Image registration and calculation of a biologically effective dose for multisession radiosurgical treatments - Technical note

The purpose of this study was to develop techniques for registering image s ets associated with staged or multifraction radiosurgical treatments of lar ge targets with the Leksell gamma knife to transform shot coordinates betwe en treatment sessions and produce cumulative dose distributions and to inve stigate the theoretical biological effects of such protracted treatments by means of such concepts as the linear-quadratic model and biologically effe ctive dose. An image registration technique based on normalized mutual information was adapted to produce one fused-image study from an imaging series acquired du ring distinct treatment sessions. A spreadsheet computer program was develo ped to determine coordinate transformations between the associated stereota ctic coordinate systems based on digitized coordinates of fiducial markers appearing on the fused images. Coordinates of shots used during one treatme nt session could then be transformed to the stereotactic space of another s ession, and cumulative dose distributions could be computed. The procedure was applied to the two-stage treatment of a giant arteriovenous malformatio n (AVM). Overall uncertainty in each transformed shot position is approxima tely 0.7 mm. An effective single-fraction dose (D-eff) was defined and computed for the two-stage AVM treatment. The simple summed dose distribution was compared w ith the D-eff distribution. Because dose values differ significantly in ove rlap regions between the individual distributions, the clinical usefulness of the simple cumulative distribution is dubious. It may be useful for a fu ture update of the GammaPlan treatment planning software to generate effect ive single-session dose distributions for such cases.