "moving fields” has been adopted.

In a symposium on moving-field therapy Dr. Felix Wachsman (Erlangen, Germany) described some of the disadvantages of overlapping adjacent fields and various forms of multiple-field patterns. It was his contention that most of these disadvantages could be overcome by a continuously moving beam. He described a number of types of moving fields. First is “rotation therapy'' with the tube stationary and the patient, seated or erect, slowly rotating about a vertical axis in a horizontal X-ray beam. The biggest advantage of this technique is the possibility of correcti the adjustment by continuous screening through a fluoroscopic visualization of the beam. He demonstrated his method of obtaining precision adjustments of even small fields. A second form of moving field is the so-called pendulum therapy. In this technique the X-ray tube swings around a horizontal axis of an immobilized patient and only describes an arc that is a portion of complete rotation. He believed that few advantages could be demonstrated for this method, although he pointed out that it made possible treatment of tumors with a longitudinal axis parallel to the axis of the body. My impression (although he did not imply such) was that the biggest advantage was the engineering of the machinery. The third technique he described was what he called convergent-beam therapy (similar to Green's horizontal-arc therapy). The central ray is always directed on a point of convergence and results in an X-ray beam comparable to a convergent lens. The technique he described was the Siemen's spiral movement over a circular skin area. He pointed out that one of the big advantages of convergent-beam therapy is that it can be based on standardized isodose curves that are adaptable to any shape and depth of tumor. For each type of motion Wachsman was using separate machines all made by the Siemen's Co. My own impression was that he was describing the advantages of three specific engineering designs rather tban the concept of a moving field.

Immediately following the congress there was a postgraduate course on movingfield therapy held at the Medical Clinic of the University of Erlangen and allegedly supported by the Siemen’s Co. In the technical exhibits Siemen's showed 250 k. v. p. apparatus which performed pendulum, spiral, and convergent beam and rotation therapy. Also shown was a 15-million-electron-volt betatron for pendulum therapy. The Muller Co. (North American Phillips) demonstrated their TV1 model for continuous 60-degree rotational therapy.

Dr. Palniere (Bologna, Italy) described a method of convergent beam therapy which he called En couronne. This method uses a stationary tube over a horizontal patient upon a moving table. It is identical with the horizontal arc therapy in England. Palniere described a number of centering and positioning devices and various kinds of filters to achieve a homogenous field of radiation in the area of the tumor. Another method Palniere pointed out as his discovery was a scanning type of therapy similar to a tomographic projection of a diagnostic machine. He was quite excited about the possibility of building up homogenous fields of concentration, especially in flat plates of tumor using tomographic scanning methods.

Dr. Jens Nielsen (Copenhagen) demonstrated his equipment and reviewed his work of many years of using a horizontal beam with a rotating patient. His results indicated sufficient improvement over ordinary stationary fields to justify further study of rotational methods. Both Wachsman and Nielsen pointed out that there are indications and contraindications for each of the various types of inoving-field techniques. Johns (Saskatchewan) described the rotational floor stand that is being used with the cobalt 60 unit in Saskatoon. He presented some of his calculated isodose pictures which showed the remarkable uniformity and precision that might be obtained with simple rotational patterns. Nakaidzumi (Tokyo) discussed much of the same material as Wachsman but included one further kind of moving field, that of the tangential approach to the body. He gave primarily a historical summary of various methods of rotation and seemed to have

« PreviousContinue »