C.126.96.36.199.4.1 Gantry Beam Line
Eye treatments on the gantry shall use all existing IEC coordinate systems with their standard definition. This applies especially to IEC BEAM LIMITING DEVICE, IEC WEDGE FILTER, IEC X-RAY IMAGE RECEPTOR.
IEC PATIENT SUPPORT, and IEC TABLE TOP coordinate systems are defined as above. Additionally, a rotation of the head fixation device is possible. The Head Fixation Angle (300A,0148) shall be defined as the angle of the head fixation device with respect to the TABLE TOP coordinate system. Positive head fixation angle is in the same direction as positive PATIENT SUPPORT pitch, i.e. backwards.
Proton eye treatments require an additional coordinate system for the placement of the fixation light. Since it is usually mounted onto the beam port the ‘natural’ coordinate system for devices mounted there is the IEC BEAM LIMITING DEVICE coordinate system. The angles for the fixation light positions shall therefore be defined as follows:
Rotation of the fixation light about the IEC BEAM LIMITING DEVICE Z-axis (Zb) is defined as Azimuthal Angle. The Azimuthal Angle is equal to 0( when the fixation light is positioned on the axis Xb of the IEC BEAM LIMITING DEVICE coordinate system. An increase in the value of the Azimuthal Angle corresponds to clockwise rotation of the fixation light as view along the axis Zb towards the virtual source.
The polar angle is always positive and defined as the angle between IEC BEAM LIMITING DEVICE Z-axis and the line connecting isocenter with the fixation light position.
Proton eye treatments require the wedge thin edge position as one additional. The wedge thin edge position allows the specification of a wedge, which does not cover the full open field. The wedge thin edge position is positive, if the wedge does not cover the isocenter position and negative, if it does cover.
Figures C.8.8.25-5 and C.8.8.25-6 show the angles and attributes as described above.
Figure C.8.8.25-5 Patient’s eye view
Figure C.8.8.25-6 Lateral view along the positive axis Xb
C.188.8.131.52.4.2 Fixed Beam Line
The coordinate systems for the treatment chair are defined above and shall also be applied to seated eye treatments.
In this case, it is recommended that a beam limiting device angle of 90º be formally applied (provided the gantry angle is defined to be 90º (and not 270º). This results in the same coordinates of the fixation light and wedge relative to the patient as in the treatment situation with the patient lying on the table.