DEVICE FOR MEASURING THE INTEGRAL DOSE IN LOCAL IRRADIATION

V. A. VOROBYEV, Yu. D. GAVKALOV, N. M. KOLBIN, A. M. KOLBIN

The integral dose is regarded as a factor determining the general condition of the patient after radiation therapy \[1\]. Under equal clinical conditions, the irradiation method chosen is the one that produces the smaller integral dose. Experimental determination of the integral dose entails measuring absorbed doses at a large number of points throughout the volume of a phantom, and calculating, by approximate formulas, all absorbed doses in the regions between the corresponding isodose surfaces.

We have attempted to construct a device that makes it possible to determine the integral dose in a single act of measurement. The design of the device is based on the construction of the Gauss quantometer \[2, 3\]. A distinctive feature of the device is that, in order to ensure conditions in the device for the absorption, scattering, and leakage of beam energy equivalent to the conditions in a “standard” human being, the external dimensions of the absorber were chosen equal to the effective radius of the “standard” human being. The effective radius of the “standard” human being was adopted, in accordance with the Recommendations of the International Commission on Radiological Protection, as equal to 30 cm \[4\]; the need for a gap taking into account the area under the tail of the transition curve is eliminated, the plates are made of tissue-equivalent plastic, and the magnitude of the annular gaps, which take into account the absorbed dose at the periphery of the beam formed for local irradiation, is calculated from the dimensions of the “standard” human being.

Graphitized plastic AKR-7 was taken as the material for the plates: the effective ordinal number \(Z_{\text{eff}} = 6.4\); density \(\rho = 1.2\ \text{g}/\text{cm}^3\). The device is designed for irradiation fields with a diameter of up to 120 mm and a limiting bremsstrahlung energy of \((20+30)\) MeV. Several variants were calculated for an integration interval of 20 cm and a Gauss quadrature formula with \((6+9)\) nodes; as a result, a quadrature formula with 6 nodes was selected. The gaps between the plates are obtained as follows: \(\Delta_1 = 1\ \text{mm}\); \(\Delta_2 = 2.1\ \text{mm}\); \(\Delta_3 = 2.73\ \text{mm}\); \(\Delta_4 = 2.73\ \text{mm}\); \(\Delta_5 = 2.1\ \text{mm}\); \(\Delta_6 = 1\ \text{mm}\); and the plate thicknesses: \(d_1 = 6.75\ \text{mm}\); \(d_2 = 27.13\ \text{mm}\); \(d_3 = 42.26\ \text{mm}\); \(d_4 = 47.72\ \text{mm}\); \(d_5 = 42.26\ \text{mm}\); \(d_6 = 27.13\ \text{mm}\); \(d_7 = 6.75\ \text{mm}\). The diameter of the smaller plates is 200 mm, the diameter of the larger plates is 266 mm. The sensitivity of the device, estimated theo-

typically amounts to \(6 \cdot 10^{-19}\) coul/MeV at the limiting energy of bremsstrahlung \(E_{\gamma \max} = 30\) MeV.

The device may be useful in planning irradiation as an experimental means for evaluating the preference for one method or another.

REFERENCES

1. H. Johns, The Physics of Radiology, Atomizdat, Moscow, 1965.

2. R. P. Meshcheryakov, On the design of a quantometer for measuring bremsstrahlung with a maximum energy below 100 MeV. Proceedings of the Interuniversity Conference on Electron Accelerators, 417–419, Vysshaya Shkola, Moscow, 1964.

3. A. P. Komar, S. P. Kruglov, I. V. Lopatin, Doklady AN SSSR 167, 785, 1966.

4. D. I. Zakutinsky, Yu. D. Parfenov, L. N. Selivanova, Handbook on the Toxicology of Radioactive Isotopes, GIML, Moscow, 1962.