In CSI treatments, the cranial field on the abutting side can be made nondiverging by using a half beam block. The size of the abutting spinal field is 40cm x 6 cm. SSD is 100 cm.
If Pheta is the angle of rotation of the collimator required for the abutting of the cranial and spinal fields, Tan Pheta is given by
In this particular example, the abutting border has no divergence. So collimator rotation to match the divergence of the spinal field would be enough to match the two orthogonal fiedlds.
The inferior border of the brain field in this case, will not diverge into the abutting border of the spine field.
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