Is a stereotactic/ thin-slice MRI of the brain absolutely necessary to determine eligibility for hippocampal sparing whole brain radiation therapy?

The small size of the hippocampal dentate gyrus (3-4cc) can render it difficult to delineate and conformally avoid if both the brain MRI and the treatment-planning CT simulation are not obtained with thin-slice axial images (not exceeding 1.5mm on the MRI, not exceeding 2.5mm on the treatment-planning CT).

If the head is positioned in the custom immobilization facemask in such a manner that the hippocampal head is coplanar with the hippocampal tail, then the impact of the slice thickness on the CT simulation becomes magnified. Siglin et al., PMID 25407858 have observed that using an incline board to variably angle the head (optimally at 30 degrees if comfortable for the patient) can “lengthen” the hippocampus in the cephalad-caudad directio...