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The apparent source position in ion therapy is not constant or can be different in x or y direction. The apparent source position (as measured from field size projections) shall be called Virtual Source, the distance from the virtual source to isocenter the Virtual SAD.

Most of the cases, no trays are used for blocks, compensators and wedges. However, the concept of trays together with the mounting position is useful for specifying exactly at which point the position of these devices shall be measured. Therefore, trays shall always be present, even though they are only virtual trays.

Figure C.8.8.25-1 shows an example.

Figure C.8.8.25-1. Virtual Source-Axis Distances

Examples: The use of the above Attributes for snout positioning and block/compensator manufacturing:

Snout Position (300A,030D) is defined at the Control Point level and can therefore vary during the beam, causing all the accessories attached to the Snout (Applicator, Block, Compensator, etc.) to move as well.

For each accessory which is attached to the Snout and has its distance to isocenter defined at the Beam level (such as Isocenter to Beam Limiting Device Distance (300A,00BB), Isocenter to Compensator Tray Distance (300A,02E4), Isocenter to Block Tray Distance (300A,00F7) ), the prescribed physical position of the accessory is defined at the first Control Point.

This is consistent with the approach used for Beam Limiting Device Boundaries and Positions (see Section C.8.8.25.3 Leaf Position Boundaries).

For accessories which are attached to the Snout whose distance is defined at the Control Point level, such as Isocenter to Lateral Spreading Device Distance (300A,0374) and Isocenter to Range Modulator Distance (300A,038A), and are attached to the Snout, both the Snout Position (300A,030D) and the accessory distance must change by the same amount.