Wk1 - SBRT/SABR Flashcards
(40 cards)
1
Q
stereotactic in RT context
A
- high precision, image guided dose delivery to the target
- highly conformal dose with steep dose gradients
- intra-fraction motion management where applicable
- requires high level of confidence in the accuracy of the entire treatment delivery process
2
Q
SBRT
A
stereotactic body radiotherapy
- for dose escalation - targets close proximity to OAR
- extra cranial eg. spine/prostate
- typically 1-5 fractions, may be up to 8
- > 8Gy per fraction but sometimes less
- high precision, image guided dose delivery
- highly conformal dose with steep gradients
- intra-fraction motion management essential
3
Q
SABR
A
stereotactic ablative body radiotherapy
- for ablation
- extra-cranial eg. lung, liver, renal
- typically 1-5 fractions, may be up to 8
- > 8Gy per fraction but sometimes less
- high precision, image guided dose delivery
- highly conformal dose with steep gradients
- intra-fraction motion management essential
4
Q
SRS
A
stereotactic radiosurgery
- single fraction
- intra/extra cranial
5
Q
SRT
A
stereotactic radiation therapy
- intracranial
- for larger lesions not suitable for SRS
- for post operative cavities
- fractionated, typically 2-5 fractions
6
Q
dose/fractionation - conventional RT
A
1.8-2.4Gy per day, 15-40 fractions over 3-8 weeks
allows
- normal cell repair
- repopulation after RT
- redistribution in cell cycle
- reoxygenation
- radiosensitivity
7
Q
dose/fractionation - SBRT
A
> 8 and up to 30+ Gy per day
1-5 fractions
1-2 weeks
- less normal tissue irradiated (smaller PTV margins used with motion management)
- anti-tumour effects not predicted by classic radiobiology
- re-oxygenation - tumours may no be hypoxic therefore no benefit from re-oxygenation
8
Q
abscopal effect
A
occurs when radiation treatment not only shrinks the targeted tumor but also leads to the shrinkage of untreated tumors elsewhere in the body
9
Q
SBRT treatment sites
A
lung, liver, spine, prostate etc
- primary and secondary disease - oligometastatic state
- <5cm max dimension
- non malignant conditions
- applied to tumours
considered radio-resistant in the conventionally fractionated scenario eg. renal cell, melanoma
- for ablation eg. lung, liver
- for dose escalation eg. spine, prostate
10
Q
oligometastasis meaning
A
an intermediate state of metastasis between purely localised disease and widespread metastasis
11
Q
why SABR
A
- inoperable due to tumour location or medical co-morbidities
- technological advances in image guidance
- highly conformal dosimetry
- dose delivery
- improvements in motion management strategies
- clinically proven to be as effective as surgery without associated cost and patient recovery
12
Q
patient contraindications
A
- prior RT
- unable to lie flat for prolonged period
- cannot receive chemo 1-4 weeks pre or post SBRT (depending on site for treatment)
- severe connective tissue disease or scleroderma
- claustrophobia
- mental status prohibitive of patient compliance
13
Q
goal of imaging
A
to provide visualisation of patient anatomy as it will appear at treatment
14
Q
why we image
A
- to delineate targets and critical normal tissues
- to calculate optimal dosimetry
15
Q
different imaging types
A
depends on the treatment site
MRI, PET, CT, 4DCT, inhale, exhale, FB, contrast, MIP, Av IP
16
Q
Average IP
A
average intensity profile
- a reconstructed data set which shows the average value from each of the 4D bins for each voxel
- averages motion effects
- use for dose calculation
17
Q
MIP
A
maximum intensity profile
- a reconstructed data set which shows the maximum value from each of the 4D bins for each voxel
- shows maximum range of motion of a tumour
18
Q
patient preparation, immobolisation
A
- consider the patient a ‘participant’ in the procedure
- RO consult - discuss goals and challenges of procedure
- consider pain management and anxiety/claustrophobia
- pre-planning discussion
- DIBH or 4DCT - information, coaching, compliance
- consider/prepare immobolisation equipment and 4D equipment inc. comfortable, reproducible vac bag
- consider location of target/s to determine arm position
- for lesions above T4/5, consider using a shell
- consider previous and future RT
- desired level of accuracy 1mm 1 degree
19
Q
patient positioning considerations
A
- comfortable and reproducible - treatment times can be long
- enable delivery of technique to achieve dosimetric goals
- allow good access to the target
- minimise dose to normal tissues
- minimise restrictions on beam angle choice
- consider arm or head position, position of tubing for body fix, position of loc bars or joins in the bed
- accommodate equipment eg. hexapod frame, compression belt/bellows
