Does the choice of reference isocentre affect the accuracy of patient set-up?

What is it about?

One of the key steps in the computed tomography (CT) simulation process is setting the reference isocentre. For head-and-neck radiotherapy, reference isocentre is set either to the patient skull or to the patient neck. The former has the advantage of being placed over a very stable anatomical structure, but it lies further away from the treatment field isocentre. Placing the reference isocentre onto the patient neck has the advantage of being closer to the treatment field isocentre, but this point may not be accessible, e.g., when tracheotomy was performed. We have shown that the difference in position shift distributions between the two groups of patients is detectable, but is nevertheless small, and neither reference isocentre set-up has shown a clear advantage over the other in terms of interfraction set-up error.

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Why is it important?

If either choice of reference isocentre would show to be advantageous to the other with respect to patient set-up errors, this could serve for establishing the priority of reference positioning criteria. In view of the results we obtained, however we believe this choice should not prevail over any clinical arguments for choosing either position.