Patent ID: 7348974

Claim:
A method of processing a wire mesh computer model of a surface comprising a plurality of triangles defined by vertices connected by edges to generate data identifying a set of points of intersection of said wire mesh model with a plane, the method comprising: (i) identifying pairs of vertices connected by edges at the boundary of said wire mesh model which lie on opposite sides of said plane; (ii) selecting one of said identified boundary edges as an initial edge for processing; (iii) storing data identifying the point of intersection of the selected edge with said plane; (iv) identifying the vertex of said wire mesh model connected to both of the vertices of said selected edge; (v) determining which of said vertices of said selected edge lies on the opposite side of said plane to said identified vertex; (vi) selecting as the next edge to process the edge of the triangle connecting said identified vertex and the vertex of said processed edge lying on the opposite side of said plane; and (vii) determining whether said newly selected edge corresponds to an edge identified as a boundary edge and if not repeating steps (iii)-(vi), wherein if the identified set of points of intersection are on a surface of a patient, the wire mesh computer model is implemented in a computer of a patient positioning system to establish where a ray from a centre of a radiation beam impinges on the surface of the patient so that the focus to skin distance can be calculated.