DETECTION OF SOURCE MISALIGNMENT IN COBALT-60 TELETHERAPY UNITS.

The distribution of ionization in the first few millimeters of tissue-like material was studied with a cobalt-60 teletherapy unit (1, 2). It was found that for short cone-to-skin distance (C.S.D.) the effect of electrons from the collimating system could be reduced appreciably by thin filters which did not noticeably affect the gamma-ray dosage pattern. Optimum results were obtained with the filter mounted over the lower end of the treatment cone. With this arrangement, however, the filter obscured the beam of light from the light localizer unless it was removed with each use. The interference of a filter with light localization is not encountered with x-ray generators in which the filter employed to harden the beam is located above the mirror of the localizer. Below the collimating system of an x-ray generator, filters to reduce electron contamination are generally not needed, for several reasons. For units operating below the supervoltage region, the average energy of secondary electrons is low, and they are removed from the beam through air scatter even for short diaphragm-skin distances. Further, the focal spot of an x-ray generator is usually small in comparison with the size of a teletherapy source. Therefore, for a given focal skin distance, long diaphragm-skin distances can be used without resulting in wide penumbras. Filters to reduce electron contamination are not needed when the diaphragm-skin distance is large. The low-activity sources employed in some teletherapy units necessitate treatment with short source-to-skin distances (S.S.D.) if the treatment time is to be kept within reasonable limits. If the penumbra width is to be kept small, this requires treatment at a short cone-to-skin distance, for which the electron contamination is appreciable. To minimize the skin dose, it was felt advisable to use an electron filter for treatment dose to the cone. It was also considered desirable to retain the light localizer without having to remove and replaces the filter with each use. Since the purpose of a light localizer is to mark the outer extremities of the field, it was found possible to leave the filter attached to the cone without hindering this function of the localizer. In fact, the method developed is believed to have improved light localization. Method of Mounting Filters and Results A Lucite cap, with bottom and walls of 1.5 mm. thickness, was constructed to fit over the end of the cone (Fig. 1). A sheet of cadmium, 0.25 mm, thick, was then attached to the Lucite so as to cover the opening of the cone except for a narrow slit (about 1 mm.) around the edges. With this arrangement, the filter can easily be removed if desired. The light pattern from the localizer is shown in Figure 2. This pattern was made with the light from the bulb of the light localizer collimated so that its image in the mirror reproduced the source in both shape and position.

Clinical applications for the liquid scintillation counting of beta particles in body fluids and tissues are under study. The method offers greater efficiencies and lower backgrounds than are obtainable with conventional gamma-ray well-counting for nuclides having both beta and gamma emissions. It is also applicable to pure beta emitters. The technic was adapted from procedures described in the literature for the counting of beta emitters on paper chromatograms. Fluid samples were placed on lengths of filter paper (Whatman 1 has proved the most suitable). After complete drying of the sample, the paper was rolled and immersed in the counting vial containing the scintillator solution. The samples were then counted in a Packard Tri-Carb Liquid Scintillation Spectrometer at the appropriate settings giving the highest merit value. For the beta particles of I131, the settings determined were 5–50 volt channel with a high-voltage tap setting of 1 (approximately 730 volts). To determine the optimum conditions for the maximum counting efficiency for I131 beta particles, various parameters were systematically investigated with the help of a factorial experimental design with one replication. An analysis of variance technic was used to test the hypotheses for the main parameters and some interactions between them. From this analysis, it was determined that maximum efficiency was obtained with standard concentrations of primary (PPO) and secondary (POPOP) scintillator solutes in toluene and with maximum quantities of scintillator fluid. The efficiency of counting I131 beta particles decreased with increasing paper size (1 3/4 inches wide and up to 18 inches in length) and with increasing quantities of serum (0.1 up to 3 ml.). Background (serum sample on paper in scintillator fluid) was not affected by paper size or serum sample size, but did increase with increasing scintillator fluid volumes. Color quenching was encountered with the larger serum volumes; however, bleaching technics (using hydrogen peroxide) served to overcome this problem and increase the efficiency. A standard method has now been evolved for the counting of I131 in a 3-ml. sample of serum which gives an efficiency of 60 per cent. A cesium-137 sample (obtained by drying CsCl on paper and immersing in the scintillator fluid) served as a standard check for daily operation of the spectrometer. A higher merit value was obtained for liquid scintillation counting of I131 beta particles than was obtained by conventional gamma well-counting technics (both integral and window counting). For the purposes of estimating total serum I131, protein-bound I131, and RISA, and for chromatographic analysis of iodinated compounds in blood, this is a practical method and allows considerable tracer dose reduction to the patient.

