SBRT/SABR Flashcards
Define stereotactic
- high precision image guided dose delivery (1mm, 1 degree)
- highly conformal dose with steep dose drop off
- intrafraction motion management
What is SBRT?
- sterotactic body radiotherapy
- dose escalation for targets close to OAR (extracranial e.g. spine, prostate) (don’t want to put OAR at risk and requires sterotactic precision)
- 1 to 5 #
- > 8Gy per fraction
What is SABR?
- sterotactic ablative body radiotherapy
- for ablative doses (extracranial e.g. liver, lung, renal)
(not limited by OAR therefore higher doses can be delivered) - 1 to 5 #
- > 8Gy per fraction
What is SRT?
- stereotactic radiotherapy
- for large cranial lesions not suited for SRS (e.g. post operative cavities)
- 2 to 5 #
- lower BED then SRS
What is SRS?
- sterotacitc radiosurgery
- historically intracranial but can be extracranial
- single fraction
- 12 to 90+ Gy per fraction
- can use gamma, cyber or linac
What does conventional dose fractionation allow?
- 1.8-2.4Gy per # over a course of 15-40# (3-8 weeks)
- normal cell repair
- re-population after RT
- re-distribution in cell cycle
- re-oxygenation
- radiosensitivity
What does SBRT dose fractionation do?
- less dose to normal tissue irradiated
- anti-tumour effects not predicted by classic radiobiology
- smaller PTV margins and motion management.
What is the patient performance criteria for SBRT?
- performance status 0-2
- life expectancy >6months (>3 months for liver)
- low metastatic burden (>5 mets, >5cm diameter)
What are the contra-indications for SBRT?
- prior RT
- unable to lie flat
- cannot receieve chemo 1-4 weeks pre and post
- sever connective tissue disease or scleroderma
- claustrophobia
- mental status prohibitve of patient compliance
What are the planning principles?
- image fusion
- increased no. beams
- non-coplanar
- small to no margin for beam penumbra
- highly conformal
- inhomogenous dose distribution
What are the simulation considerations?
- increased immobilisation
- 4DCT
- breath hold
What body areas move?
- skeletal/muscle: stabilisation
- respiratory (lungs, ribs, abdomen): 4DCT, breath hold or gating
- cardiac: remains
- peristalsis: compression
- bladder and bowel: preparation or catherisation
What are the sources of error (non-patient)?
- image resolution (size of structures)
- accuracy of image fusion
- accuracy of target delineation
- accuracy of mechanical isocentre
- accuracy of treatment isocentre
- resolution of couch position
- resolution of infrared camera
What lung tumours are considered for SBRT?
- inoperable
- central tumour
- > 5cm diamter
- no tissue diagnosis
- T3 tumour with chest wall invasion
What is the interfraction interval?
- 40 hrs
What is dose fractionation for lung?
- ITV is >1.5cm from ribs: 54Gy in 3#
- ITV is <1.5cm from ribs: 48Gy in 4#
Purpose of test runs/dry runs
- small rotational corrections can require large translational moves
poorly con structured immobilization can result in gantry collisions - reviews breath hold reproducibility for DIBH/EEBH
- check tumor excursion
- check visibility of lesion and surrounding anatomy on CBCT (limited FOV)
Dosimetry
- dependent on site
- increased number of beams/arcs
(non-coplanar beam arrangements to create isotropic dose fall off) - must be highly conformal
- in-homogeneous dose distributions
- small or no beam margins for pneumbra
- dose painting techniques
- VMAT/conformal arc/FFF 6MV or 10MV
Prescriptions and dose (conventional vs stereo)
Conventional
- PTV covered by 95% isodose line
- dose range 95-105%
- fall off outside PTV 95% - 0
- up to 10mm margins depending on number of fields
- homogeneous distribution
Stereo
- PTV covered by 100% isodose
- acceptable max dose is prescribed covering isodose is a % of this max dose
- no margin or very small on PTV
- fall off outside PTV 60-80% - 0
- heterogeneous distribution
Multiple lesion plans
- if it cant fit in a 10cm radius (need to use separate isocentres - why? can’t correct rotationally with 1 isocentre)
R50
Ratio of volume covered by the isodose representing 50% of the prescription dose to the volume of the PTV
Gradient index
Ratio of volume of half the prescription isodose to the volume of the prescription isodose
- differentiates plans with similar conformity but with different dose gradients
When is the R50 and gradient index used?
Useful for targets completely surrounded by OAR (e.g lung and brain) where isotropic low dose is desired
D2cm
Point at any point 2cm from the PTV (isotropically defines that dose is less than 2cm from the target)
Mechanism for evaluating dose fall of geographically
Hexapod
Common issues
Optimal distance = 30-50cm Issues - arm position - patient height - patient BMI - indexing on vacbags
Organ motion
- Skeletal/muscular (i.e voluntary)
- Respiratory motion - evaluate with 4DCT, manage with compression, breath hold or gating
- Cardiac motion
- Peristalsis - managed with compression
- Bladder and bowel - filling and empty (enemas, medications and catheterisation)
Motion management strategies
- breath hold - DIBH/EEBH
- elekta bodyfix
- compression belt
- compression plate
- gating
Sources of positional error
- resolution of imaging (affects size and appearance of structures)
- accuracy of image registration
- target delination
- mechanical isocentre
- isocentre of radiation/treatment isocentre
- resolution of couch positioning
- resolution of infrared camera for movement verification
Interplay effect and VMAT lung
leaf interplay - interaction between movement of the MLC shapes for segments in an IMRT beam or VMAT arc and the motion of a tumor with the respiration cycle
- dose is scattered more in soft tissue rather than lung
- segments in the arcs and IMRT beams may shield the tumor to even out the dose across the PTV
- ways around is to plan on Av IP data set for dosimetry as it is more representative of total respiratory cycle
- motion management (gated or breath hold)
- treat with FFF beams
Requirements for IGRT
- all treatment require pre-treatment CBCT guidance
- RO present for all treatments (responsible for approving CBCT guidance)
- RT credentialing (specialized training)
- initial image registration is to be made utilizing large clip box to exclude gross error; clip box is subsequently reduced to the ROI for final image guidance as determined by RO
- CBCT translations >1mm are actioned
- CBCT rotations - 3 degrees
