CT Simulation

goal of radiation therapy

kill tumor and protect normal tissue

requires high precision and accuracy

ct simulation

allows decrease in PTV, dose escalation where indicated, less morbidity, improved outcomes/survival/tumor control

goal of CT sim

establish a reproducible pt position

immobilization devices

pt education

identify isocenter

image pt with appropriate technical factors

mark pt (tattoo)

document pt position/photos

modalities used with CT

MRI

PET

CT

primary data set used because of hounsfield units

equipment requirements

CT scanner

fast CT acquisition time

multislic technology

axial and helical scanning capabilities

wide bore >75 cm

external laser marking system

virtual simulation software

other options: gating technology (4D CT), surface guidance

field of view

actual width of imaging capability

SFOV bigger than DFOV

benefits of ct sim

accurate delineation of 3D volumes

isocenter can be placed quickly and accurately

virtual pt provides flexibility to create/change tx plan

more info/data for measurements post sim

conedown or boost fields accomplished w/o pt

BEV display allows anatomy to be views from the perspective of the xray beam

allows field shaping electronically

virtual sim allows comparison of beams and DRR construction

allows for downstream calc and viewing of dose distribution based on pt anatomy

able to mitigate intrafraction motion with 4D CT

easier procedure for pt d/t decrease table time

all info stored digitally

CT exposure can be quantified and recorded

CT sim considerations

size of aperture must be large enough to accomodate pt w/ immobilization devices

SFOV and DFOV must be large enough so that pt entire contour can be visualized

couch must be flat and level

an external marking laser system

table time should be minimized to reduce pt movement and localization errors

CT numbers used for dose inhomogeneity corrections

machine parameters, blocks, MLCs cannot be verified on CT sim

monitor CT dose

reproducibility and quality imaging

pt must be cooperative and relaxed; good communication

take extra care to construct an immobilization device to ensure accuracy

straighten the pt, restraighten and repeat topogram if needed

contrast media

used to help differentiate anatomic structures or highlight an abnormality

introduced IV, PO, intrathecally, intraarteriorly

uses power injector

complete medical hx/questionnaire

mild contrast reaction

pruritus, itchy throat, sneezing

limited N/V, altered taste

Tx: observe, reassure pt

moderate contrast reaction

diffuse erythema, facial edema

wheezing, protracted N/V

Tx: careful observation for possible progression

severe contrast reaction

diffuse edema, erythema

wheezing, bronchospasm

anaphylactic shock

convulsions

Tx: requires prompt recognition and tx, almost always requires hospitalization

pt positioning in CT scanner

center pt in CT bore and SFOV (helps to eliminate artifacts, provides best image quality)

ensure FOV encompasses entire external contour (reduces pt dose)

CT Simulation procedure

topogram/scout: scan limits, assess pt alignment, plan start/stop, xray tube is stationary while table moves

data acquisition: axial, helical (reduces scan time)

protocols: kVp, mAs, DFOV, slice thickness, increment, scan length, rotation speed, window width, window level

documentation

accurate daily set up notes

photos

shifts

special instructions

integration with record and verify systems (ARIA, Mosaiq)