Abstract
The theory put forward in this paper is based on the assumption that in a given type of mitotically active cells the application of a radiation dose of 1 rad results in a specific chance ε of any of the daughter cells to die off. This assumption will be shown
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to imply:
1. 1. that the dose required to obtain a reasonable chance of a cure by radiotherapy of a given tumour in a given overall time is rather sharply defined and
2. 2. that it depends on the size of the tumour.
Both conclusions are likely to agree with facts.
In certain cases, e.g. in the basal cells of normal epidermis and of epidermal tumours, the values of ε can be derived from empirical data. These values will be considered in connection with a particular hypothesis on the mechanism by which mitotically active cells die off in radiotherapy.
The main points of this hypothesis are as follows:
1. (a) In a mitotic active cell the electrons of part of the hydrogen bound N- and O atoms of the DNA-strands are targets.
2. (b) A target-electron is radiosensitive when the hydrogen-bound of its atom has been released, that is in the periods of involvement of this N- or O atom in DNA synthesis or in RNA formation.
3. (c) When a target electron is detached from its atom in a radiosensititive period, the daughter cell corresponding to the DNA strand of this atom will not develop or be unviable.
A quantitative development of this hypothesis will be shown to suggest the following conclusions on the basal cells considered:
1. (A) A considerable fraction, perhaps about 0·5, of the electrons of hydrogen-bound N- or O atoms in the strand of DNA molecules are targets; the number of target-electrons per daughter cell is therefore of the order of 1010.
2. (B) In a given cell the mean number of periods in which a hydrogen bound N- or O atom of a DNA strand is involved in RNA formation is comparatively small —5 to 20— in normal epidermis and rather larger —20 to 70— in epidermal tumours.
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