C. Fiducial triplets	Distances (mm)			Angles (degrees)
	a	b	c	A	B	C
#1 (1,2,3)	15	21	21	69	70	41
#2 (1,2,4)	15	32	21	31	127	22
#3 (1,3,4)	21	32	20	38	101	41
#4 (2,3,4)	21	21	20	57	60	63

(A) The coordinates for the four implanted fiducials. (B) The calculated distances between fiducials. (C) Assessment of four possible fiducial triplets. In this case, the distance between fiducials 1 and 2 is less than the spacing threshold of 20 mm (bold). Thus, fiducial triplets #1 and #2 failed the spacing criteria. However, fiducial triplets #3 (fiducials 1,3,4) and #4 (fiducials 2,3,4) exceeded both the spacing and collinearity thresholds, thus allowing 6D target tracking.