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RT DICOM objects

Alongside the image series the viewer parses the RT objects below and resolves the DICOM reference chains between them.

RTSTRUCT — structure sets

Parsed per file: ROI names, display colors, interpreted types (PTV, CTV, GTV, ORGAN, EXTERNAL, …) and all planar contours in patient coordinates. ROIs are ordered EXTERNAL → PTV → CTV → GTV → alphabetical, and a fallback 12-color palette fills in for structure sets without stored colors.

Axial views draw the native closed contours; sagittal and coronal views show the reconstructed cross-section silhouette of each ROI (even–odd crossing pairing of the contour stack). Per-ROI visibility toggles live in the sidebar, with All/None shortcuts. Every structure set in the folder is loaded (e.g. one per 4DCT phase) and selectable; the set referencing the active image series (RTReferencedSeriesSequence) is chosen automatically and follows series switches. Structure sets also feed the 3D surface view — see segmentation.md.

SEG — DICOM Segmentation objects

A Segmentation instance is a multi-frame image of binary masks, one frame per (segment, slice) pair, placed in patient space by the per-frame functional groups rather than a slice index. Reading one rebuilds a lattice from the frame positions: frames are grouped into slice levels along the stack normal, the slice spacing is the median level distance, and the in-plane geometry comes from PixelMeasuresSequence / PlaneOrientationSequence (shared group first, first per-frame group as a fallback).

Supported: BINARY (1 bit per pixel, packed across all frames as one continuous stream) and FRACTIONAL (8 bit, thresholded at half MaximumFractionalValue). Segment labels come from SegmentSequence, colors from RecommendedDisplayCIELabValue via CIELab → XYZ (D65) → sRGB, with the 8-color segmentation palette as fallback. Compressed (encapsulated) Pixel Data is reported as a load warning.

Each SEG file becomes one segmentation series in the data tree, linked to the image series named in ReferencedSeriesSequence. The masks keep their own lattice and are resampled onto the displayed volume only when their own image series is shown, so a study can carry segmentations of several series at once. Writing is the reverse: only the slices a segment occupies become frames, so a ten-slice structure on a 200-slice CT costs ten frames. See export-and-tools.md.

RTDOSE — dose grids

16- and 32-bit dose grids with DoseGridScaling applied at load, GridFrameOffsetVector handled in full generality (uniform or not, ascending or descending — descending grids are re-ordered) and the frame offsets re-based onto ImagePositionPatient. Multiple dose files (plan and/or per-beam) are listed and selectable.

Sampling is trilinear in patient space (bilinear in-plane, linear across the possibly non-uniform frame offsets), with an incremental affine fast path when resampling a whole display plane. Display offers:

  • a translucent colorwash with adjustable opacity and a lower threshold (in % of the reference dose);
  • isodose lines at configurable percentages, extracted per level with marching squares (parallelized across levels).

The reference dose defaults to the plan's TargetPrescriptionDose and can be overridden; the status bar shows Gy and % of reference at the crosshair for both datasets.

RTPLAN — photon and ion plans

Photon (BeamSequence) and ion/proton (IonBeamSequence) plans are summarized: label, date, prescription and fractionation, and a per-beam table with radiation type, delivery type, scan mode (for scanned ion beams), gantry/couch angles, energy range, meterset and control-point count. Beam isocenters are marked in all three views (toggleable).

REG — spatial registration objects

Rigid Spatial Registration files are parsed into their 4×4 frame-of-reference matrices, shown with the decomposed translation/rotation and frame-of-reference hints (matched against the loaded studies' FoR UIDs). A matrix can be applied as the active registration in either direction, optionally inverted, so a TPS-exported registration drives the fusion overlay and the cross-study crosshair link without running the optimizer; it is validated (orthonormality, no reflection/scale) before being accepted.

Deformable Spatial Registration objects are read the same way, grid included: the displacement lattice becomes a transform applicable in either direction, and everything downstream — fusion, the crosshair link, the analytics, the vector-field display, structure propagation — works on it unchanged. The panel reports the lattice size, spacing and largest displacement, and which loaded dataset the grid's frame of reference matches.

A registration recovered here can be written back out as a Deformable Spatial Registration (Image registration ▶ Vector field ▶ 💾 Save as DICOM…); the IOD's pre- and post-deformation matrices are written as the identity and the grid carries the whole mapping. See registration.md.

RTRECORD — treatment records

RT (Ion) Beams Treatment Records are summarized per session: fraction number, date, machine, and a per-beam table of specified vs delivered meterset with percentage difference and termination status (non-NORMAL highlighted).

Reference chains

The viewer parses and preserves the standard chain

CT series ◀ RTSTRUCT ◀ RTPLAN ◀ RTDOSE

and uses it to select the structure set matching the displayed series, pair doses with their plans (and hence the prescription dose), define tree copy/move semantics (a series carries exactly its dependent RT objects) and drive DICOM export (the chain is written back out). Frame-of-Reference UIDs associate objects spatially; RT objects with a different FoR still load and display, but patient-space overlays are only meaningful within one frame of reference (or through an explicit registration).