DICOM PS3.3 2023a - Information Object Definitions

C.36 RT Second Generation Modules

The following Attribute Macros and Modules are used by the RT Second Generation IODs.

C.36.1 RT Second Generation Concepts

This section discusses general concepts used in RT Second Generation Modules.

C.36.1.1 RT Second Generation Radiation Concepts

C.36.1.1.1 Control Points

A Control Point represents the state of a delivery device in a sequence of states defined at a given Cumulative Meterset (300A,063C) value.

A Control Point contains geometric and radiological parameters. Control Points are used by the delivery device to implement a planned delivery and to record the actual delivery.

C.36.1.1.2 Nominal Energy

Nominal energy characterizes the penetration of the beam into a material. The values are defined by the manufacturer to label a specific beam spectrum. For photon beam delivery, the maximum energy of the delivered photon spectrum is typically used. For electron beam delivery, the most probable energy of the spectrum is typically used.

C.36.1.1.3 Meterset

A Meterset is a single parameter from which the absorbed dose delivered can be calculated through a calibration procedure with additional information. The Meterset is used to measure the progress of radiation delivery during treatment, or report on progress after treatment.

See [IEC 60601-2-64] for more information on using monitor units as the unit for the Meterset.

C.36.1.1.4 Radiation Dose Point

A point chosen in space, or in the patient treatment volume, to measure or plan for a specific amount of radiation. The point usually is placed at a significant location, such as within a tumor (where radiation will be delivered), or within healthy tissue (where radiation will be minimized) or where a measurement device can be positioned.

C.36.1.1.5 Continuous Rotation Angle

A Continuous Rotation Angle is an angle in the range (-∞,+∞).

Continuous Rotation Angle represent a rotation direction and magnitude. The magnitude is not limited to be between 0 and 360 degrees.

All rotations are defined in a right-handed coordinate system, thus the direction of a positive rotation is seen as clockwise when viewed in the positive direction of the axis of rotation.

C.36.1.1.6 External Contour

The External Contour is the spatial extent that is taken into account for dose calculation. The External Contour includes the Patient Anatomy Model, Bolus, Patient Positioning Devices, Patient Immobilization Devices or other devices in the path of the radiation.

C.36.1.1.7 C-Arm LINAC

A C-Arm LINAC is a linear accelerator that follows the coordinate definitions of [IEC 61217]. Any hardware belonging to this category may or may not represent an actual C-Arm gantry.

C.36.1.1.8 Virtual Simulation

Virtual Simulation is a form of Radiotherapy treatment simulation that uses volumetric imaging studies in a computer to model the geometry of a radiation beam with respect to a patients anatomy. The spatial relationship between beam and anatomy is verified in Digitally Reconstructed Radiograph (DRR) images that conceptually represent actual beam portal images.

C.36.1.1.9 Beam Modifier Coordinate System

Beam modifiers, e.g., beam limiting devices, compensators and blocks, are specified by geometric coordinates.

A Base Beam Modifier Coordinate System is defined with respect to a parent coordinate system, which may be the Equipment Coordinate System (see Section 10.39.1.1) or another coordinate system derived from the Equipment Coordinate System. Section C.36.12.2 specifies several Well-known Frames of Reference used as the Equipment Frame of Reference, and specifies how a Base Beam Modifier Coordinate System is related to each as a child coordinate system.

The x/y plane of the Base Beam Modifier Coordinate System is referred to as the Base Beam Modifier Definition Plane. The orientation of the Base Beam Modifier Coordinate System is such that the Base Beam Modifier Definition Plane is parallel to the x/y plane of the parent coordinate system. The origin of the Base Beam Modifier Coordinate System is offset from the RT Device Distance Reference Location by the RT Beam Modifier Definition Distance (300A,0688) as shown in Figure C.36.1-1.

Base Beam Modifier Coordinate System and a rotated Beam Modifier Coordinate System

Figure C.36.1-1. Base Beam Modifier Coordinate System and a rotated Beam Modifier Coordinate System


Each beam modifier is defined in its own Beam Modifier Coordinate System with the following characteristics:

  • Defined with respect to the Base Beam Modifier Coordinate System.

