DICOM PS3.3 2020b - Information Object Definitions
C.8.8.15.13 Reference Point for Calibration of Beta Emitting Isotopes

For beta emitting isotopes, Source Strength (300A,022B) shall be defined at reference point (r0,θ0), where r0 is the radial distance of 2 mm from the source longitudinal axis, and θ0 is the angle of 90 degrees between the source longitudinal axis and the line defined by the center of the source and the reference point. Refer to:

DICOM PS3.3 2020b - Information Object Definitions