In craniospinal irradiation, what is the collimator rotation needed to align the skull field with the divergent spinal field?

In craniospinal irradiation, precise alignment of the treatment fields is critical to ensure that both the cranial and spinal areas receive the appropriate radiation dose while minimizing exposure to surrounding healthy tissues. The collimator rotation is essential to account for the divergence of the radiation beams, particularly when treating two distinct anatomical regions – the head and the spine.

The correct collimator rotation needed to align the skull field with the divergent spinal field is determined based on the angles at which the beams diverge as they move from the treatment machine to the patient. A rotation of 8.53° has been established as the optimal angle in practice for achieving this alignment, allowing the beams to converge in such a way that coverage of the intended treatment volume – the entirety of the craniospinal axis – is established effectively.

This value is derived from geometric considerations involving the positioning of the patient, the treatment machine's capabilities, and the arrangement of treatment fields to ensure maximum dose delivery to the target while minimizing impacts on normal tissue.