Dose Selection

Abstract

After a treatment plan is optimized, the radiation dose is selected. In general, the dose is expressed in the unit called gray (Gy). One hundredth of a Gy (centigray or cGy) is equivalent to an older unit of absorbed radiation, the rad. The dose is usually prescribed to the isodose line (or shell, in reality) that conforms to the periphery of the target (i.e., the lesion to be treated). For example, the AVM shown in Figure 7.1 was treated with 12.5 Gy to the 80% isodose line. As seen in the figure, the 80% isodose line corresponds to the periphery of the lesion nidus. In this situation, the maximum delivered dose, or 100% of the dose (which lies near the center of the lesion), is 25% higher than the prescribed dose at the 80% isodose line, or is equal to approximately 15.6 Gy. The lower the isodose line to which the treatment dose is prescribed, the greater the difference between the prescribed treatment dose and the maximum dose; in other words, the greater the dose inhomogeneity across the target. For example, if the dose is prescribed at the 50% isodose line, the maximum (100%) dose is twice as high. All doses mentioned in this chapter are those prescribed to the periphery of the lesion.