The Co-Relation of Physical and Clinical Data in Radiation Therapy

THE more important problems of radiation therapy can be roughly summarized as follows: A. The estimation of the radiosensitivity (1 and 2) of a given lesion (by biopsy study). In terms of + − skin unit dose. B. The selection of the proper quality of radiation according to the location and depth of the lesion. By absorption curves, by the 10 cm. depth dose, or by the half value layer correlated to phantom measurements. C. Delivery to the lesion of the required radiation dose intensity. In r, or in fractions or multiples of the unit skin dose. D. The number of portals (by crossfire) to be selected (with skin tolerance the limiting factor) to build up the required depth dose. E. The choice of treatment method: a. Multi-unit single dose (destructive to superficial lesions). b. Sub-unit with recovery interval (applicable to the more radiosensitive lesions). c. Fractional cumulative (for all deeper seated and less radiosensitive lesions). To bring to realization the importance of the subject matter under consideration, it might be well to put this in the form of two questions, namely: a. Of what significance are quality variations? b. Just what does it mean when one records a certain number of r as dosage in radiation therapy? A basic fact ever to be kept clearly in mind is that ionometric dose calibration in r is a purely physical measurement, and has value in radiation therapy only when properly correlated to biological reaction. With due respect for much important biological research and experimentation, nevertheless, in clinical radiation therapy, the human skin still remains the most practical guide as a biological index of dosage. The variants of quantity, quality, and time, as well as the area, exclusive of the obvious inverse square distance rule, are under consideration insofar as these may influence the r dose in relation to a given reaction. In this connection, it might be well to state that these variations are not hairsplitting, small percentage differences, but sufficiently large to afford clinically observable effects. Clinical experience soon forces one to the conclusion that the r dose is not purely a mathematical compilation of the obvious physical factors. Observation indicates that with all other factors constant, there is a considerable difference in the r dose required for a given reaction, depending upon at least five important variants: 1. Voltage and filtration (wave length); 2. Size of portal (area exposed); 3. The intensity of dose administration, i.e. r/min., or 4. The rhythm or frequency of a given dose application; 5. The depth at which the calculated dose is delivered. Notwithstanding experiments which indicate that the biological effect is independent of the wave length (3), there appears to be a definite difference in the reaction of thin layers of biological test material and thick human tissue mass.