Relative Depth Dose Research Articles

Dosimetric properties of neutron beams from the D-D reaction in the energy range from 6.8 to 11.1 MeV

The tissue kerma in air, the tissue dose at maximum build-up, the relative depth dose on the central axis and the dose build-up characteristics were measured for neutrons produced by 6.8, 8.9 and 11.1 MeV deuterons on deuterium. The neutron beams were produced by a variable-energy cyclotron with a fully stopping deuterium gas target 20 cm long. Measurements were made in a 11.1 cm*11.1 cm field 126 cm from the target entrance window.

Experimentally determined tissue air ratios and scatter air ratios for collimated beams of 14 mev neutrons.

Tissue air ratios (TAR) and scatter air ratios (SAR) for collimated beams of 14 MeV neutrons were determined from depth dose curves measured in air and water using a tissue equivalent proportional counter. Central axis depth dose data were obtained as a function of field size and depth in a water phantom for an SSD or 125 cm. The collimator was composed of steel and polyethylene and was continuously variable from a 3 times 3 cm to a 20 times 20 cm field size at 125 cm SSD. The field scans for this collimator were flat within plus or minus 2 per cent over 80 per cent of the field dimension in air and were reduced to 20 per cent of the central axis dose at 1-2 field radii in air. Relative depth dose data in air and phantom were measured with an accuracy of plus or minus 2 per cent and were used for the TAR and SAR calculations. The resulting scatter air ratios for 14 MeV neutrons have roughly the same characteristics as 60-Co SAR, but are 60 per cent larger at maximum build-up and reach that maximum at about 60 per cent of the peak depth for 60-Co for all field sizes. The effects of collimator throat scatter on the central axis depth dose characteristics of this particular collimator design are discussed in detail using the measured tissue and scatter air ratios.

Dependence of central axis depth dose on field size for 35, 40 and 45 MeV electrons

Doses have been measured in a water phantom using an ionization chamber. The data have been obtained for a large selection of field sizes including elongated and circular fields. A strong dependence of depth dose on field size is observed. The values of the electron absorption coefficient ( mu ) for water were calculated for various field sizes at the above energies. The terms absorption number (n) and 90% depth (D90) were introduced and mathematical relations among n, D90 and mu were obtained using the empirical relation I(x)=110-10 exp ( mu (x-dm)) to describe the relative depth dose, I(x), as a function of depth x. Depth dose data were analysed using the above parameters and variations of D90 with beam energy and field size were discussed.

Comparisons of 10-40 MeV electron and X-radiation dosimetry methods

Comparisons have been made between the dosimetry methods for 10-40 MeV electron- and X-radiation used at the radiation treatment centres in Essen and Villejuif by the respective resident staff and that used by a visiting group from the centre in Umea. The absorbed doses from both electron- and X-radiation and the relative electron depth doses were measured using thimble ionization chambers. Electron isodose curves were derived from data obtained with photographic films. In both comparisons, Essen-Umea and Villejuif-Umea, the absorbed dose and relative electron depth dose data showed good agreement while the electron isodose curves showed significant discrepancies.

Results of comparative studies in a man-like phantom between measured and computer-determined dose distributions with different radiation methods.

