A basis for blocking the spinal cord in opposed field irradiation

The concept of partial tolerance is applied to the equivalent dose formula for spinal cord tolerance to allow for the change in dose when open and blocked fields are used in a course of radiotherapy. The field arrangement considered is parallel opposed irradiation. Certain assumptions are made regarding the contribution of dose to the spinal cord from open and blocked fields, which allows for the development of an equation to calculate the number of fractions that require spinal cord shielding in the posterior field. The use of a form to facilitate these calculations is described.