Image-guided radiation therapy Flashcards by Ali Alcala

A radiation therapy procedure that uses image guidance at various stages of treatment workflow: patient data acquisition, treatment planning, treatment simulation, patient setup, and target localization before and during treatment

Image-Guided Radiotherapy (IGRT)

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IGRT identifies and corrects problems due to inter- and intra- fractional variations in patient setup and anatomy
Inter-
Intra-

inter- (between fractions)
Intra- (during fraction)

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What are some examples of IGRT?

CBCT vs planning CT (inter)
Electronic portal images vs. DRRs (inter)
ExacTrac (intra)

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Use of imaging modalities to define the target and normal tissues for treatment planning

Image-based RT

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Use of imaging modalities to monitor, guide, and modify treatment

Image-guided RT

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Why use IGRT? Larger planning target volumes (PTV) margins can be used to compensate for localization errors during treatment BUT this means more healthy tissue is exposed
These are reasons to use IGRT:

Radiation delivery accuracy
Precision
Intra-fraction motion management
Adaptive RT

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Why use IGRT?
Radiation delivery target

target coverage and normal tissue sparing

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Why use IGRT? Precision

patient specific ptv margins

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Why use IGRT? Intra-fraction motion management

respiratory gating/tumor tracking
abdominal compression

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Why use IGRT? Adaptive RT

correct and moderate setup errors
assess anatomical changes/re-plan

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Despite advancements in radiotherapy, technical challenges are still present in accurately delivering radiation:

Variation in setup positioning
Geometrical organ displacement
Deformation/volume changes of organs

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IGRT technologies

ultrasound, kV radiographic, portal imaging, markers (active and passive), kV Ct-based, MV ct-based, kV CBCT based

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Setup techniques
HexaPOD

Elekta – 440lbs
Six degrees of freedom (translational errors – x, y, z & rotational errors – roll, pitch, yaw)
Provides remote positioning correction
Look at powerpoint (slides 13-14)

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Advantages to MRI Guided RT

superior soft tissue contrast
functional and physiological imaging
Real time dynamic imaging
No radiation imaging dose
Online adaptive radiotherapy (On-ART)

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Challenges of MRI guided RT

MR safety
Lack of electron density information on MRI images for dose calculation
Magnetic field interference on dose distribution

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MRIdian

MRI Guided Linac
0.35 T MRI with 6 MV linac
4D, real-time MR imaging
MLC based
Software for Adaptive Planning
(split superconducting magnet, gantry, patient handling system, source heads, split gradient coil)

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Unity by Elekta

Digital Linac with MLCs, continuous rotation
1.5T Phillips MRI with 7 MV linac
Software functions that allow motion management and adaptive online planning

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Intra-fraction motion
Intra-fraction: Clarity 4D-US

4D US based intra-fraction motion monitoring
3D US reconstruction
Fast refresh rate
Prostate-approved only

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Visual tracking of a patient’s position on the couch during setup before treatment and constant monitoring of the patient’s surface movement and comparing to pre-recorded reference surface (detects intra-fraction motion)

Surface Guided Radiation Therapy (SGRT)

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Advantages to surface guided radiation therapy

No extra irradiation
Shown to significantly reduce radiation heart damage for left breast cancer patients
Patient surface is constantly under surveillance

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Intra-fraction Motion AlignRT

Real-time 3D surface imaging technique with high-speed tracking to determine the position of a patient in three dimensions

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How does the Intra-fraction Motions Align RT work?

2 or more camera pods mounted in the treatment room and in the simulation room (optional)
Each pod contains one camera that projects a speckle pattern on the patient and cameras to image the reflected pattern
Image captured on treatment day is aligned with the reference image to calculate couch translations needed to correct a patient’s position

Intra-fraction motion CatalystHD

3 optical systems provide real-time patient surface mapping
Projects a known sequence of patterns onto the patient and records the reflected patterns
Reconstruction compares projected and reflected patterns to identify the coordinates of each pixel on the captured image and errors used to correct the patient position
Intra-fraction motion detection
Respiratory gating

Non Radiation based systems

HexaPOD for Setup techniques
MRI Guided RT
MRIdian by Viewray
Unity by Elekta
Intrafraction: clarity 4D-US
Intrafraction: clarity
SGRT
AlignRT
CatalystHD

Radiation based systems

port films EPIDs CBCT ExacTrac

These are taken start of treatment and once a week to ensure proper radiation positioning Verifies the position of external radiation fields to the target volume Do not track patient’s progress during treatment

Port films

limitations to port films

Delay in viewing due to time required for processing Impractical to do port films before each treatment Image quality is poor (lack of contrast)

This is Offered as a standard on nearly all linacs Image acquisition is 2D Use bony landmarks to compare to DRR Gives translational shifts but not rotational shifts

Electronic Portal imaging

Electronic portal imaging uses either kV or MV x-rays for imaging:

kV: better image contrast and average dose is 1-3 mGy MV: less distortions from metal implants and average dose is 30-70 mGy

With this, Planar projection images are acquired from multiple angles as the source with an opposing detector panel rotates around the patient Acquired images are used to reconstruct patient anatomy Mounted on accelerator gantry

CBCT

CBCT can use kV or MV x-rays. True/false

TRUE

CBCT enables:

localization of PTV and critical structures before each treatment

With this, the x-ray tube is mounted on a retractable arm orthogonal to therapy beam direction and detector mounted opposite to x-ray tube 2D radiographic and fluoroscopic modes verify portal accuracy, manage intra-fraction motion, and make positional corrections before and during treatment

kV CBCT

What is the average dose of kV CBCT?