We describe the commissioning of the first dedicated commercial total body irradiation (TBI) unit in clinical operation. The Best Theratronics GammaBeam 500 is a Co‐60 teletherapy unit with extended field size and imaging capabilities. Radiation safety, mechanical and imaging systems, and radiation output are characterized. Beam data collection, calibration, and external dosimetric validation are described. All radiation safety and mechanical tests satisfied relevant requirements and measured dose distributions meet recommendations of American Association of Physicists in Medicine (AAPM) Report #17. At a typical treatment distance, the dose rate in free space per unit source activity using the thick flattening filter is 1.53 × 10−3 cGy*min−1*Ci−1. With a 14,000 Ci source, the resulting dose rate at the midplane of a typical patient is approximately 17 and 30 cGy/min using the thick and thin flattening filters, respectively, using the maximum source to couch distance. The maximum useful field size, defined by the 90% isodose line, at this location is 225 × 78 cm with field flatness within 5% over the central 178 × 73 cm. Measured output agreed with external validation within 0.5%. End‐to‐end testing was performed in a modified Rando phantom. In‐house MATLAB software was developed to calculate patient‐specific dose distributions using DOSXYZnrc, and fabricate custom 3D‐printed forms for creating patient‐specific lung blocks. End‐to‐end OSLD and diode measurements both with and without lung blocks agreed with Monte Carlo calculated doses to within 5% and in‐phantom film measurements validated dose distribution uniformity. Custom lung block transmission measurements agree well with design criteria and provide clinically favorable dose distributions within the lungs. Block placement is easily facilitated using the flat panel imaging system with an exposure time of 0.01 min. In conclusion, a novel dedicated TBI unit has been commissioned and clinically implemented. Its mechanical, dosimetric, and imaging capabilities are suitable to provide state‐of‐the‐art TBI for patients as large as 220 cm in height and 78 cm in width.

Purpose: To characterize the photon spectrum from a Theratronics T1000 cobalt-60 teletherapy unit using Compton spectrometry and compare to primary beam measurement and Monte Carlo (MC) simulation. Methods: A well-shielded high-purity germanium detector was used to measure the photon spectrum scattered at 22.5 and 70 degrees from the central axis of a 10cm×10cm beam of a cobalt-60 teletherapy unit. The central-axis photons were scattered using an aluminum rod (6mm diameter) positioned 100cm from the source, and the scattered photons were admitted to the detector through a 30cm-long collimating aperture (2mm diameter). The measured Compton-scattered spectra were corrected for detector response, determined using a model of the measurement apparatus in the MCNP5 MC code, and Compton relations (energy-angle and Klein-Nishina) to determine the central-axis spectrum at 100cm from the source. The primary photon spectrum was measured with the collimating aperture aligned to the central axis and corrected for detector response. A MC model of the cobalt-60 unit was developed in MCNP5 and used to generate the central-axis spectrum for a 10cmx10cm field. Results: The primary photon peaks from the cobalt-60 source were appropriately shifted to lower energies in the Compton-scattered spectra. The resolution of the shifted peaks degraded with increasing scattering angle. The corrected scattered spectra compared well with the primary measurement and MC simulation for peak position, but disagreed in the lower-energy continuum where source and collimator scatter are present. Differences are likely due to resolution loss with scattering and incomplete measurement correction. Conclusions: The spectrum from a cobalt-60 teletherapy unit was measured using Compton spectrometry. This spectrum was compared to direct measurement and MC simulation of the unit. Results indicate that small scattering angles should be used with this measurement technique and that further work is required to reconcile differences between measurements and simulations for source and collimator scatter.

