When a plaque is offset and/or wobbled, when Central AXis (CAX) points are specified and doses tabulated, when seed coordinates are displayed for COMS and other plaques, and when a plaque is exported as an .stl file for rapid prototyping, the origin of its coordinate system is at the center of its face, ie the concave surface of the plaque (or carrier) as illustrated above. This center was established when the plaque shell was created in the Shell Editor window.

The X axis of this coordinate system is the Central AXis (CAX) of the plaque. In the Plaque and Shell Editor windows you are looking at a 2D projection of the plaque onto the YZ plane.

When designing a plaque in the Shell and Slot editors, the origin of the plaque's coordinate system is shifted along its Central Axis (CAX) (ie the X axis) to the center of a sphere of radius R that matches the concave surface curvature of the plaque, or of the seed carrier if it has one.

The default center of a linear source (eg an I-125 seed) or a beta patch source is initialized as tangent to a sphere of radius R' = R + Source Offset, with the X axis passing through the center of the source at X = -R'. The initial orientation of the linear source is parallel to the Z axis (tilt = 0°). Plaque Simulator first orients the source by rotating it about its center by its tilt angle, and then uses the angles alpha (or r, its projection onto the YZ plane) and beta to rotate the source center to its final location on the R' sphere.