Stereotactic radiosurgery XVI: Isodosimetric comparison of photon stereotactic radiosurgery techniques (gamma knife vs. micromultileaf collimator linear accelerator) for acoustic neuroma--and potential clinical importance.

PURPOSE: Two stereotactic photon radiation therapy methods are currently in practice for the treatment of acoustic neuroma. In the 1990s, our data and those of others demonstrated isodosimetric advantages for gamma knife technology over linear accelerator methodology. Since then, the introduction of micromultileaf collimator technology has improved the conformity of the linear accelerator method such that the isodosimetric differences between the two techniques have narrowed.
MATERIALS AND METHODS: In this study, modern gamma knife isodosimetry was compared to that of modern linac technology (conformal fixed fields and dynamic arcs) for the therapy of acoustic neuroma. This is an unusual target in that a special sensory nerve (holding the key to hearing preservation) frequently runs through the targeted volume, unlike the majority of other stereotactic radiation therapy targets. This was a single-dose prescription comparison; the perceived extra benefit of fractionation (a technique not routinely available to the gamma knife) was thereby abrogated.
RESULTS: Although the gamma knife technique maintained a slight, but statistically significant, advantage with regard to dose conformity (p < 0.02) (at the debatable cost of a lower minimum target dose), the much higher internal dose gradient (high maximum dose to prescription dose [MD:PD] ratio) could be interpreted as a disadvantage with respect to hearing preservation, although advantageous with regard to tumor ablation. Of the two linac methods, the dynamic arc method gave a statistically significant advantage over the fixed-field method as regards conformity (p < 0.05), at the expense of a slightly higher brainstem dose (an average of 12.4 Gy, cf. 11.7 Gy for fixed fields), but this result was not statistically significant. No significant difference was seen in the MD:PD ratio for the two single-isocenter linac techniques.
CONCLUSIONS: Gamma knife methodology remains well validated, with very good isodosimetry, but when hearing preservation is important, the improving linac technologies will compete with the gamma knife for optimal therapy. In these circumstances, the minor differences in isodosimetry between the two techniques will become important.