Abstract: In one form, a bolus is configured to fit over a target body position and includes an internal relatively rigid endoskeleton structure and a surrounding relatively non-rigid skin interfacing layer. A positioning and locking system may include an indexing plate mountable to a fixture. The indexing plate includes a plurality of multiposition-enabling formations and corresponding reference locations and is configured to engage with at least one locking mechanism. The at least one locking mechanism is movable between and lockable relative to the multiposition-enabling formations. The locking mechanism is also configured to interface between the indexing plate and accessories/devices that require immobilisation, accurate and/or repeatable positioning, e.g. the bolus.