Source to External Contour Distance Attribute
| Tag | (300A,0132) |
|---|---|
| Type | Conditionally Required, Empty if Unknown (2C) |
| Keyword | SourceToExternalContourDistance |
| Value Multiplicity | 1 |
| Value Representation | Single (FL) |
Distance in mm along the central beam axis from the nominal Radiation Source location to the external contour including devices associated with the patient anatomy model. For dosimetric purposes this value may differ from the Source to Patient Surface Distance (300A,0634).
See Section C.36.11.1.4.
Required if the conditions in Section C.36.2.2.5.1.1 are satisfied.
Section C.36.11.1.4
C.36.11.1.4 Radiation Dose Source to External Contour Distance
The Radiation Dose Source to External Contour Distance (300A,0628) is the distance to the beam entry point, which may include Bolus, Patient Positioning Devices, Patient Immobilization Devices or other devices. This value is useful for including the attenuation effects of external devices on the dose calculation and for patient setup.
Section C.36.2.2.5.1.1
C.36.2.2.5.1.1 Requirements for Changing Values within RT Control Point Sequence Attributes
This Section specifies when individual Attributes shall be present in a Sequence.
The RT Control Point Sequence specifies a certain order of execution.
At each RT Control Point the value of various Attributes may be specified as an explicit value (which in the case of a type 2C Attribute may be a null value) and if absent remain at the same value as specified previously. There are physical and mechanical implications of specifying a new value as opposed to staying at the same value, for example gear lash, floating point jitter, etc.
At the first Sequence Item in RT Control Point Sequences (i.e., with an RT Control Point Index (300A,0600) equal to 1) all Attributes affected by this Section shall be present if applicable conditions are met.
For Sequence Items other than the first Sequence Item, Attributes shall be present if applicable conditions are met and the value is different from the previously populated value for the same Attribute (in the case of a type 2C Attribute, a null value is considered as a value). The previously populated value is the value from the Item where the Attribute was present with the greatest value of RT Control Point Index (300A,0600) less than the value of the RT Control Point Index (300A,0600) in the current Item.
This means that for an Item in which an Attribute is absent, the application stays at the value of the previously populated Item.
For Sequences inside a RT Control Point Sequence Item, the Sequence shall be present if any of the nested Attributes affected by this Section differ from the corresponding previously populated Item.
For multi-valued Attributes, such as Parallel RT Beam Delimiter Positions (300A,064A), all values shall be present if any value changes.