The Calculate 2D Matrices item of the Dosimetry menu begins calculation of the currently active 2D dose matrix set. If this item is dimmed and can not be selected it means there are no sources in the currently active plaque.
A 2D matrix set consists of dose matrices for three surfaces; a meridian plane, a coronal plane, and a spherical surface representing the retina. There are four matrix sets available. By default, each matrix set is associated with a plaque: matrix set 1 is associated with plaque 1, matrix set 2 with plaque 2, and so on... When you switch from one plaque to another, the current matrix set is switched as well. This allows you to perform side-by-side dosimetric comparisons of different plaques and loadings.
If Auto. Update 2D Matrices is checked, the 2D matrices will update automatically in the background whenever a change occurs. For example, adding or removing a seed or moving the plaque would trigger an update. The calculation time for plaques that use seeds is generally fast enough that this feature can be enabled, but Ru plaques require a few seconds to calculate so you may wish to leave it disabled until you gain experience with the software.
Isodose plots of the 2D matrices can be displayed in the Dosimetry, Retinal Diagram, and Setup windows, and in several documents.