In radiation therapy it is generally appreciated that tumor effect is dependent on tumor dose, and on the total time during which this dose is administered. For a particular patient and series of treatments, determination of tumor dose requires the following procedures: 1. Construction of a life-sized cross-sectional diagram of the individual at the proper level. 2. Indication on this of the exact position, size, and shape of the tumor, in so far as possible, and of the portals of entry of each field. 3. Application of isodose charts for each field to obtain the dosage contribution in various parts of the tumor—not the central point alone —and for any other points of concern, such as skin portals or vital organs. 4. Addition of contributions from all beams to obtain total doses, for all points in question. Frequently before a series of treatments is undertaken, several different dosage schemes need to be studied, to see how the desired tumor dose can best be administered. It is evident that the use of...