Cranial Field
The brain fields are centered at the outer canthus to reduce divergence into the eyes. To cover the optic nerve and the cribriform plate in the treatment field, while protecting the anterior portion of the orbit, tight blocking is used.
The distance between the cribriform plate and the field edge should be 0.5 cms.
Collimator is rotated to match the divergence of the spinal field; the amount determined by the upper spinal field jaw.
Geometrical arrangement of fields and beam divergence:
Cranial field:
Lateral POP whole brain fields, centered at outer canthus to minimize divergence in region of eye and cribiform plate
Brain fields treated at 100cm SAD centered on outer canthus
Multileaf collimator or poured blocks can be used to protect the eye, face and anterior neck.
The brain field should extend to at least 1 cm beyond the outside of the scalp.
The image below shows the isodose distribution for the cranial component of CSA RT. It is possible to see that the lens are just outside the edge of the field and will receive a significant amount of scattered RT.
Spine Field:
Posterior spine field is set to SSD 100cm/ 130cm to highest midline point on spine and couch moved longitudinally to field center
Width of the spinal field should include the vertebral body with a 1 cm margin on either side (so lateral portion of theca is treated).
In the imaging below (courtesy of Dr. Roger Taylor) the CTV (clinical target volume) is in purple and the PTV (planning target volume) is in blue:
Inferior border is generally at S3/4 for a margin on the dural sac as determined by MR scan. The dural sac usually ends at S2.
The junction is accomplished by tilting the collimator of bilateral cranial fields to match the divergence of the adjacent posterior spine field.
It is best that the spine be treated with a single posterior field.
An extended SSD is preferable to the use of adjacent fields,
If 2 spinal fields have to be used then it is better that they be junctioned below L2 – inferior to the spinal cord.