1-10cGY

Advantage of kV CBCT:

good soft-tissue contrast

disadvantage of kV CBCT

Artifacts due to high-Z materials

This uses MV x-ray beam of linac and EPID mounted opposite to the source Good image quality for bony anatomy Less susceptibility to artifacts caused by high-Z materials

MV CBCT

2 kV x-ray units and a 6D fusion provide fast and accurate positioning Unintended shifts detected and displayed to user Non-coplanar verification Detects intra-fraction motion, regardless of couch and gantry angle Patient’s initial position set by linac-based IGRT system continuously verified with X-ray imaging Frameless cranial radiosurgery Noninvasive, time-saving, and increases patient comfort

Exactrac x-ray 6D stereotactic

object placed in the field of view of an imaging system which appears in the image produced, for use as a point of reference or a measure. It may be either something placed into or on the imaging subject, or a mark or set of marks in the reticle of an optical instrument (fiduciary tracking system)

Fiducial marker or fiducial

3 Beacon electromagnetic transponders are implanted into the prostate before starting the treatment Transponders work with the tracking system to monitor prostate motion throughout the treatment If motion is detected, Calypso sends out an alert and stops the radiation until the target is properly aligned

Calypso 4D

Advantages to calypso 4D

Direct target tracking 4D and real-time Fast feed back No imaging dose Signal can be used for gating

Disadvantages to calypso 4D

Transponders need to be implanted Can’t see anatomy in images Can have interference

3D scan is taken at multiple breathing phases. We can do a 4D CT sim. What is the fourth dimension?

time

Visualize motion of the tumor at different parts of the breathing cycle At specific parts of the breathing cycle, a CT is taken Only a few slices of the patient are imaged every time we take a CT (single couch position) Need to image patient over a range of couch positions Images captured at the same parts of the breathing cycle at different couch positions are grouped together

4D CT simulation

Often used as a technique for left breast cancer patients Raises the chest wall and expands the lung volume, which pushes the heart away from the chest wall and increases the distance between the left breast and heart

Deep inspiration breath hold (DIBH)

Location of structures in the abdomen (liver, kidneys, pancreas, and tumor volumes) affected by respiratory motion Minimizes respiratory motion A tolerable pressure is applied to the abdomen to restrict internal organ motion

Abdominal compression

Normal breathing – record breathing cycle (amplitude of chest wall) with Sentinel or RPM or ABC Essentially gives you a little movie of how the tumor moves Take the movie and look at all the slices & see where the tumor is Make a large ITV

Free breathing

Patient free breathes Breathing is monitored to know when the tumor is in the window When the tumor is in the window, the beam is turned on

Gating-free breathing

Beam is on during a certain window Patient has to hold their breath to get into the window Something like CatalystHD can be used to monitor for the window Adjust the active breath hold process to maximize compliance Short, normal inspiration breath hold Deep inspiration breath hold (fill the lungs up with air to increase the separation between breast tissue and heart

Gating-breathhold

Breath hold methods for tracking breathing cycle

Active Breathing Coordinator (ABC) *Spirometry-based *Elekta Real-Time Position Management (RPM) *Video-based *Varian

Uses a spirometer to track the patient’s lung volume Consists of snorkel, a nose clip, a green button, and a computer Used to reduce anatomical movement in the chest and abdomen caused by breathing Specifically used for left breast treatments to reduce dose to heart, lung and to reduce tumor target margins Patient holds and maintains an accurate breath hold to a specific lung volume

Active breathing coordinator (ABC)

Consists of infrared tracking camera, reflective marker block, and predictive filter Respiratory gating for respiration-synchronized treatment Camera and marker measures patient’s respiratory pattern and range of motion Gating thresholds are set after determining how the tumor moves with respect to the patient’s breathing Thresholds placed when tumor is in the desired part of the respiratory cycle Determines when the gating system turns the beam on and off 3D real-time patient position monitoring Tracks the position of the marker block in 3D Detects unexpected movement of the of the marker block Ensures that the target is positioned accurately

Real-time position management (RPM)

Gating advantages

Minimal effect on healthy tissue *Smaller margins Patient comfort

Gating challenges

Challenge to assess the interval or window of delivery *Large window – no gating *Small window – specific phase threshold? Predictive algorithm – reproducible pattern Patient specific motion Significant increase in overall treatment time *Duty cycle (fix-field-based IMRT) Potential Solution – Gated VMAT delivery