Many designs of radiation therapy rooms miss the correct shielding requirements due to improperly selected equipment or incorrectly chosen design. These designs may have been established based on the most common devices in the market at design time or based on previous work or experiences. Furthermore, the device upgrading from low energy to high energy or from cobalt tele-therapy gamma-ray units to linear accelerator may result in an imperfect shielding. Through implementation, problems may be faced to keep the required safety levels, furthermore, to satisfy the needed protection. In this paper, we illustrate through selected case studies, some of the problems faced and how it could be overcome in order to reach the required protection levels. This study includes seven bunkers at different places whose designs prevented to establish a linear accelerator due to room areas, room surrounding spaces or type of existing shielding: wall or layers thickness and material. Three rooms were predesigned for a cobalt tele-therapy system with an overall wall thickness of 70 cm; two rooms were inadequately designed with irregular walls of 1 m thickness, and the other two rooms were designed for low energy devices and are to be upgraded to high energy, in-addition to different inadequate conditions surrounding these bunkers. Combined solutions were used to overcome the faced problems. The presented solutions incorporate using other shielding materials than concrete as lead, borated polyethylene, gamma-600 together with changing and/ or controlling the obliquity factor (changing the incident angle of radiation). The existing shielding may be also be modified by adding an internal/external layer of lead. Moreover, the position of the device inside the room was re-allocated with a certain angle to the incident rays in order to reduce leakage radiation. The resulting design solutions were validated via the atomic energy commission at those countries.

Autografts of Bone Marrow in Dogs After Lethal Total-Body Radiation

Strictly speaking, the term megavoltage therapy should be applied to the use of x rays and gamma rays which have an effective energy of 1 million electron volts or more. However, for our purposes the term can be applied to cover all x rays generated by voltages greater than 1 megavolt and terminating, for medical purposes, at 43 megavolts, as well as cobalt 60 gamma radiation. From the practical point of view, the apparatus in regular use for providing high energy radiation are cobalt teletherapy units (1.2 megavoltage gamma radiation), Van de Graaf generators (1 to 2 megavoltage x rays), linear accelerators (4 to 8 megavoltage x rays), and betatrons (18 to 43 megavoltage x rays). Both the linear accelerator and the betatron have advantages over a cobalt unit in the sense that they can produce both electrons and x rays; they are capable of carrying a higher patient load

Total body photon irradiation (TBI) may be delivered with a number of standard techniques, typically using extended SSD geometries to obtain large field sizes. Since 1982 we have treated over 620 patients (adult and pediatric) mostly in the floor-located prone/supine treatment position with AP/PA beams using a column mounted 4 MV linac, and more recently with a fixed field, extended SSD, Cobalt teletherapy unit. This treatment position has many advantages for TBI including ease of delivery (especially for pediatric or compromised patients), improved dose uniformity, simplicity for partial or complete organ shielding, and imaging capabilities to name a few. In an effort to transfer this technique to a conventional isocentric linac, we have undertaken a feasibility study for RapidArc based extended SSD total body irradiation. Proof of principle was first explored using the DBD (Dynamic Beam Delivery) toolbox to configure a 6 MV beam with an 80 degree arc, centered on the 12 o'clock gantry angle with a sliding-window beam. This was followed by the development of an ECLIPSE generated 80 degree RapidArc plan. Initial measurements were conducted with a Varian 21EX using the 6 MV DBD beam to explore characteristics such as PDD, surface dose, off-axis ratios, output, dose per MU, and linearity. Subsequently ECLIPSE generated RapidArc TBI plans using similar partial arcs were also evaluated. Encouraged by our results, we believe this technique shows potential for making floor-located AP/PA total body photon irradiation possible for any standard RapidArc enabled isocentric linac.

60Co teletherapy units featuring 100 cm source-to-axis distances and with nominal source activities of 481 TBq (13,000 Ci) are available as replacements for older model 80 cm source-to-axis teletherapy units that generally have source activities of less than 370 TBq (10,000 Ci). We have redesigned and renovated two vaults--one of which required supplemental shielding--designed for older 80 cm source-to-axis units with 333 TBq (9,000 Ci) sources and installed modern 100 cm source-to-axis units with 481 TBq (13,000 Ci) sources. Renovation costs (in 1991 U.S. dollars) were about $40,000 and $160,000; the latter, including shielding costs, was about two-thirds of projected costs for installation of a 6 MV electron beam linear accelerator (linac). We have designed the treatment heads of these 100 cm 60Co units to accept the secondary field shaping blocks used on 100 cm source-to-axis linacs, allowing the 60Co units to be used as emergency back-up units for linacs when they are inoperable.

