Teletherapy Machine Research Articles

An exposure timer for use with telecobalt units.

The influence of mains frequency variation on the exposure output of teletherapy machines has been investigated. As the correction to be applied for such variations during routine treatment is cumbersome, a silicon diode timer, which works directly by the action of radiation, is described.

Field size definition on cobalt teletherapy machines

Field size definition on cobalt teletherapy machines

Beam Anomaly for a Cobalt-60 Teletherapy Machine

A beam anomaly due to trimmer bars for a cobalt teletherapy machine and the possible consequences in clinical work are reported.

Electronic dual timer for a teletherapy machine.

The timer described here complies with the requirements of RASMP/75 (Department of Health and Social Security) for existing gamma-ray machines. The unit has been in continuous operation since August 1976 as the exposure control of a TEM Mobaltron 100 cobalt unit.

Possible complications in the use of a light field matched to the 80% isodose line of a cobalt teletherapy machine

Possible complications in the use of a light field matched to the 80% isodose line of a cobalt teletherapy machine

Experience with an automatic isodose plotter

We have recently completed comprehensive beam measurements on three teletherapy machines, an SL75−10 Linear Accelerator, an F100 Mobaltron Cobalt unit and an A.E.C.L. Theratron. The S.H.M. 3-D plotting tank system* (Pitmans Ltd.) was used under the control of a PDP-8 system with extended hardware (DEC Ltd.). The controlling program was a modified version of that used at the Western General Hospital, Edinburgh for the determination of dose distributions for their planning programs (Bottrill et al., 1975; Redpath et al., 1976). The S.H.M. tank may also be used as a stand-alone analogue system with its own plotter; both diode detectors and ionization chambers are available. Our measurements concentrated on the use of the ionization chambers which showed significant polarity effects. As several of these systems are now available in the U.K., readers may find some comments on this and other problems helpful.

A study of dose distribution patterns from cobalt-60 and Varian Clinac-4 beams

Dose distribution patterns of 4 MV x-rays from a Varian Clinac 4 linear accelerator are compared with those from a 60Co teletherapy machine. Field flatteners of lead and depleted uranium were used. Dose distribution maps display the off-axis high dose or "horns" with lead filters and circular dose distribution patterns. The uranium field flattener clearly "clipped the horns" and eliminated circular dose distribution patterns with larger fields.

Calibration in water versus calibration in air for Cobalt‐60 γ rays

In the United States it is common practice to calibrate Cobalt-60 teletherapy machines "in air," despite recommendations by the International Commission on Radiation Units and Measurements (ICRU) and other organizations that calibration be accomplished by measurement at 5-cm depth in a water phantom. A comparison has been made between the results of ionization measurements in air at 80.5-cm distance from the source and in water at 80-cm source-skin distance (SSD) for the determination of absorbed dose at three depth (5, 10, and 15 cm) for each of three fields sizes (6 X 6, 10 X 10, and 20 X 20 cm2), for a total of 42 Cobalt-60 machines. The mean of the ratio, absorbed dose from in-water measurements to absorbed dose at the same depth calculated from in-air measurements, ranged frt 5-cm depth for a 20 X 20-cm2 field size. Reasons for the differences are offered, and compliance with ICRU recommendations is suggested.

Use of auxiliary collimating devices in the treatment for breast cancer with 60Co teletherapy units. I. Dosimetric considerations

The dosimetry of tangential field treatment for breast cancer using an auxiliary collimating device on 60Co teletherapy machines is reviewed. The position of the device with respect to the source axis has negligible effect on either the dose rate or the dose distribution. Measurements with thermoluminescent dosimeters in an anatomic phantom verify the accuracy of the method of correction of isodose distributions for sloping skin. Possible systematic errors in dosimetry are identified.

Letter: Electron contamination of telecobalt beams.

Baum and Phillips (1974), using an experimental set-up, showed that electron contamination could make the surface dose as much as 50 per cent greater than the dose at a point 0·3 cm deep in tissue, even with a collimator-skin distance as large as 20 cm. They emphasized the importance of such contamination in radiotherapy, and showed that the overdosage could be removed by a Perspex (or better still, a copper) filter at the end of the collimator. The measurements of Baum and Phillips were made under conditions not directly comparable to those in most teletherapy machines.

A block and wedge holder for the Picker Series 8 cobalt-60 teletherapy machines.

A rigid, easily demountable, and versatile device combines the function of three separate accessories for the Picker Series 8 cobalt-60 teletherapy machine. Wedges and lead blocks are held simultaneously, and firmly so that the blocked field may be directed at any angle.

