What is Image Guidance
- Not the delivery of dose
- Used to ensure QA and to check no movement has occurred
- Most tumours are radioresistant if they are not in the treatment beam
Key features of IGRT
o Availability of high-quality imaging in the equipment in the treatment room
o Ability to visualise the target (not just external markers or bony anatomy) with the patient in treatment position
o A protocol to act on the findings
This could be done on-line, i.e., prior to turning the radiation beam on, or off-line between fractions if more complicated decision-making is required
Offline IGRT matching
taking image within certain amount of tolerance and review following treatment
Online IGRT Matching
taking image while the patient is still on the treatment couch
Workflow of IGRT
o Take images 3 consecutive days at the commencement of shift
o Average shifts to account for systemic setup error
o Apply average displacement to the isocentre shifts and this becomes the new isocentre
o Image on Day 4 to confirm Image weekly and if images are beyond threshold action level, repeat each day until a trend is observed or they are within threshold tolerance for two consecutive days
Difference between inter-fractional vs Intra-fractional
- Inter-Fractional varies day to day (e.g., prostate)
- Intra-Fractional varies within the same day (e.g., lung)
Role of IGRT on Motion management
- IGRT facilitates the detection and management of exceptional deviations
o Including emergent or spurious changes, such as gross positioning errors, weight loss, substantial organ deformations, systemic changes in internal organs, changes in respiratory motion
Skin render
- Important of every plan that many RT’s do not value highly enough
- Skin render or images of the field position on skin generated from the planning system is one of the most important steps that should be checked everyday. Prior to imaging.
Transponder
- Magnetic implants allow for target delineation without ionising radiation both for setup and also for live tracking during delivery of treatment
Rules of matching anatomical points
- Match at least 3 structures within the field outlined
Choose the most stable features
Respiratory Gating
- Machine measures peak volume and peak expiration to determine when to turn beam on and off
- Can sometimes assist with beam regulation
DIBH
Deep Inspiration Breath Hold
- Camera watches box sitting on the chest of the patient (if moves, beam is turned off)
What is EPI
Electronic portal imaging
Methods of using IGRT
Original diagnostic Imaging • Simulation scanning on CT • Image fusions MRI, PET/CT, SPECT • DRR Digitally reconstructed radiograph • MV epi • kV epi • CBCT (Cone beam computed tomography) • Ultrasound • Transponders (magnetic implants) • Fiducial markers (Gold seeds) • Respiratory Gated breathing • Active breathing control (Compression) • Surface tracking (Surface Guided Radiation Therapy) • MR-Linac
What is ITV
Internal Target Volume
Is the volume encompassing the CTV, which takes into account the fact that the CTV varies in position, shape and size