- avoid creation build up with equipment
20
Q
patient considerations
A
- comprehension/understanding - greater chance of success if the patient is a contributing participant
- pain control/management - should be well controlled before attempting simulation
- mobility/co-morbidities
- need to hold treatment position
- positioning aides/accessories need to be made so that the patient can readily get into and out of the treatment position, without leaning on and potentially displacing or deforming the equipment eg. vacbags
- breath hold, coaching
21
Q
patient education process
A
- patient information sheets and pre-CT checklist
- long consultation with RO
- pre-CT consultation with RT (motion management, fasting, organ filling/voiding alongside normal education for RT)
- post CT and pre treatment consultation with nursing staff and RT
22
Q
is the patient suitable for SBRT
A
- can they manage DIBH/EEBH
- can they manage organ filling/voiding instructions
- can they manage an appropriate treatment position
- has there been previous RT
- do they have pain
- are they claustrophobic or anxiety/stress
23
Q
prescriptions and dose - conventional
A
- PTV covered by 95% isodose
- dose range 95-105%
- fall off outside PTV 95%-0
- up to 10mm margin depending on number of fields
- homogenous distribution
24
Q
prescriptions and dose - stereo
A
- PTV covered by 100% isodose
- acceptable max dose is prescribed, covering isodose is a % of this max dose
- little to no margin on PTV
- fall off outside PTV 60-80%
- heterogenous distribution
25
plan evaluation
- check PTV cover
- checking min doses to GTV and CTV, especially in cases where there is a critical normal structure abutting or impinging on the target
- consider what is surrounding the target - heterogeneities can make it difficult to achieve all plan goals, as will proximity of OAR
- dose fall off beyond target - should be as rapid as can be achieved
- communicate with the RO (you will not always get 95-99% target cover)
26
R50
ratio of the volume covered by the isodose representing 50% of the prescription dose to the volume of the PTV
this is a function of the size of the PTV - smaller for larger PTVs
27
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 dose gradients
28
D2cm
the dose at any point 2cm from the PTV is recorded and is expected to meet set criteria
mechanism for evaluation dose fall off geographically
29
What must be considered when positioning a patient for SBRT treatment?
1. Comfortable and reproducible
2. Need good access to the target
3. Minimize dose to normal tissues
4. Accommodate equipment needed on treatment - hexapod frame
30
Describe leaf interplay effect
Interaction between the movement of the MLC shapes for segments around the motion of a tumour with the respiration cycle. Gating the treatment minimises the effect of the interplay.
31
List rationale for SBRT
1. Less normal tissue irradiated because IGRT and motion management allows smaller margins
2. Hypoxic cells respond well
3. Comparable results to surgery in some cases
32
What is a stereotactic dose to the liver?
48 Gy in 3 fractions or 60 Gy in 6 fractions
33
What is a stereotactic dose to the spine?
18 Gy in 1 fraction
34
What is a stereotactic dose to the prostate?
19-21 Gy in 2 fractions post EBRT of 46 Gy in 23 fractions
35
List indications for liver SBRT
- more than 3 months to live
- unfit for surgery
- 800cc of uninvolved liver
- less than 3 HCC primary and less than 5 Mets
36
motion management strategies for SBRT
- breath hold (DIBH/EEBH)
- bodyfix
- compression belt
- compression plate
- gating
37
sources of positional errors (non-patient)
- resolution of imaging
- affects size and appearance of structures
- accuracy of image fusion
- depends on user experience
- accuracy of target delineation
- accuracy of mechanical isocentre
- accuracy or radiation/treatment isocentre
- resolution of couch positioning
- resolution of infrared camera for movement verification
38
sources of error (patient factors)
- position
- immobolisation
- organ motion
- respiration
- cardiac function
- peristaltic activity
- organ filling/emptying
- correct use of immobolisation equipment
39
lung SBRT dose
depends on the location of the lesion
GTV/ITV >1.5cm from ribs 54Gy in 3#
GTV/ITV <1.5cm from ribs 48Gy in 4#
lesions approaching central zone or larger lesions 50Gy in 5# or 60Gy in 8#
small favourable tumours 26-33Gy in 1# (SRS)
40
3DCRT lung SBRT
- 6MV
- 8 to 10 beams
- non opposing
- non coplanar component included
- beams do not overlap at entry
- minimum 5 non coplanar beams at 3 couch angles
- beams should have sufficient separation to minimise overlap of beams away from the PTV
