John B. Massey

Radium dosage: SI units and the Manchester system

The book Radium Dosage: The Manchester System Meredith, 1967) describes in detail a practical system, often referred to as the Paterson Parker system, for interstitial, mould and intracavitary radiotherapy. The system, which is used in many centres throughout the world, consists of two parts: there are rules stating how the radium is to be distributed in treating various geometries, and there are tables that allow the user to calculate either the total quantity of radium required in milligrammes or the duration of treatment in hours for a given dosage prescription, assuming the distribution rules have been followed.

I. The problem of air spaces

In clinical practice the most usual method of estimating the magnitude and distribution of dose within a patient is by means of standard isodose curves. We know very well that these curves are strictly valid only for a homogeneous water or water equivalent medium. The effects caused by heterogeneities such as bone and lung tissue have been the basis of many previous discussions and investigations; these will, no doubt, continue in the future. One thing is certain, however, namely that for megavoltage radiation the magnitude of these effects is very much smaller than for kilovoltage qualities. So much so, in fact, that the effect of bone is almost small enough to be negligible, and is certainly easily estimated. The two heterogeneities which it is wished to consider here are lung tissue and the airfilled cavity. Due to its lower density and, therefore, absorption there is a greater transmission of radiation by the lung than by the equivalent thickness of soft tissue which it replaces. There is also a reduc...

EXPOSURE DOSE MEASUREMENTS IN MEGAVOLTAGE THERAPY

Arguments are presented to show that an ionization chamber in a scattering medium measures the exposure dose at its own center provided it is at a depth sufficient to be on the exponential part of the depth dose curve. The minimum depth for the establishment of electron equilibrium is defined. This is not in accord with the usual assurnption that the equilibrium is established at the depth dose curve maximum, and it is shown that the difference in expcsure dose measured according to the two definitions can be as much as 5% for 20 Mv radiations. At 2 Mv the difference is negligibly small. Use of the N.P.L. roentgen calibration factor leads to a statement about the dose at the center of the hole made in the phantom to receive the chamber. The correction factors to be applied to yield the exposure dose at the center of the filled hole are discussed. For 2 Mv radiation the measured factor, which is independent of field size and chamber depth, is 0.991. (auth)

Some measurements on the absorption of 4 MV x rays in concrete.

Measurements have been made on the absorption, in concrete, of the primary and secondary radiation from a 4 MV linear accelerator. The results give tenth-value layers for primary and secondary radiation of 12 in. and 9 in. respectively, in concrete of density 2.2 g/c.c. The distribution of tube leakage radiation, and of secondary radiation scattered from a phantom in the beam have also been examined, and it is shown that scattered radiation is only significant in the forward direction.

A defence of the use of the concept C

In view of some recent publications in which the concept of Cλ seems to have been misunderstood and criticized, the basic equation for the calculation of Cλ is derived. The assumptions in this derivation are discussed and the errors which arise from them in practice are estimated. It is concluded that the use of Cλ remains the present method of choice for converting ion chamber readings into absorbed dose in water.

\lambda

In the previous symposium on the relative biological efficiency of 4 MV X rays, this term was defined as the ratio: The purpose of the present series of experiments is to compare the efficiencies of 4 MV and 20 MV radiation. The relative biological efficiency in this case is defined by the ratio: In accordance with this definition, it was necessary to deliver a series of doses, measured in rads, to the various biological specimens, using 4 MV and 20 MV radiations. As an interim measure, a series of equal ionization doses was delivered at the two qualities, and these doses were subsequently converted to rads. This was possible because the R.B.E. as here defined does not require that equal doses be delivered at the two qualities. As is required in experiments of this type, all variables, oter than that being investigated, the radiation quality, were made the same for the to machines. There is one exeption to this statement, namely, that the 4 MV linear accelerator operates at 350 pulses second, while the 20...