A Nomographic Technique for the Isocentric Computation of Cobalt 60 Central Ray Depth Dose

Abstract A nomographic technique for computation of depth dose of 60Co at areas along the central axis is presented. A calculator composed of 2 sets of graphs and 2 pointers connected by an elastic cursor is used to obtain the depth dose rather than working inverse square law calculations and using TAR and TMR tables as has been previously required for isocentric teletherapy computations. The feasibility of extending this calculator design to other high energy teletherapy machines is discussed.

A film technique for determining the absorbed dose at exit from the patient during radiation exposure.

A film technique for measuring the absorbed dose at exit from a patient during radiation exposure was studied with the 60CO and Van de Graaff teletherapy machines. Film response was investigated at several field sizes and depths. Results indicate that a standard film calibration curve can be obtained to calculate absorbed dose. The treatment verification film is obtained by inserting the film in a thin vinyl flexible cassette which remains beneath the patient throughout the complete radiation exposure. The absorbed exit dose can be obtained from the verification film with the calibration curve.

Dependence of exposure rate upon field size and shape

It has been previously reported that the exposure rate of teletherapy machines for a given SSD is a linear function of the variable loge (A/P) where A is the area and P is the perimeter of the field (Patomäki, 1968).

Results of cobalt 60 rotation therapy in carcinoma of the cervix.

1. A method of treating carcinoma of the cervix with a cobalt 6o teletherapy machine, using 360° rotation, and irradiating a larger and then a smaller volume in the pelvis is described.2. Isodose curves are given for the distribution of dose in a body cross section, in per cent of tumor dose and in r for the following conditions: when a tumor receives (a) 4,000 r, (b) 3,000 r with the smaller field, (c) a combination of these two, and (d) a combination of 5,000 r to the larger field and 3,000 r to the smaller field.3. An analysis is made of the patient material, 55 cases, giving the pathology of the tumor, the stages of the disease, the marital status, the number of pregnancies, the symptoms which brought the patient to the physician, and the age distribution.4. The results of this method of therapy are given. The survival with no evidence of disease is 86 per cent in Stage I, 80 per cent in Stage II, 67 per cent in Stage III and none in Stage Iv. The over-all survival is 67 per cent. The more advanced ca...

A DOUBLE-HEADED FIXED BEAM TELETHERAPY MACHINE FOR LOW COST MEGAVOLTAGE THERAPY.

Compared to kilovoltage radiation, megavoltage radiation has important advantages for external radiation therapy, but the much higher costs of the machines and treatment rooms have prevented more general use. Figure 1 shows an arrangement which greatly lowers the costs of megavoltage therapy. Two identical treatment rooms are served from one double-headed machine mounted in the wall between the two rooms. The teletherapy source, either cobalt 60 or cesium 137, is in the middle in the “off” position and can be moved into either head by a steel cable operated manually by a control wheel. Automatic return of the source to the “off” position at the end of the treatment time is provided by a spring. While one patient is treated in one room, the next patient is set up in the adjoining room, an arrangement similar to Failla's layout for the Memorial Hospital 4 g radium bomb. The radiation beam is always directed straight down. Its size and shape are adjusted with shielding blocks placed on a tray below each head. The obvious disadvantage of this arrangement is that the radiation beam cannot be angled or rotated. However, our large experience with movable megavoltage machines indicates that the majority of radiation therapy patients can be satisfactorily treated with “straight-down” beams. The lack of mobility of the head in our design is compensated for by the possibility of using field sizes up to 45 × 30 cm at 70 cm source distance. With such large fields, most tumors and lymph node areas can be treated in a single field. In our experience, this is most desirable for therapy machines, since it is difficult to join adjacent fields properly. In principle, movable teletherapy machines can also be equipped with large fields, but the difficulties of room protection and beam collimation have limited the field size in most commercially available teletherapy machines to much smaller dimensions. The low cost of the arrangement shown in Figure 1 is due to the following features: 1. The simplicity of the machine and of the controls. 2. The elimination of a special stand for the head because it is supported by the wall between the treatment rooms. 3. The use of shielding blocks on a tray for collimation and beam-shaping. 4. The ease of room protection since, with the machine installed on the lowest floor, only 20 in. of concrete for the walls and 15 in. of concrete for the ceiling are required for satisfactory protection with a cobalt-60 source up to 2,000 c. 5. The possibility of using low-curie teletherapy sources since with a double-headed machine, no treatment time is lost while a patient is set up, and thus approximately twice as many can be treated as with standard single-headed teletherapy machines of the same source activity. The suggested arrangement appears especially suitable for hospitals which already have one rotational megavoltage machine and desire to treat more patients with megavoltage radiation.

COMPARATIVE CHANGES IN RADIOSENSITIVITY BETWEEN BONE MARROW AND A RAT TUMOR IRRADIATED UNDER ANOXIA.

