Session 8 - IGRT_ Guidelines for Matching _ Alignment, Methods & Workflows.pdf

IGRT: Guidelines for Matching/Alignment
Methods and Workflows
Justus Adamson, PhD
ROLE
November 9, 2024
INSTITUTION

Outline
1. IGRT Basics
2. IGRT Clinical Application
4

IGRT basics
o Introduction to IGRT and its importance
o IGRT modalities and technologies
o Matching and alignment strategies

Why is Image Guided Radiation Therapy (IGRT) Important?
Conventional
2D/3D Radiotherapy IMRT IMRT with IGRT

Why is IGRT Important?
Conventional 2D/3D
Radiotherapy: IMRT: IMRT with IGRT:
• Irradiation volume larger than target
• Less risk of missing the target
• Ability to better conform to the target
• Risk of target miss if setup uncertainties aren’t
accounted for
• Ability to better conform to the target
• IGRT minimizes setup uncertainty & enables
better conformity
Margins too small:
Margins increased to account for
setup uncertainty:

IGRT Modalities
o MV Imaging System
• 2D imaging
– portal image
– cine imaging
• MV CBCT (MV CT for tomotherapy)
o kV On Board kV Imager (OBI)
• 2D imaging
– radiograph
– fluoroscopy
• kV CBCT
OBI kV source
MV flatpanel
OBI flatpanel
OBI flatpanel
OBI kV source
MV flatpanel

Other IGRT Modalities
External orthogonal kV-x rays
Electromagnetic transponders
Ultrasound
Each of these are uncommon &
require extra external hardware Cureus. 2015 Jun; 7(6): e280.

Image Guidance Process
Reference image derived from patient
geometry at simulation
Image acquired at linear accelerator
Patient set up for treatment at linear
accelerator
Image fusion
Adjust patient setup
“source” or “moving” image
“target” or “fixed” image

IGRT Roles & Responsibilities
Reference image derived from patient
geometry at simulation
Image acquired at linear accelerator
Patient set up for treatment at linear
accelerator
Image fusion
Adjust patient setup
Review & approve fusion
approval takes place after treatment
for some cases
Treat patient
(intrafractional imaging?)
Therapist
Other Roles:
Radiation Oncologist: prescribes type & frequency of IGRT, defines
CTV to PTV margins accounting for localization uncertainty
Dosimetrist/physicist: prepares plan with DRRs, add IGRT details in
setup notes, consults with RadOnc on appropriate IGRT & CTV to
PTV margin
Therapist,
Physicist (SRS / SBRT)
Therapist
(Physicist present for SRS/SBRT)
Therapist
Therapist
Physicist (SRS / SBRT),
Radiation Oncologist

Image Registration:
Target image
o 2D image matching
• “Digitally Reconstructed
Radiograph” (DRR) from simulation
CT
o 3D image matching
• Simulation CT
o 2D/3D image matching
• Orthogonal kV/MV radiographs
Source (moving) image
o 2D image matching
• MV port image, kV radiograph
o 3D image matching
• kV-CBCT, MV-CBCT, MVCT
(tomotherapy)
o 2D/3D image matching
• DRR from simulation CT (new DRR
for each iteration of registration)

2D Matching Target Image: Digitally Reconstructed
Radiograph (DRR) from simulation CT
Source (moving) Image: kV radiograph or
MV portal image

Patient positioning and treatment
o Align patient marks to lasers (get close)
o Verify and fine-tune correct patient position by matching ‘live’ daily radiograph (MV or
kV) to the DRR
Digitally reconstructed radiograph (DRR)
calculated as projection through CT data in
treatment planning system
kV radiograph acquired just prior to
treatment to verify position

Patient positioning verification and treatment QA
o Example: Verify patient alignment by matching DRR to daily
image
o This is 2D/2D match, but 3D/3D also possible with CBCT
DRR calculated from CT data in
treatment planning system
kV set-up field acquired just prior to
treatment on linac

2D/2D match: orthogonal images matched (AP
and Lateral) kV and MV
DRR radiograph
Lateral View
DRR radiograph
AP View

3D Matching
Target Image: simulation CT Source Image: kV CBCT, MV CBCT, or MVCT

Diagnostic x-ray CT for volume definition (PTV
etc), CBCT for positioning verification
o Diagnostic CT imaging for volume definition
o Fan-beam, best image quality
• Cone-beam CT imaging for
positioning verification prior to Tx
• High scatter, lower quality

2D/3D Matching
Target Image: orthogonal kV radiographs
Source (moving) Image: new DRRs from simulation
CT for each geometric transformation
Medical Physics, Volume: 35, Issue: 5, Pages: 2180-2194, First published: 29 April 2008, DOI: (10.1118/1.2903431)

Comparison of 2D/2D match with 3D/3D match
for position verification
2D/2D match of
orthogonal
radiographs
3D/3D match of CT
and CBCT data sets
• CT acquired at
planning Sim
• CBCT acquired
prior to Tx

Matching Strategies:
Soft Tissue
Fiducial Markers
Bony Anatomy

Matching Strategies
o Bony Anatomy
• Advantage: High contrast, can be used for matching with most / all imaging modalities
• Disadvantage: Soft tissue and bony anatomy motion do not correlate perfectly, leading to
CTV localization uncertainty
o Soft Tissue
• Advantage: Can match GTV / CTV / PTV
directly
• Disadvantage: Contrast may be too low
for accurate matching with some patient
geometries & imaging modalities
o Fiducial Markers
• Advantage: highly accurate (when placed
in the PTV) and high contrast- visible in
many imaging modalities
• Disadvantage: Requires that fiducial
markers be implanted prior to CT
simulation

