4 1 8 D O C . 4 3 8 R A D I A T I O N B I O L O G Y A N D T H E R A P Y For X-ray radiation, a) predominates and it is practically the only one that is ef- fective in medical applications. For γ-rays, b) plays an at least equivalent role.[4] In all cases, the physical end result of the radiation constitutes a number of ion- ization acts, whose secondary chemical processes would constitute the biologically effective component of the irradiation. II. Do the individual kinds of radiation (α, β, and γ), and particularly those within the X-ray spectrum, have specific characteristics with respect to physico-chemical processes, or is the radiative effect only dependent on the quantity per unit volume of the absorbed dose of radiation? If one disregards α-rays, which owing to their low penetration power (order of magnitude 1/10 mm in tissues) can only serve medical purposes as surface irradi- ation, the effect of radiation depends solely on the absorbed energy per unit volume. It surely may be taken for granted that this energy is principally expended on the ionization acts, which are practically randomly distributed throughout the volume. III. Is the effect of the energy of radiation within certain limits independent of how we change intensity and time when keeping their product constant? Does the effect on physico-chemical processes depend on the absorbed radiation dose (stimulation–initial destructive effect)? The reactive substances produced by irradiation are in all cases in vanishingly small concentrations. That is why it seems to me settled that (for a fixed total ab- sorbed energy) the irradiation time has no influence, albeit with one constraint: if the irradiation time is made so large that within the given time noticeable regener- ation of the tissue can occur through growth or material restoration, then this can diminish the total effect. The upper limit within which, according to this consider- ation, the irradiation time would have to be chosen in order to reliably attain the maximal effect would be the time needed for the restoration of the sick tissue’s fluid content. However, it is a priori probable that this upper limit of time is far greater, as only those chemical effects ought to be damaging that directly hit the structural substances of the tissue, which restore themselves much more slowly. Professor Lazarus told me that, according to the opinion of some researchers, cancer cells are especially sensitive in the state of division. If this were true, a lon- ger time of irradiation could possibly be advantageous at a given total absorbed en- ergy. It is, of course, entirely beyond my ability to assess whether there is any truth to this opinion. I would only take this possibility into account, though, if strong em- pirical reasons for it were put forward. [p. 2]
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