Radiation Therapy Simulation and Planning Vocabulary

A comprehensive set of vocabulary flashcards covering radiation therapy simulation practices, coordinate systems, positioning/immobilization technology, target volume terminology, and brachytherapy principles.

Isocenter

The point determined during the simulation process where the sagittal, lateral, and vertical laser lines converge.

Localization

The delineation of the treatment target and the placement of the isocenter relative to that target using an XYZ coordinate system.

x Axis

The transverse axis in the XYZ coordinate system that extends right to left in the patient.

y Axis

The longitudinal axis in the XYZ coordinate system extending head to foot.

z Axis

The axis in the XYZ coordinate system extending upward from the table top.

EPIDs

Electronic Portal Imaging Devices used to acquire portal images on the treatment machine to verify beam path and patient anatomy.

CBCT

Cone-beam CT; technology available on on-board kV imagers of linear accelerators used in IGRT to determine setup accuracy or needed shifts.

Aquaplast

A thermoplastic molding product that becomes pliable when warmed in water and is used to create rigid immobilization masks for head and neck treatments.

Positioning Aids

Devices such as head holders, knee sponges, and wing boards that help facilitate a specific position consistently but do not limit patient movement.

Immobilization Devices

Devices designed to limit the movement of the patient during treatment, thereby decreasing setup and targeting errors.

Vac-Loc

An immobilization system consisting of a bag filled with Styrofoam beads that retains the shape and contour of the patient when air is removed.

Alpha Cradle

An immobilization system using two liquids that create a thermal reaction to expand into a plastic bag, conforming to and then solidifying around the patient.

Belly Boards

Devices used in prone pelvic treatments to move the small bowel anteriorly out of the treatment area.

Surface Fiducials

Small radiopaque wires placed on the patient during CT simulation corresponding to the XYZ coordinates to allow visualization of the preliminary isocenter.

BEV

Beam's-eye view; a computer-generated radiograph used to visualize patient anatomy from the perspective of the proposed treatment beam.

DRR

Digitally reconstructed radiograph; a computer-generated radiograph created from CT data to visualize patient anatomy for treatment planning.

Gated Imaging

A technique in $4 ext{D}$ CT simulation where the patient holds their breath at maximum inspiration or expiration while the CT couch moves.

Free Breathing Imaging

A technique in $4 ext{D}$ CT simulation where the patient breaths freely during a helical scan, acquiring images through several respiratory cycles.

Image Fusion

Also known as image registration; the process of superimposing two sets of images on the same coordinate system to be viewed simultaneously.

GTV

Gross tumor volume; the portion of the tumor that is visible by imaging modalities such as CT, MRI, or PET.

CTV

Clinical target volume; the GTV plus a margin of up to $2.0 ext{ extbackslash,cm}$ to encompass unseen subclinical disease.

ITV

Internal target volume; a volume that compensates for internal physiologic movements such as respiration or bladder filling.

PTV

Planning target volume; the CTV plus an additional margin of $0.5 ext{ extbackslash,cm}$ to allow for patient movement and setup error.

OAR

Organs at risk; normal tissues whose radiation sensitivity significantly influences treatment planning or the prescribed dose.

Dmax

The depth of electronic equilibrium or the maximum build-up depth for a given photon beam energy.

Bolus

Any tissue density equivalent material used to fill in deficits for a homogenous dose distribution or to bring $D_{max}$ closer to the skin surface.

Wedge Angle

The angle of the $50 ext{ extbackslash,}$ isodose line at the central axis for low-energy beams or the dose line at $10 ext{ extbackslash,cm}$ depth for high-energy beams.

Hinge Angle Formula

$$ext{Hinge angle} = 180 - 2 ext{ extbackslash,( ext{wedge angle})}$$

DVH

Dose-volume histogram; a graph showing the volume of a particular organ and the amount of radiation exposure it receives by percentage.

Fluence Maps

Quick reference tools used in IMRT to show the concentration of dose.

Parallel Opposed Pair

A field arrangement that provides even dose distribution for centrally located targets like the brain or humerus.

4 Field Box

A setup concentrating dose near the intersection of four beams, typically used for deep-seated targets like the prostate or bladder.

Mayneord F Factor

A factor calculated at extended SSD to accommodate for the change in PDD as the SSD changes.

HDR

High dose rate brachytherapy; delivering a dose rate equal to $20 ext{ extbackslash,cGy/min}$.

LDR

Low dose rate brachytherapy; delivering a dose rate between $0.5 ext{ extbackslash,cGy/min}$ and $2.0 ext{ extbackslash,cGy/min}$.

Fletcher Suite

A combination of tandem and ovoid applicators used together in gynecologic brachytherapy.

Afterloading

The practice of installing an applicator into the patient first and then loading radioactive sources later to avoid excess dose to personnel.

Patterson-Parker System

Also known as the Manchester system; it uses a nonuniform source distribution to achieve a uniform dose distribution.

Quimby System

A brachytherapy system that uses a uniform source distribution yielding a nonuniform dose distribution.