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RT Plan

The focus for this Radiotherapy Plan IOD (RT Plan IOD) is to address the requirements for
transfer of treatment plans generated by manual entry, a virtual simulation system, or a treatment
planning system before or during a course of treatment. Such plans may contain fractionation
information, and define external beams and/or brachytherapy application setups. [DICOM PS 3.3]

It uses the following RT-specific modules:
  1. RT General Plan: General information about the RT Plan object, giving the plan a name, and referencing e. g. doses, structure sets or any related RT plans. Mandatory Module.
  2. RT Prescription: Describes some prescriptions regarding the dose delivered to the patient pertinent to the complete plan (maximum/minimum doses, warning levels etc). Optional Module.
  3. RT Tolerance Tables: Describes maximum tolerances pertinent to the complete plan; tolerances between planned and applied treatment regarding table position, gantry angle, beam limiting device angles (jaws/leafs) etc. Optional Module.
  4. RT Patient Setup: Describes the positioning of the patient with respect to the treatment machine, along with any fixation devices and shilding devices, used. Optional Module.
  5. RT Fraction Scheme: Describes single or multimple scheme of dose descriptions, ie which dose should be delivered at which day. Optional Module (but will be found in most object probably)
  6. RT Beams: Contains information defining equipment parameters for delivery of external radiation beams. Conditional module.
  7. RT Brachy Application Setups: For Brachy Application, this module describes the application of a brachytherapy radiotherapy treatment, containing one or more sources each associated with one or more channels. Conditional Module.

The different modules are discussed below.

RT General Plan Module

This module handles some core information of an RT Plan object, giving the plan a name, and referencing e. g. doses, structure sets or any related RT plans. Because it is mandatory it must be included in any RT Plan object.

The only mandatory (type 1/2) information in this module is a user-defined RT Plan Label, modification date/time of the plan, Operator's Name and the attribute RT Plan Geometrie, which denotes which geometric system applies for interpretation of this plan. If the value (defined terms) is PATIENT, then there must be some Structure Set objects (referenced in the Referenced Structure Set Sequence) which then serve as a base for the plan geometry. If the value is TREATMENT_DEVICE, then no custom patient-based is available and the "IEC FIXED Coordinate System" applies.

Optional information includes references to other, related RT Plans (defined terms PRIOR, ALTERNATIVE, PREDECESSOR), referenced RT Dose objects (TODO: not clear what for), Treatment Protocols, Treatment Intent.

So mainly the RT General Plan Module is for referencing the structure sets for plan geometry and for some basic meta informations like a plan label, for referencing related RT Plans and for referencing related RT Dose objects.

RT Beams Module

The RT Beams Module contains information defining equipment parameters for delivery of external radiation beams. It consists of one very large sequence, the Beam Sequence, which contains one or more items each describing exactly one beam. That does not mean that a single beam is static, ie. is delivered from a fixed position; e. g. a single beam can include changing collimator or wedge setups (see Control Point Concept). The module is mandatory in that moment any "real" radiation treatment should be applied with this plan.

There are different functional parts in this module (not all attributes listed):

  • General beam/equipment information: Unique Beam Number, Beam Name, Beam Description, kind of beam (dt: PHOTON,ELECTRON,NEUTRON; PROTON), Treatment Machine Name, Manufcacturer, Institution information etc.
  • Beam Limiting Devices: Beam limiting devices (collimator) used for this beam (kind of device, distance from radiation source, etc.)
  • Referenced Reference Images: Number of images that can be used for verification of current beam later on; referenced by SOP Class/SOP Instance UID and identified by unique Reference Image Number. It is also defined when the validation images during treatment were taken (during simulation?) (Start/End Cumulative Meterset Weight, see meterset.
  • Planned Verification Image Sequence: Defines progress times (per meterset definition) when verification images should be taken to compare them later on with images from the Referenced Reference Images Sequence. An attribute "Referenced Ference Image Number tells which reference image must be taken for comparison. Also there are other optional informations for each image like RT Image Plane and RT Image Orientation as well as X-Ray Image Receptor Angle etc.
  • Treatment Type: Attribute for kind of treatment, defined terms: TREATMENT (normal patient treatment), OPEN_PORTFILM (portal image acquisition with open field), TRMT_PORTFILM (portal image acquisition with treatment port), CONTINUATION (continuation of interrupted treatment)
  • Wedges: Specification of wedges associated with this beam.
  • Compensators: Specification of compensators associated with this beam.
  • Boli: Specification of any Boli associated with this beam (references corresponding ROIs)
  • Blocks: Specification of any Blocks associated with this beam
  • Applicator: Specification of an electron applicator if used
  • Control Points: Definition of various control points which are used for specifying wedge/collimator/gantry/table movements while treatment is applied. Control points also offer a measure of progress (called meterset).
There are also references to Doses, Tolerance Tables and Patient Setups defined in other modules:
  • Referenced Dose Sequence: References any RT Dose Objects associated with this beam.
  • Referenced Tolerance Table: References any associated Tolerance Tables for this beam. See RT Tolerance Tables Module.
  • Referenced Patient Setup: References any associated Patient Setup tables for this beam. See RT Patient Setup Module.