Fractional exposure rate due to scattered radiation from a Co 60 teletherapy unit

Scattered radiation from linear accelerator and cobalt-60 collimator jaws

In recent years there has been a growing interest in total body, hemibody, total lymphoid irradiation. For refractory leukemia or lymphoma patients, various techniques and dose regimens were introduced, including high dose total body irradiation for destruction of leukemic or bone marrow cells and immunosuppression prior to bone marrow transplantation, and low dose total body irradiation for treatment of lymphocytic leukemia or lymphomas. Accurate provision for specified dose and the desired homogeneity are essential before clinical total body irradiation. Purposes of this paper are to discuss calibrating Cobalt Unit in 3m distance using Rando Phantom, to compare calculated dose, calibrated dose, and compensating filters for homogeneous dose distribution in the head and neck, the lung, and the pelvis. Results were following. 1. Measured dose on the lung was 6% higher than on the abdomen. Measured dose on the head (10%) and neck (18%) were higher than the abdomen because of thinness. Pelvic dose was measured 12% less than the abdomen. Those data suggest that compensating filter was essential. 2. Measured dose according to distance was 3% less than calculated dose which suggest that all doses in clinical use should be compared with calculated dose for minimizing error.

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.

THE ADVANTAGES of intracavitary and interstitial radiation therapy have been long acknowledged. In both oral and pelvic carcinomas, which are a considerable fraction of the curable lesions seen by the radiation therapist, the most significant therapeutic contribution toward control is exemplary intracavitary or interstitial radiation therapy (1–6). Neither peroral nor transvaginal intracavitary teletherapy, nor the interstitial injection of colloidal radioactive suspensions has been shown to be other than a poor substitute for carefully planned and executed intracavitary applicator or interstitial needle treatment of the same conditions. In spite of technical advances, i.e., afterloading (7), Ir192 (8), etc., the use of these forms of radium or isotope therapy has not progressed commensurately with other advances in radiation therapy. This is most probably because of the therapist's difficulty in learning the technics and dose distribution of implants without subjecting a number of patients to less than adequate therapy while expert skills and judgment are being acquired. While there seems to be a marked swing back to interstitial and intracavitary therapy from dependence on external radiation alone (9–11), it is probable that this transition to the methods and the concepts inherent in short focal distance therapy (and thus implicit in intracavitary and interstitial treatment) would be more rapidly achieved if a critical evaluation of implants were possible. This should permit correlation of dosage with successor failure to assist in training the beginner and in correcting the misapplication of the expert. Because of the difficulty and confusion concerning the dose distribution in the region of an intracavitary device or in the volume of the sources of an interstitial application, the need for meaningful expression of dosage estimation has been without adequate solution until recently. This need is concerned with the cases undergoing treatment, with the planning of future cases, and, more particularly, with the instruction and teaching of the art and technics of interstitial and intracavitary radiation therapy. Though it is simple to remember that if the dose at 1 em from a point source is 80 roentgens per hour, the dose at 2 em is 20 roentgens per hour, and the dose at 4 em is 5 roentgens per hour, it is difficult to convey in a meaningful form the inhomogeneity of dose that occurs in the region of an applicator consisting of multiple linear sources. The tremendous difficulty of imagining this dose distribution arises from the inability to think “inverse square.” In a given radium application, it is not hard to demonstrate by rapid calculations that the dose at two points may vary by a factor of two or more, resulting in overirradiation of one region and underirradiation in the other. It is not easy to recognize this difference from looking at an anterior and lateral view of the implant unless a fairly sophisticated understanding of dose distribution has been developed.

A novel printed antenna configured with circular loop geometry of approximately one wavelength perimeter is designed for operation at 5.8 GHz with near omni-directional radiation pattern. The loop is partitioned into multiple segments and loaded with capacitive elements at selected locations so as to decrease phase variations in the current flow and thereby increase the radiation efficiency. A total of five capacitors are used to achieve stable current flow, resulting in phase variations of <; 12°. The performance of the loop antenna is first analyzed as a transmitter in free space using a method of moments (MoM) solver for thin-wire structures, and then validated using 3-D finite element method (FEM) and MoM solvers. The simulated radiation pattern for the thin-wire model in the plane of the loop is close to omni-directional with directive gain of 1.46 dBi. A printed circuit antenna model is then designed with alternating top- and bottom-layer conductors of annular geometry on a thin substrate, with regions of conductor overlap functioning as physical capacitors. Simulations of the printed antenna demonstrate omni-directional radiation in the azimuthal plane, with peak directive gain of 1.66 dBi, peak directivity of 1.69 dB, radiation efficiency of 0.98, and input impedance close to 50 Ω. Performance measurement data for a fabricated printed circular loop antenna (in process) will be shown during presentation for comparison and validation.