Image-guided radiation therapy Flashcards
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)
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)
What are some examples of IGRT?
CBCT vs planning CT (inter)
Electronic portal images vs. DRRs (inter)
ExacTrac (intra)
Use of imaging modalities to define the target and normal tissues for treatment planning
Image-based RT
Use of imaging modalities to monitor, guide, and modify treatment
Image-guided RT
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
Why use IGRT?
Radiation delivery target
target coverage and normal tissue sparing
Why use IGRT? Precision
patient specific ptv margins
Why use IGRT? Intra-fraction motion management
respiratory gating/tumor tracking
abdominal compression
Why use IGRT? Adaptive RT
correct and moderate setup errors
assess anatomical changes/re-plan
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
IGRT technologies
ultrasound, kV radiographic, portal imaging, markers (active and passive), kV Ct-based, MV ct-based, kV CBCT based
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)
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)
Challenges of MRI guided RT
MR safety
Lack of electron density information on MRI images for dose calculation
Magnetic field interference on dose distribution
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)
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
Intra-fraction motion
Intra-fraction: Clarity 4D-US
4D US based intra-fraction motion monitoring
3D US reconstruction
Fast refresh rate
Prostate-approved only
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)
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
Intra-fraction Motion AlignRT
Real-time 3D surface imaging technique with high-speed tracking to determine the position of a patient in three dimensions
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 thefield of viewof animaging systemwhich 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 thereticleof 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