Control Point Concept

Definition of various control points which are used for specifying wedge/collimator/gantry/table movements while treatment is applied. Control points also offer a measure of progress (called meterset).
TODO: in-depth explanations

RT Prescription Module

The RT Prescription module provides for the inclusion of dose prescription information pertinent to the complete plan (and therefore, not specific for an actual beam), which may comprise several fraction schemes, themselves consisting of many beams. The RT Prescription Module offers a user-defined description of the treatment prescription (attribute Prescription Description) as well as the a number of doses definintions in the Dose Reference Sequence (one item per dose), which makes up the rest of the module. Both attributes (description and sequence) are optional.

In the Dose Reference Sequence different doses can be specified. Each item (and so each dose definition set) is identified by a Dose Reference Number, which then can be referenced from other modules. One set of dose definitions include (among others) the following information:

  • Dose Reference Structure Type: Describes what kind of structure is associated with this dose (DT: POINT, VOLUME, COORDINATES, SITE).
  • Referenced ROI Number: The Region of Interest (part of Structure Set referenced from General Plan Module) this dose definition set applies to
  • Different dose information: Information about Target Minimum/Maximum Dose, Organ at Risk Limit/Maximum Dose, etc.

RT Tolerance Tables Module

The RT Tolerance Tables module provides information concerning machine tolerances as they apply to the whole treatment plan (but a tolerance table can be referenced by a specific beam). Tolerances are applied by reference to a tolerance table within the RT Tolerance Tables module for beams contained within the RT Beams Module.

The RT Tolerance Table module is composed of the Tolerance Table Setup Sequence, with each item carrying one tolerance table, identified by the "Tolerance Table Number" attribute. A tolerance table describes information about tolerances permitted for the treatment equipment positioning during treatment, ie there are attributes for specifing the Gantry Angle Tolerance, the Beam Limiting Device Angle Tolerance, the Patient Support Angle Tolerance, various Table Top Position tolerance attributes etc.

Tolerance tables may be used to compare planned with delivered machine parameters. If the absolute difference between the planned and delivered values exceeds the Tolerance Table value, treatment may be inhibited or the operator may be warned.

RT Patient Setup Module

The RT Patient Setup module provides information concerning patient setup parameters and fixation devices as they apply to the whole treatment plan. Patient setup information within the RT Patient Setup module is referenced by beams contained within the RT Beams module.

This module contains information describing the positioning of the patient with respect to the treatment machine, along with any fixation devices used. It also describes the shielding devices applied to the patient. The module contains a sequence of patient setup descriptions, each of which may be referenced by one of more beams or brachy application setups.

RT Fraction Scheme Module

The RT Fraction Scheme module contains attributes that describe a single or multiple scheme of dose descriptions. Each sequence item contains dose specification information, fractionation patterns, and either beam or brachytherapy application setup specifications. The design of the RT Fraction Scheme module allows a beam or brachytherapy application setup to be used in multiple fraction schemes.

As a consequence, this module offers a schedule of treatment sessions. One treatment is called fraction in the context DICOM RT context, and the scheduling of fractions is called fractionations. The schedule plan is defined in a fraction(ation) pattern. The Fraction Scheme module can reference(sometimes optionally) RT Dose objects, Patient Setups (from the Patient Setup module), a number of beams (from the RT Beams module), a number of Brachy Application Setups (referenced from the Brachy Application Setup module).

All information of the Fraction Scheme module is enclosed by the Fraction Group Sequence which includes all further attributes. One item of the Fraction Scheme module represents one fraction group, which again is a number of treatment applied to a patient. The schedule of such a fraction group is defined for each item in a so-called fraction pattern (see below).

Every fraction gruop is identified by a Fraction Group Number and references (optionally) a Patient Setup from the Patient Setup Module and one or more RT Dose objects (thus linking additional dose information like dose grids, isodose curves or dose points). Also doses from the RT Prescription module can be referenced in the Referenced Dose Reference Sequence for verifying what dose (actually one referenced dose means here several dose prescription information - target maximum/minimum dose and so on) actually is prescribed. In the same sequence the attributes from the referenced prescription dose definitions are repeated (not copied) for specifying the actual dose parameters applying for this fraction group.