The technical development of equipment for radiotherapy in the direction of new technics of administering highenergy radiation, such as cobalt units, linear accelerators, and betatrons, has in our opinion decisively changed the role of deep therapy, even though in radiobiological effectiveness there is no difference between 200 kV radiation and high-energy radiation. The increase of relative depth dose, the overcoming of the barrier of injurious effects to the skin, the leveling of differences in absorption in bones and soft tissues, and the reduction of lateral scattering are basic steps toward an old ideal: making the dose distribution within the patient's body conform to the actual dimension of the individual tumor. In the era of conventional or 200-kV deep therapy, despite the possibilities of the moving field, extensive areas of the body were given a fairly uniform dose in the treatment of deeply situated tumors. This dose was often kept too low for the tumor, even though it was rather strong for healthy tissue. The therapeutic effect of such irradiation was based almost entirely on the difference in radiosensitivity between tumor and normal tissue. Only with the advent of high-energy irradiation with its especially designed moving-field irradiation and wedge filters did maximal extended dosages to deep areas of the body become possible, with clear reduction of the dosage to the surrounding tissue. By "selective" dose distribution, one may now confine the radiation effect to the tumor tissue to a degree which may be decisive for therapeutic success. This selective dose distribution is satisfactory, however, only when there is satisfactory coincidence between the dimension and form of the maximum dose and those of the tumor and its local area of expansion. Successful results, moreover, depend not only upon the accurate localization of the tumor but also upon a tremendous amount of mathematical calculation of the dose distribution, which necessarily varies from case to case. In routine work such calculation cannot be handled by ordinary reckoning. It is our belief that the use of computers is inevitable today if we would fully utilize the capacity of modern high-energy equipment. Our method of dose calculation with cobalt-50 therapy by means of a digital computer was developed in the Institute for Cancer Research, Robert-Roessle Hospital, Berlin, by Richter and Schirrmeister (1–4). It does not employ fixed-field distributions, but is based on an equation originated by Pfalzner (5):

New developments in high energy electron beam therapy with the 35 Mev Brown Boveri betatron.

Electron beams have now been used several years for therapy. The clinical results up to this time are favorable and, therefore, interest in this new mode of radiotherapy increases more and more, not only for superficial but also for deep therapy. In the Radiation Laboratory of the Swiss Brown Boveri works we have been interested for a long time in the physical and technical problems of electron therapy. The intention of this paper is to give a short survey of the 35-Mev betatron and the latest development of electron-focusing apparatus for therapeutic application. Deep therapy with electrons demands an electron energy of up to 30 to 40 Mev. Figure 1 represents the depth dose curves in water for electron beams with energies between 5 and 35 Mev. An energy of 35 Mev, for example, is necessary for a relative depth dose of 80 per cent at 11 cm. The Brown Boveri betatron produces x-rays and electron beams of continuously adjustable energy from 5 to 35 Mev and is also equipped with a 125-kv diagnostic x-ray unit for beam localization purposes. Fixed-field, pendulum, rotational, and tangential therapy may be carried out. Figure 2 is a schematic diagram for electron beam therapy. The electrons leave the accelerating tube through a window, where they are scattered and suffer an energy loss of 1 Mev. Dose rate and dose are measured by means of a thin-walled transmission type ionization chamber. To collimate high energy electron beams effectively, a sandwich-like composition of aluminum, brass, and lead layers, giving sharp beam boundaries with only low production of stray electrons and secondary x-rays, is used. Depth dose obtained with high-energy electrons is good, as long as the radiation field is not small. The curves in Figure 3 illustrate the dependence of depth dose on diameter of the irradiation fields for 30-Mev electrons. A relative depth dose of 72 per cent, corresponding to a diameter of 10 cm. and a depth of 10 cm., is decreased to 33 per cent if the diameter is reduced to 3.5 cm. In order to improve the depth-dose curves and isodose distributions for small fields we have developed magnetic focusing electron lenses. Figure 4 shows schematically the construction of such a focusing device. Because of the technical difficulties in focusing the entire beam, only part of the beam, about 10 to 20 per cent, is used. The electrons to be focused pass through the air gap of a strong permanent magnet and are therefore deflected toward the axis. A small motor rotates the apparatus at a constant speed about the axis. The entire circular area within a diameter of approximately 8 to 12 cm. at the skin of the patient is sprayed with electrons coming through the air gap of the magnet. A cone of electrons is thereby produced in the body, focusing at a depth of about 10 to 20 cm. Thus, depth-dose curves with a reduced skin dose and increased depth dose are achieved.

Measurements of Relative Beta Depth Doses in 'Tissue Equivalent' Material

Measurements of Relative Beta Depth Doses in 'Tissue Equivalent' Material