The protective effect of low oxygen concentrations in the radiosensitivity of cells has been well established by numerous experiments both in vitro and in vivo (1, 4, 5, 12). Because some tumors probably contain areas of relative hypoxia or anoxia (10), there is a possibility of variation in radiosensitivity within the tumor volume, depending on the state of oxygenation of its individual cells. One way to eliminate this variation would be to remove all oxygen, thereby equalizing the radiosensitivity of all tumor cells as far as the oxygen effect is concerned. At the same time, the surrounding normal tissues would be rendered anoxic. The question to be determined is whether or not induced anoxia would protect tumor and normal tissues to the same degree; if so, nothing has been gained. If anoxia, however, protects the normal tissue to a greater degree, than the tumor, the net effect would be a relative increase in tumor radiosensitivity. The present experiment was carried out to test this hypothesis. The data demonstrate that anoxia has a greater radioprotective effect on a normal tissue (bone marrow) than on a tumor. Materials and Methods The experimental plan was to give radiation to the lower extremities of rats bearing tumor in both legs, while the vascular supply to the left lower extremity was temporarily occluded to render it anoxic. The radiosensitivities of the femoral bone marrow and tumor in the right leg (normally oxygenated) and left leg (anoxic) were measured by determining the duration of depression of DNA synthesis in these tissues. Animals and Tumor: Male CFN rats raised in our laboratory and weighing between 150 and 300 gm. were employed. The tumor was the Walker carcinosarcoma-256. When the tumors attained a diameter of 2 to 4 cm., the animals were anesthetized with intraperitoneal sodium pentothal (50 mg./kg.), and a laparotomy was performed. The left common iliac artery was dissected free from the vein, and an arterial clamp was applied. The laparotomy incision was partially sutured, and, after waiting fifteen to twenty minutes for the left leg to become anoxic, the animals were irradiated. The clamp was always released one hour after its application; the abdominal incision was then completely closed and the animals were returned to their cages. Irradiation: The anesthetized rats were given radiation in a specially made plastic cage and positioned under a cesium-137 teletherapy machine. A collimator formed a 6 × 6-cm. square field which covered the lower extremities, but part of the pelvis was also included in the radiation field. The source-to-top-of-cage distance was 9.5 cm. Halfway during irradiation the animals were turned over to equalize the tissue dose as much as possible. Measurements made in a water-filled phantom cage indicated that the tissue dose at the mid-plane of the irradiated part of the animal was 15 per cent lower than the surface dose, and in this report all doses are expressed as midplane.

DOSE DELIVERED TO THE MEDIASTINUM: AN INTERCOMPARISON BETWEEN CENTERS.

In November 1961, the opportunity arose of developing a program of co-operative radiotherapy trials which had originated with the National Cancer Institute in the late 1950's. Fully documented, randomized radiotherapy with classification of disease was to be studied statistically. The first treatments of preoperative lung irradiation began in 1963. As part of the overall program it was thought essential to include a comparison of dosage at each center involved. A survey of the first group of therapy centers was undertaken; this paper presents the factors that were considered and the results obtained. A Victoreen r-Meter was carried to nine different centers to compare the method of calibration at those institutions, and to calculate the tumor dose in a standard phantom, comparing the local dosimeter readings and calculations with the transported Victoreen r-Meter. The results are presented in this paper. Correction Factors Timers and integrators should be discussed before considering the factors for the Victoreen. Timers used with x-ray and teletherapy machines may have errors of seconds; there may be errors due to the time required for voltage build-up. This factor has to be taken into account when exposures are short, as with a 25-r chamber. The difference between time set and time registered is usually constant, if the timer is set in the same manner each time it is used, and can be determined by exposing a chamber for different times and extrapolating to zero dose. Integrator systems also may have “end” effects which can be determined in a similar manner. The Victoreen r-Meter model 570 serial 359 employed in this study is one of four used at the M. D. Anderson Hospital. The following routine checks are made on their constancy. The electrometers are given voltage checks by measuring the voltage required to produce full and half-scale deflections; the chambers are intercompared monthly with x-rays of h.v.l. 1.26 mm. Cu. Intercomparisons are also made with radium, cobalt, and cesium gamma and x-rays or other half-value layers at less frequent intervals. Routine leak tests are performed on all chambers and desiccators are changed frequently. A model 70-5, 25-r chamber is normally used for cobalt-60 with Lucite build-up of 4 mm. wall thickness. At times of intercomparison 100-r and 250-r chambers which were employed in this work are also re-checked with suitable build-up caps. The correction factors are shown in Table I. Two of the four Victoreens at M. D. Anderson Hospital are nonlinear to the extent that it is worthwhile to make a linearity correction when the readings are beyond certain limits. For instance, a correction of +3.5 per cent is required for the instrument in this study, and −4 per cent for another for a full-scale deflection reading (Fig. 1).

