Stereotactic radiosurgery, X: clinical isodosimetry of gamma knife versus linear accelerator X-knife for pituitary and acoustic tumours.

Several review articles have compared gamma unit versus linear accelerator (linac)-based radiosurgery systems, concluding that the dose gradient 'fall-off' at the margin of the target (expressed as the distance between isodoses) is very similar for both techniques as far as single isocentre treatment volumes up to 1.5 cm diameter are concerned, and that the two radiosurgical systems are, in general, comparable. 'Fine tuning' of the gamma unit can be carried out by using multiple isocentre plans, the differential use of small collimator sizes (down to 4 mm) and field weightings, and adroit use of the gamma angle, and selective beam blocking. Multiple isocentre plans, beam modification, restriction of gantry angles and arc lengths, and microcollimation can similarly improve the isodose gradients from linac units. In both instances, the dosimetric advantages occur along selected aspects of the target perimeter border. However, the more frequent use of multiple isocentred 'shots' on the gamma unit achieves greater conformity indices for more complex target volumes, but at the expense of steeper internal dose gradients. We studied two patients with tumours close to or arising from radiosensitive special sensory nerves (optic and cochlear) to compare and contrast fine tuning of the two technologies. In a previously irradiated patient with a pituitary adenoma, the dose gradient achieved at the rostral margin, adjacent to the optic chiasma, was steeper on the gamma unit (due to the concentration of small collimator shots rostrally and beam blocking), which was therefore the dosimetrically preferred technique. In contrast, the vastly smaller internal dose gradient (11% for linac/X-knife versus 100% for Gamma Knife) and the ability to fractionate on the X-knife system, gave a large dosimetric advantage to the X-knife plan in the treatment of an acoustic neuroma, where the intracanalicular component of the cochlear nerve traversed the target volume. This advantage also pertains to the cochlear ramus of the internal auditory (labyrinthine) artery and the facial nerve. Our published work on X-knife radiosurgery of acoustic neuroma has documented improvement of hearing after therapy and may be relevant in this regard. That there are advantages in physical dose distribution and fractionation, producing a reduction in the biological dose in normal tissue, argues for the use of linac technology in acoustic neuromas. Craniopharyngiomas enveloping the optic nerve/chiasma will similarly be better treated by the linac X-knife system. It is apparent that different radiosurgery systems may be indicated in particular neuro-oncological situations.