SU‐E‐J‐30: Intuitive Display Coordinates for Six‐Degree‐Of‐Freedom Treatment Couches

Abstract

Purpose: To define couch display coordinates of six‐degree‐of‐freedom treatment couches in a manner that is more intuitively linked to the position of isocenter in the patient, permits easy identification of irregularities in couch positioning, and permits the tightest possible override tolerances for all couch motions. The display coordinates should be calculated from the IEC table‐top coordinate system with no patient‐specific information. Methods: Couch display coordinates are proposed as the position of radiation isocenter in IEC table‐top coordinates. Coordinate systems are compared across multiple radiation therapy machines and then compared to the proposed display coordinates. Examples are provided showing how the proposed definition can be applied across a variety of different couch designs. Results: Consistent with IEC standards, most machines with six‐degree‐of‐freedom couches, and particularly those with robotic couches, measure rotations about a point fixed with respect to the couch and independent of all translational motions. This convention has the undesirable consequence of eliminating the one‐to‐one relationship between the translational coordinates displayed and the position of the radiation isocenter in the patient. By defining the display coordinates as the position of radiation isocenter in IEC table‐top coordinates, the one‐to‐one relationship is reestablished. Examples provided show that the proposed definition insures that a one‐to‐one relationship between the position of isocenter in the patient and the three translational display coordinates regardless of the values of the three angular degrees of motion. Conclusion: The examples illustrate how the proposed couch display coordinates lead to easier identification of irregularities in couch positioning such as identifying laterality of isocenter or shifts in multi‐isocenter treatments. The proposed display also allows tighter tolerances for couch overrides. These advantages suggest a revised IEC display standard following these conventions has the potential to reduce setup errors at the treatment machine and enhance the utility of the couch display coordinates.