  • Right-handed Cartesian coordinate system, with the positive z-axis pointing towards the nominal Radiation Source location.

  • The Beam Modifier Coordinate System rotates about the z-axis of the Base Beam Modifier Coordinate System. The orientation at a zero angle about the z-axis is the same as the Base Beam Modifier Coordinate System, i.e., the x- and y-axes are aligned.

The x/y plane of the Beam Modifier Coordinate System is referred to as the Beam Modifier Definition Plane.

C.36.1.1.10 Radiation Source

A Radiation Source is a generalized source of radiation that encompasses linear accelerators, brachytherapy sources, etc. Each source is associated with a nominal Radiation Source location which is a point in space from which the radiation is considered to be emanating.

C.36.1.1.11 Imaging Source Coordinate System

TheImaging Source Coordinate System describes the location of the imaging source with respect to the Equipment Frame of Reference coordinate system identified by the Equipment Frame of Reference UID (300A,0675) , i.e. the Equipment Frame of Reference coordinate system is the parent system of the Imaging Source Coordinate System.

The Device Position to Equipment Mapping Matrix (3002,010F) relates the two coordinate systems, and when it is identity:

  • The origin of Imaging Source Coordinate System is located at the origin of the Equipment Frame of Reference coordinate system

  • The axes of Imaging Source Coordinate System are aligned with the axes of the Equipment Frame of Reference coordinate system

The Imaging Source Coordinate System is aligned with the imaging source as follows:

  • The origin of the Imaging Source Coordinate System is the nominal location of the imaging source.

  • The z-axis is aligned with the central ray of the diverging rays of the imaging source

  • The positive z-axis is in the direction from the image receptor to the imaging source

Beam modifying devices attached to the imaging source, such as Beam Limiting Devices, use a Base Beam Modifier Coordinate System, if they use coordinates in their specification.

The Base Beam Modifier Coordinate System, defined in Section C.36.1.1.9 Beam Modifier Coordinate System, is related to the Image Source Coordinate System as follows:

  • The parent system of theBase Beam Modifier Coordinate Systemis the Imaging Source Coordinate System.

  • The Base Beam Modifier Plane is located at a distance specified by RT Beam Modifier Definition Distance (300A,0688) along the z-axis from the reference location specified by RT Device Distance Reference Location Code Sequence (300A,0659).

  • If the radiation used for imaging is generated by an imaging-specific source, the RT Device Distance Reference Location Code Sequence (300A,0659) shall have the value (130789, DCM, "Nominal Imaging Source Location")

  • If the radiation used for imaging is generated by the therapeutic source ("MV Imaging") , the RT Device Distance Reference Location Code Sequence (300A,0659) shall have the value (130358, DCM, "Nominal Radiation Source Location")

C.36.1.1.12 Image Receptor Coordinate System

The Image Receptor Coordinate System describes the location of the image acquisition receptor device with respect to the Equipment Frame of Reference coordinate system identified by the Equipment Frame of Reference UID (300A,0675) , i.e. the Equipment Frame of Reference coordinate system is the parent system of the Image Receptor Coordinate System.

The Image Receptor Coordinate System is also used when describing the location of an acquisition plane of a virtual imaging device without presence of physical image receptor, e.g. in case of a digital reconstructed radiograph (DRR).

The Pixel Spacing (0028,0030) is measured on the x/y plane of the Image Receptor Coordinate System at z = 0.

The Device Position to Equipment Mapping Matrix (3002,010F) relates the two coordinate systems, and when it is identity:

  • The origin of Image Receptor Coordinate System is located at the origin of the Equipment Frame of Reference coordinate system

  • The axes of Image Receptor Coordinate System are aligned with the axes of the Equipment Frame of Reference coordinate system

The Image Receptor Coordinate System is aligned with the image receptor as follows.

  • The z-axis passes through the center of the image receptor

  • For rectangular receptors, the x-axis and y-axis are aligned with the edges of the image receptor.

The alignment shall be documented in the Conformance Statement for the device.

DICOM PS3.3 2023a - Information Object Definitions