Use this menu to select a prescription point. The most common prescription point for medium to tall tumors is the apex of the tumor.
A prescription point of historic interest is the COMS 5 mm point which was defined to be 5 mm from the inner surface of the sclera on the central axis (CAx) of the plaque. Assuming that the sclera is 1 mm thick, this is equivalent to 6 mm from the concave surface of the plaque. The COMS protocol required that for tumors less than 5 mm tall, the Rx height be set at 5 mm from the inner sclera. This helped to assure an adequate margin surrounding the base of small tumors in the era before image guided planning.
Since PS uses fundus images whenever possible to delineate the tumor base and surrounding margin, the Rx for small tumors can be set at whichever point yields the desired dosimetric coverage of the tumor apex, base, and surrounding margin. For instance, the plaque CAx might not pass through the tumor apex. To address that situation, PS also offers Rx points on the tumor axis (TAx), a ray originating from the geometric center of the tumor base in the direction of the tumor apex.