During a radiosurgery procedure, the center of the targeted lesion will receive a certain amount of radiation.  There will be a spot in the lesion which will receive 100% of the maximum dose.  For example, a part near the center of the lesion may receive 20 Gray (a unit of radiation).  There will be a curve or surface which will receive 30% or this, and one which will receive 50%, etc.  These are known as isodose lines, much like the isothermal lines of temperature on a map of the United States.  The 50% isodose line will receive 50% of the maximum dose, or half the dose.  In the example above, if the maximum dose is 20 Gray, then the 50% isodose line will receive 10 Gray. 

In the figure, we are looking at a metastatic tumor (spread from another part of the body).  The 55% isodose line is seen in yellow, and this will receive 55% of whatever the maximum dose is to the tumor.