IGRT Flashcards
What is IGRT?
What is its ultimate goal?
Image guided radiotherapy: it’s an approach to radiotherapy where decisions about the validity of the plan and its optimization are based on the frequent imaging of the patient.
Together with CRT is one possible solution for increasing the therapeutic window, meaning the dose range between TCP and NTCP for 50% complication probability. CRT is based on the generation of a dose distribution which is highly conformed to the shape of the tumor, whereas IGRT aims to decreasing the PTV margins. If we include less NT is the PTV we reduce the NTCP.
How do we reduce the PTV margins?
1) Frequent imaging
2) Models and assumptions about the structure and the motion of the organ
3) Consider geometric uncertainties
4) Constraints or a priori defined criteria that we have to meet
–> systematic uncertainties weight more than random ones
Why is the PTV margin with online adaptation smaller than with offline ART?
online ART is based on adapting the plan the same day the patient comes to the treatment facility to get the fraction. The imaging delivers the real-time positioning of the tumor volume and the patient doesn’t move, therefore the motion and anatomical errors are strongly reduced.
Offline adaptation is done for the fraction of the next day, so the uncertainties about the motion and positioning are going to be higher.
Explain the clinical workflow and possible improvements
1) Patient positioning and immobilization
2) Daily imaging: New image reference (non-invasive but can be ionizing or non-ionizing) or time-to-time tracking (invasive and non-invasive)
3) Image fusion:
4) Corrections to set-up: table shifts, shifts+rotation, aperture-based corrections (gantry rotation, field shape corrections, new target and fluence calculations)
5) Dose delivery
Improvements: we could replan the dose every time instead of doing it only in the worst cases of major changes. This also would advantage us in terms of the dose accumulation monitoring.
We could also use non-radiation imaging devices like MRI but then the imaging time and the costs would increase…
Online ART for protons could also be useful.
What are typical uncertainties?
1) motion and positioning of the patient (R)
2) changes in the anatomy (S)
3) machine-related (S)
4) RBE/tissue response (R/S: they are related to both deterministic and stochastic effects…)
5) CT artifacts (S)
List some IGRT techniques with advantages and disadvantages
EPID: 2D imaging, beams are perpendicular to each other
A) easy, kV beam can be used in fluoroscopy mode, matching done with bony surrogates and fiducials
D) poor contrast, no soft-tissue details
CBCT (oV, MV): 2D projection, 3D image through rotating gantry. Energy is emitted as pulses to reduce the dose to the patient.
–> different from the diagnostics CT because:
- beam is cone-shaped and produces an image on a flat detector
- the diagnostics CT has a fan-shaped beam and arc-shaped detectors.
A) Rapid acquisition, sub-mm accuracy, high contrast and sp. res., soft tissue visibility, good for onlina ART
D) limited FoV, lower quality than planning CT, beam hardening
MVCT: Megavoltage beam+helical scanner
A) IMRT and IGRT with same device, few artifacts, full FoV
D) low contrast resolution, low detail visibility
IN-ROOM CT: separated from the treatment machine but they are in the same room.
A) high accuracy and precision, high contrast and spatial resolution, full FoV, high soft tissue visibility
D) BIg equipment –> big room, increased costs, travel time
optical surface imaging/marker-based optical tracking: images the surface/images the marker assuming spatial correlation between marker and anatomy (useful for positioning and pre-alignment.
A) sub-mm accuracy, no dose delivered, signal acquired DURING the treatment
D) assumes direct correlation with organs/invasive, limited to tracking 1 marker per time
EM tracking: transponder is implanted, excited by set of coils, sends EM signal to track marker.
D) don’t image the target directly
