SBRT Flashcards
SBRT meaning
Stereotactic Body Radiation therapy
Does not involve any stereotactic treatment to the brain
Why SBRT
Dose escalation - targets in close proximity to OAR
Extra cranial
Around 1-5#
More than 8Gy per fraction
Highly conformal with steep gradients
SABR
Stereotactic ablative body Radiotherapy
Used for ablation - not many OARs surrounding
Extra cranial
SRS
Single fraction
For brain
Gamma knife is an example of SRS
SRT
Stereotactic radiotherapy
Intra cranial
For larger lesions that are not suitable for SRS
Fractionated typically 2-5#
Radiobiology in SBRT fractionation
Anti-tumour effects can not be predicted by classic radiobiology
Tumours may not be hypoxia therefore no benefit from reoxygenation
Hypo-fractionated alters micro environment which leads to more death of tumour cells
Dead tumour cells released quantities of antigens which stimulated antitumour immunity
The abscopal effect
Distant tumour regression after localised irradiation
What can we treat
- inoperable tumours
- oligometastic state
- <5cm max dimension
- non-malignant conditions
- for ablation e.g. lung, liver
- for dose escalation e.g. spine, prostate
Patient contraindications
- prior RT
- unable to lie flat
- cannot receive chemo 1-4 weeks pre or post SBRT
- severe connective tissue disease
- claustrophobia
- mental status prohibitive of patient compliance
Average intensity profile versus maximum intensity profile
Maximum - sharper image
Average - more fuzzy, easier to match in treatment
Image fusion issues
Image artefacts - from motion, metal implants
Image distortion - for PET and MR delineation
Registration errors - can be the largest source of error in the entire treatment process
SBRT techniques
Mohawk, flipper, seatbelt
Dosimetry for SBRT
Always highly conformal
Inhomogenous dose distributions
170% max dose
Dose painting techniques
Stereo dose distribution
Heterogenous
Fall off outside PTV 60-80%
PTV covered by 100% isodose
Prescription
Prescribe at 80% iso line
Prescribe at an acceptable maximum point dose dose (e.g. 125%)
Plan evaluation priority
Check OAR goals - different OAR prescribed to conventional RT
Check PTV cover
Dose fall off beyond target
R50
Ratio of the volume covered by the isodose representing 50% of the prescription dose to the volume of the PTV
Function of the size of the PTV - smaller for large PTVs
This is volumetric not geographical
Gradient index
Ratio of the volume of half the prescription isodose to the volume of the prescription isodose
Differentiates plans with similar conformity but with different gradients
D2cm
The dose at any point at 2cm from the PTV is recorded and is expected to meet set criteria
Geographical evaluation
Elekta hexapod
Patient immobilisation equipment must fit inside it
High sensitivity optical tracking
Difficulties
- must be positioned 30-50cm distance of iso
- arm position
- patient height
- patient BMI
- Indexing on vac bags
When to use rotational corrections
Spine
Pancreas
For intercranial lesions, particularly if located at the base of the skull
For lesions abutting, overlapping with or within 2cm of critical normal tissue structures
For treating multiple lesions
Anything that is not round
Motion management strategies
Breath hold
Elekta body fix
Compression belt and plate
Gating
Sources of positional error
Resolution of imaging
Accuracy of image fusion
Accuracy of target delineation
Accuracy of mechanical iso
Accuracy of radiation/treatment iso
Resolution of couch positioning
Sources of error - patient factors
Position
Immobilisation
Organ motion - respiration, cardiac function
Level of accuracy
Dependent on sources of error and how they are managed
Dependent on clinical protocol (fractionation, margins, pathology)
TPS technique
FFF - fast treatment
Collimator angle choice
VMAT - not ideal for single fraction due to leaf interplay
MLC leaf interplay
Interaction between the movement of the MLC segments in VMAT or IMRT and the motion of a tumour with the respiration cycle
Best practice to plan on AV IP data set for dosimetry more representative of the total resp cycle
Spine treatment
For oligometastatic disease
Very steep dose gradients around SC
Most commonly 1 vertebrae but can be multiple
PRV 1.5-3mm
Larger cord structures or the cal sac will impact PTV cover
Lung patient conditions
Inoperable
Central (fractionate)
>5cm diameter
No tissue diagnosis
Salvage after prior RT
T3 tumour with chest wall invasion
Synchronous or multi-focal tumours
Lung techniques
VMAT most common - except for single fraction regimes due to leaf interplay
Use FFF and DIBH
Target delineation for lungs
ITV to be marked to encompass full range of tumour excursion
If using DIBH - do 2 CTs to check variability in breath holds
When an OAR dose goal conflicts with covering iso lines, what happens
The dose to OAR and covering isodose should run like parallel or concentric
Liver SBRT
Common site of Mets
For patients unsuitable for surgery
Can treat up to 5 lesions if Mets
Treats up to 3 lesions if HCC
Must have 800cc of uninvolved liver
liver dose
48gy in 3
40-50Gy in 5
Lower dose used where there is underlying liver dysfunction
Plan techniques for liver
Shorter arc lengths to avoid entry dose through liver
Partial arcs - slightly higher than mid range dose but lower low dose
Usually co-planar VMAT to avoid increasing low dose wash in liver
Prostate SBRT
Mono therapy
As a boost
Requires fiduciary markers for accurate localisation
Space OAR needed
Pancreas SBRT
Fidcucials useful
Use of oral contrast to distend duodenum
Critical structure. - duodenum
Very similar to liver protocol
Desired accuracy
1mm 1 degrees
Patient positioning considerations
Arm position
Comfortable and reproducible
Allow good access to target
Minimise dose to normal tissues
Minimise restriction on beam angle choice
Avoid creating build up with equipment
Consider head position, position of tubing for body fix, loc bars
Check whether patient comprehends and understands instructions
Pain control and management
Morbidity/co-morbidity
What moves?
- Skeletal/muscular (i.e. the patient) – mitigate through stabilisation
- Respiratory motion – evaluate with 4DCT, manage with compression, breath hold or gating
- Cardiac motion – remains
- Peristalsis – manage with compression
- Bladder and bowel filling and emptying – manage with protocols,
enemas, medications, catheterisation