Matching Strategies: Best Practices
o During planning, CTV to PTV margins should account for the
anticipated target localization uncertainty
• Larger for 2D-2D, bony anatomy matching, infrequent matching
• Smaller for CBCT, soft tissue, fiducial based, daily matching
o When matching, define a region of interest that incorporates
the PTV
o Match can be 3D (translation in each axis), 4D (include couch
rotation), or 6D (include rotation in all axes with a specialized
couch)
• Caution for potential collisions when couch rotation is included!
o When scheduling patients, be sure to account for extra time
needed for image guidance!

Cautions:
o For spine treatment- careful to avoid matching incorrect
vertebral bodies
o For matched fields- special care should be taken to avoid
overlap
• Match anatomy or use a fixed isocenter shift?

Methods of patient set-up for Tx
Patient alignment
method
Equipment Accuracy of
bone alignment
Accuracy of tumor
alignment
Skin marks lasers 2-8mm Poor (2-10mm+)
Bony landmarks (2D) MV or kV radiographs 1-2 Modest (2-6mm+)
Implanted seeds of
surgical clips (2D)
MV or kV radiographs N/A Good (1-2mm)
soft tissue and bones
(3D)
MV or kV On-board
CBCT imaging
~1mm Very good (~1mm)

ZOOM POLL
What is a key consideration when choosing between different
IGRT matching strategies (e.g., bony anatomy, soft tissue,
fiducial markers)?
A. Bony anatomy is always preferred due to its high contrast.
B. The strategy should be chosen based on which provides the
highest image quality, regardless of patient needs.
C. Matching strategies should consider patient-specific factors,
such as target localization uncertainty and anatomy.
D. Fiducial markers are avoided due to their poor visibility in
imaging modalities.
26

IGRT Clinical Application
o Clinical examples
o Imaging dose

IGRT Example: Right Axilla
o Merkel cell carcinoma of right upper limb,
including shoulder
o 60Gy, 2Gy/fraction
o 2 field plan (RPO / LAO), 6X photons
o Non uniform CTV to PTV margin
o Wing board + Vac lock bag, head to left,
hands up on pegs, knee sponge
o Imaging: MV day 1; daily kV, match to
clips

IGRT Example: 2D Spine (T8-L1)
o Bone metastasis
o 2 fields (AP/PA), 15X photons
o Short wingboard, Arms Up, Knee Sponge,
Feet Taped
o Imaging: MV Weekly

IGRT Example: Mediastinum VMAT
o Mediastinum
o 2 VMAT arcs, 10X photons
o Wingboard, Sponge under each arm,
Knee Sponge
o Imaging: orthogonal kV radiographs
daily

IGRT Example: 3D Rectum
o 3 fields, 15X photons
o Wingboard, Sponge under each
arm, Knee Sponge
o Imaging: CBCT day 1 and weekly

IGRT Example: Lung SBRT
o 2 VMAT Arcs, 6X photons
o Treated free breathing (CTV
defined using MIP)
o Bodyfix, arms up
o Imaging: orthogonal kV + CBCT
prior to treatment at each
fraction

Imaging Dose: IGRT can add significant dose!
Data summarized from:
0 20 40 60 80 100 120 140
planning CTs (Adaptive RT)
CBCT-Linac (Adaptive RT)
planning CT
kV-stereoscopic
CBCT-Linac
MV planar
Dose (cGy)
prostate head and neck
Note: Dose varies greatly by location, see full publication for details!
Cumulative Imaging Dose

Take Home Points
o IGRT Enhances Treatment Precision and Patient Safety:
oBy aligning the treatment target in real-time, IGRT minimizes setup
uncertainties, improving outcomes and reducing the risk of irradiating
healthy tissues.
o The Right Matching Strategy Matters:
oChoosing between bony anatomy, soft tissue, or fiducial-based
matching requires careful consideration of each patient’s needs, with
the goal of ensuring the most effective and accurate treatment.
o Plan for the Extra Time and Dose of IGRT:
oIntegrating IGRT into clinical workflows may increase session times and
imaging doses, but proper planning ensures these are managed without
compromising patient safety or treatment quality.

ZOOM POLL
Which of the following is a true statement about the impact of
IGRT on treatment planning and session time?
A. IGRT does not affect the session time and can be
implemented without additional planning.
B. Integrating IGRT may require adjustments in the treatment
plan due to increased session times and imaging dose.
C. Imaging dose from IGRT is negligible and does not need
consideration.
D. IGRT is used exclusively for emergency cases and not routine
treatments.
35

References
o Hanley, Joseph, et al. "AAPM Task Group 198 Report: An
implementation guide for TG 142 quality assurance of
medical accelerators." Medical physics 48.10 (2021): e830-
e885.
o Bissonnette, Jean-Pierre, et al. "Quality assurance for
image-guided radiation therapy utilizing CT-based
technologies: a report of the AAPM TG-179." Medical
physics 39.4 (2012): 1946-1963.
o Murphy, Martin J., et al. "The management of imaging dose
during image-guided radiotherapy: report of the AAPM Task
Group 75." Medical physics 34.10 (2007): 4041-4063.

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