Isocenter in aerial photography: A point where a line meets at 90-angle of tier/2. It is the point on the aerial photo platform that falls on a line half-way between the Principal point and the Nadir point. In imaging physics and radiation oncology, the isocenter is the point in space through which the central ray of the radiation beams pass.
In radiation oncology there is typically talk of two isocenters:
Radiation isocenter is the point in space through which the central beam of radiation passes whereas mechanical isocenter is the point where optical beams intersect.
Isocenter definition in Radiotherapy:
The point in space relative to the treatment machine about which various components of the linac rotate. The gantry rotation defines a horizontal axis which intersects a vertical axis defined by the rotation of the treatment couch. The treatment collimators also rotate about an axis pointing through the isocentre.
The placement of the radiation isocenter plays an important role in treatment planning because ideally the isocenter should be placed in the center of the target volume, usually a tumor. Patient positioning, usually via 3D imaging methods, is thus essential for the successful irradiation of cancerous tissue and - consequently - the treatment outcome.
The source to isocenter distance (SIsoD) can be an important parameter to control in determining patient exposure and image quality in diagnostic computed tomography and fluoroscopy.