Radiation Factors Research Articles

General equations for the calculations of biologic effect ratios for parallel opposing fields.

The tolerance level of normal tissue, which is the concept of NSD, is the limiting factor in radiation therapy. It is well known that the two parallel opposing fields should be treated at each session instead of alternating one field per session. The biologic effect ratios between the normal tissue at the depth of maximum build-up and the midline for parallel opposing fields were published by ELLIS et coll. General formulae are now presented providing biologic effect ratio at any two locations in terms of per cent depth doses in the treatment volume for parallel opposing fields. Examples at the depth of maximum build-up and certain depth of the connective tissue at the tumor site are also given.

Physical Factors in Radiation Therapy and Their Clinical Application

IF the response to radiation therapy in the treatment of malignant disease is due to the direct action of the roentgen ray on the cell structure, it is then of paramount importance to adhere to some technic possessing a certain degree of flexibility and yet deliver to the tumor area the maximum amount of radiation. The necessity for a flexible technic is to allow for adjustments to the requirements of a particular case. The age, duration, location, and extent of involvement are so variable that the clinical judgment of the radiologist must alter any usual procedure of radiation management. The so-called “erythema dose” does not necessarily exist as a therapeutic dose The problem of a radiation therapy technic, therefore, becomes more complicated, due to its inherent variability. If we could have one standard therapeutic dose, then the problem would be solved. Every radiologist would know dosage technic in the same terms. There are a great many fractional dose technics described by various workers, most of which can roughly be put into one of two classes: a divided series of radiation treatments given within a period of about three weeks, or smaller doses of radiation extending over a longer period of time (4–8 weeks). Usually no systematic scale of dosage is adhered to, and more often the condition of the skin or the clinical response is the only guide. The erythema dose will probably always be a variable factor, because the degree or threshold determination must necessarily depend upon the personal judgment of the radiologist. It is almost generally agreed that there is a variability of sensitivity to radiation in different patients, so that an absolute erythema factor is difficult to establish. It is important, therefore, to have some standard to which all technics can be compared. The exponential curve (1) representing the probable biologic changes taking place in the tissue aims to keep up a continuous radiation or saturation within the limits of skin tolerance. This curve may not be an accurate representation of the true biologic changes in the tissue, but this should not contra-indicate its use. The saturation curve can act as a guide in giving radiation as well as a standard for comparing all fractional close technics, and any errors in adjustments will always be relative. It is important to know the quality and quantity of radiation used when treating patients. Sphere gap and meter controls are essential, but must be controlled by careful ionization measurements. Different types of rectification may give intensities of radiation with as much as 100 per cent variation. The variable fluctuation of voltage and current necessitates a system of ionization measurement that will enable one to know the exact output of radiation during an actual treatment.