Craniospinal irradiation (CSI)

While treating a patient for craniospinal irradiation (CSI), two orthogonal fields must abut at the desired depth in the patient. If both fields are divergent, for the abutting of lateral and spinal fields:

Both collimator and couch must be turned through the angle of divergence of abutting spinal and cranial fields respectively.

See a figure in the Dosimetry Review Book.

The figure shows the rotation of the collimator by an angle to make the inferior border of the lateral field tangent to the P/A spinal field for the abutting of two orthogonal fields.