Easy method for defining intracranial target volumes on orthogonal simulation films using magnetic resonance images.

A technique is described for simulating intracranial radiation fields. The method involves the projection of images from magnetic resonance (MR) scans to orthogonal simulation radiographs. This approach often can supplement and occasionally obviate the need for computerized tomographic (CT) assisted treatment simulation for co-planar beam arrangements in brain irradiation. Because magnetic resonance imaging can be performed in virtually any plane and is routinely carried out in sagittal and coronal planes, transferring tumor volumes to orthogonal simulation films using this technique can be done simply and accurately. The method employs an inexpensive photographic enlarger to superimpose sagittal MR scans on the lateral skull simulation films. Tumor volumes along with other structures of interest are treated onto the film and appropriate fields and blocks are contoured. The technique is performed easily in conjunction with routine simulation sessions, provided that the appropriate MR images are available. It is most suitable for planning treatment of tumors located at midline (e.g., medulloblastomas, craniopharyngiomas, brainstem gliomas, germinomas, etc.). The technique has also been used to verify the accuracy of CT-assisted treatment planning. With relatively minor modifications, eccentric tumors also can be accurately planned, and although we have restricted our description to brain tumours, the technique may be extrapolated to tumors in other sites.