Determination of isodose curves for supervoltage and cobalt-60 teletherapy machines with x-ray film.

Treatment planning with supervoltage and cobalt-60 machines is often hampered by a surprisingly limited supply of suitable published information. Table I summarizes dosage references from four major radiologic journals. Supervoltage data are represented by only one treatment distance (100 cm.) and three square fields. Although there are more curves between actual and nominal field size is accurately known for the particular machine and treatment distance used, considerable errors are possible. A further difficulty is introduced in teletherapy machines by lack of standardization of penumbra factors, such as source sizes and collimator design (17). The effect of penumbra size on dosage for cobalt-60, they must be employed with care because of the variability of field-defining systems in such machines.3 Even the term “field size” is quite ambiguous. Table II shows actual measured separations in air of 50 per cent of maximum dose locations, for nominal 10 × 10-cm. fields, on several machines in the Philadelphia area. A range of 9.1 to 11.3 cm. is observed among commercial units. Unless the relationship distribution across the field is shown in Figure 1 comparing small and large penumbra situations for identical field size. Achievement of a good penumbra requires both sources of high specific activity and collimator blocks placed relatively close to the patient. The latter requirement normally necessitates rotation through less than 180° to prevent collision of the collimator assembly and the treatment table. It is clear from the above considerations that one must check the applicability of published data to the planned use of a particular teletherapy machine. It appears desirable that field size and penumbra characteristics of such units be checked routinely at installation as well as when significant changes occur in optical system, collimator, or source size. In addition, considerably more isodose curve information is evidently needed, for both single and multiple fields. This paper discusses the development of film methods for obtaining isodose curves in a Masonite or Mix-D phantom. If sufficient accuracy can be achieved, the use of film offers certain advantages over alternative ionization chamber and scintillation probe technics (19, 20, 27). For example: Equipment: Suitable film densitometers cost as little as $550; they are relatively easily maintained and serviced, and normally are quite dependable. On the other hand, small ionization chamber and scintillation probe assemblies are best used by persons with adequate physics and electronics background to keep them operating satisfactorily. Resolution: Films are conveniently scanned with light slits 1 mm. or less in width. Probe units are usually 5 to 8 mm. in diameter, with about 3 mm. as a practical lower limit, and thus tend to measure less accurately near the edge of the field.

Co60 isodose curves for 240degrees rotation, showing displacement of the center of dose from the center of rotation.

This investigation was undertaken in order to find the dosage distribution within the tumor and surrounding tissues when a patient is treated by a partial rotation technic with a Co60 teletherapy machine. The published isodose curves and data (1–6) were not sufficiently extensive for our purposes, especially when the rotation was limited to an arc of 240°, the maximum traversed by the Picker C-3000 machine. Partial rotation is valuable in many problems of therapy; for example, treatment around the bladder, uterus, and rectum. Hence, it may be used on machines with which full rotation is possible. This study, therefore, includes 240° rotation for the 360° rotation Picker C-1000 machine. Method A sheet of du Pont Adlux film was placed between two elliptical sections–each 1 cm. thick–of a unit-density Masonite Presdwood phantom. The assembly was made light-tight by pasting black Scotch electrical tape over the edge of the boards. Films were exposed to 10 r, 25 r, 50 r, 100 r, 150 r, and 200 r. These, together with an unexposed film, were pan developed, fixed, and washed. The maximum density at the center of the exposed film was measured on a Photovolt transmission densitometer set for zero reading for the unexposed film density. Seven sets of films were exposed, developed, and measured similarly. Density-dosage curves were plotted. All seven curves had essentially the same shape but differed in slope due to variable temperature and time conditions of developing. At the point of greatest spread, the 200 r exposure, the greatest variation in density was 8 per cent. An average curve was plotted from these seven curves. This became the density-dose calibration curve (Fig. 1). Each section of the Presdwood phantom had a long axis of 29.5 cm. and a short axis of 23 cm. The boards containing the film were placed in the middle of the phantom with 11.5 cm. thickness of Presdwood on either side of the film. The boards making up the phantom were held together by small plastic pegs at the edge of the sections and sand bags at both ends. The phantom was placed on the table, lined up so that the plane of the film was in the plane of rotation and the center of rotation at the center of the phantom section containing the film. The set-up is shown in Figure 2. The exposed film, together with an unexposed portion of the same film, was developed and measured on the densitometer. The density was read at centimeter intervals along axes from the center of rotation, spaced 30° apart. By the use of the density-dose calibration curve the roentgen values were found. From these the per cent of maximum dose at all the measured points was calculated. Each family of curves involved about 750 determinations. Curves of per cent of maximum dose versus distance from the center of rotation for the different axes were plotted. Three separate films for each field size were exposed and worked up.