For every fraction group a number of beams is specified and referenced from the RT Beams Module.

Alternatively, for Brachy Application Brachy Setups from the Brachy Application Setup module can be referenced and for each setup a Dose in Gy can be defined and a Dose Specicification Point (x,y,z coordinates of point in the patient based coordinate system at which that Dose is applied).

Fractionation Patterns

A fractionation (or fraction) pattern (or scheme) defines at which days and how often a day a patient should be treated. This is specified for one week (from Monday to Friday). It also can be specified how many weeks this fractionation pattern applies. All those information is stored in the following attributes:
  • Fraction Pattern: Contains a string of 0s (no treatment) and 1s (treatment) describing the treatment pattern.
  • Number of Fractions per Day: The number of fractions (treatments) applied per day. A time cannot be specified.
  • Repeat Fraction Cycle Length: Number of weeks needed to describe pattern, ie how many weeks the Fraction Pattern specifies.
    Overall the resulting Fraction Pattern length is: 7 x Number of Fractions Per Day x Repeat Cycle Length.
Examples from the Standard:
  • a) 1 fraction group, 1 fraction per day (Monday to Friday):
    Number of Fraction Pattern Digits per Day = 1, Repeat Fraction Cycle Length = 1, Fraction
    Pattern = 1111100
  • b) 2 fraction groups, 1 fraction per day, first fraction group Monday, Wednesday, and Friday, second fraction group Tuesday and Thursday:
    Fraction Group 1: Number of Fraction Pattern Digits Per Day = 1, Repeat Fraction Cycle Length = 1, Fraction Pattern = 1010100
    Fraction Group 2: Number of Fraction Pattern Digits Per Day = 1, Repeat Fraction Cycle Length= 1, Fraction Pattern = 0101000
  • c) 2 fraction groups, 1 fraction per day, alternating fraction groups every day of treatment (Monday to Friday):
    Fraction Group 1: Number of Fraction Pattern Digits Per Day = 1, Repeat Fraction Cycle Length = 2, Fraction Pattern = 10101000101000
    Fraction Group 2: Number of Fraction Pattern Digits Per Day = 1, Repeat Fraction Cycle Length = 2, Fraction Pattern = 01010001010100
  • d) 1 fraction group, 2 fractions per day (Monday to Friday):
    Fraction Group 1: Number of Fraction Pattern Digits Per Day = 2, Repeat Fraction Cycle Length = 1, Fraction Pattern = 11111111110000
  • e) 2 fraction groups, 2 fractions per day, alternating fraction groups every treatment (Monday to Friday):
    Fraction Group 1: Number of Fraction Pattern Digits Per Day = 1, Repeat Fraction Cycle Length = 1, Fraction Pattern = 1111100
    Fraction Group 2: Number of Fraction Pattern Digits Per Day = 2, Repeat Fraction Cycle Length = 1, Fraction Pattern = 11111111110000

The number of fraction groups is reflected by the number of items in the Fraction Group Sequence, ie if there are 2 fraction groups then there must be 2 items defined in that sequence. One Fraction Pattern attribute always describes only one group.

RT Brachy Application Setups Module

From the PS 3.3 of the standard:

The RT Brachy Application Setups Module describes the application of a brachytherapy radiotherapy treatment. It contains one or more sources, each associated with one or more Channels. A Channel is a device by which a source is placed in its intended treatment position or positions. A Channel may consist of a Source Applicator plus a Transfer Tube, a Source Applicator alone, a rigid or flexible linear source, or a seed. A number of Channels (for example applicators, sources or seeds) are generally arranged in an Application Setup which may be considered a “logical” device. It is important not to confuse Application Setup with Applicator. The model used here has been primarily built around the concept of remote afterloading, but extended to support other brachytherapy applications such as manual applicators and molds, seeds, and sources. Additional devices that are not Channels are described as Brachy Accessory Devices. Examples of Accessory Devices include shields, which modify the dose distribution from all sources in the treatment. However, Channel shields modify the dose only for the source(s) in that Channel.

The data in the module is arranged as follows:
Data Explanation
Treatment Machine Sequence treatment machine information (single item)
Source Sequence library of sources used in brachy application
Application Setup Sequence one or more applicators, sources, seeds etc.
> Brachy Accesory Device Sequence application leve shields etc.
> Channel Sequence applicator, line source(s) seed(s) etc.
> > Channel Shield Sequence channel-specific shields
> > Brachy Control Point Sequence mechanism to support individual source dwell times

TODO: Add description (or pages) for brachy process and different terms (channel etc., maybe explanatory images)