We have noticed that during assembly of a marker according to our previous application GB2512416, i.e. one having a solid first component and a liquid second component, a small amount of air is sometimes trapped within a housing of the marker. Precautions can obviously be taken to limit this, but it would be preferable if the structure was resilient to this. Accordingly, the present invention is directed to a marker for use in calibration of a radiotherapeutic apparatus, comprising a first component, preferably spherical, and having a first hydrogen proton density and a first mass density, and being solid, a second component having a second hydrogen proton density and a second mass density, and being liquid, wherein the first component is substantially completely surrounded by the second component within a void of the second component to form an interface between the first and second components, the marker further comprising a housing enclosing the first and second components, and defining a region into which the second component can extend at a location which is displaced vertically from the interface between the first and second components, thereby to allow trapped air to be collected. Thus in use any trapped air is encouraged to migrate to a position away from the first component, allowing the interface between the first and second components to be free of contamination from such trapped air.
