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CSB094 > SBRT/SABR > Flashcards

SBRT/SABR Flashcards

1
Q

Define stereotactic

A
  • high precision image guided dose delivery (1mm, 1 degree)
  • highly conformal dose with steep dose drop off
  • intrafraction motion management
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2
Q

What is SBRT?

A
  • 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
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3
Q

What is SABR?

A
  • 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
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4
Q

What is SRT?

A
  • stereotactic radiotherapy
  • for large cranial lesions not suited for SRS (e.g. post operative cavities)
  • 2 to 5 #
  • lower BED then SRS
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5
Q

What is SRS?

A
  • sterotacitc radiosurgery
  • historically intracranial but can be extracranial
  • single fraction
  • 12 to 90+ Gy per fraction
  • can use gamma, cyber or linac
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6
Q

What does conventional dose fractionation allow?

A
  • 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
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7
Q

What does SBRT dose fractionation do?

A
  • less dose to normal tissue irradiated
  • anti-tumour effects not predicted by classic radiobiology
  • smaller PTV margins and motion management.
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8
Q

What is the patient performance criteria for SBRT?

A
  • performance status 0-2
  • life expectancy >6months (>3 months for liver)
  • low metastatic burden (>5 mets, >5cm diameter)
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9
Q

What are the contra-indications for SBRT?

A
  • 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
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10
Q

What are the planning principles?

A
  • image fusion
  • increased no. beams
  • non-coplanar
  • small to no margin for beam penumbra
  • highly conformal
  • inhomogenous dose distribution
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11
Q

What are the simulation considerations?

A
  • increased immobilisation
  • 4DCT
  • breath hold
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12
Q

What body areas move?

A
  • skeletal/muscle: stabilisation
  • respiratory (lungs, ribs, abdomen): 4DCT, breath hold or gating
  • cardiac: remains
  • peristalsis: compression
  • bladder and bowel: preparation or catherisation
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13
Q

What are the sources of error (non-patient)?

A
  • 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
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14
Q

What lung tumours are considered for SBRT?

A
  • inoperable
  • central tumour
  • > 5cm diamter
  • no tissue diagnosis
  • T3 tumour with chest wall invasion
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15
Q

What is the interfraction interval?

A
  • 40 hrs
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16
Q

What is dose fractionation for lung?

A
  • ITV is >1.5cm from ribs: 54Gy in 3#

- ITV is <1.5cm from ribs: 48Gy in 4#

17
Q

Purpose of test runs/dry runs

A
  • 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)
18
Q

Dosimetry

A
  • 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
19
Q

Prescriptions and dose (conventional vs stereo)

A

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
20
Q

Multiple lesion plans

A
  • if it cant fit in a 10cm radius (need to use separate isocentres - why? can’t correct rotationally with 1 isocentre)
21
Q

R50

A

Ratio of volume covered by the isodose representing 50% of the prescription dose to the volume of the PTV

22
Q

Gradient index

A

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

23
Q

When is the R50 and gradient index used?

A

Useful for targets completely surrounded by OAR (e.g lung and brain) where isotropic low dose is desired

24
Q

D2cm

A

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

25
Q

Hexapod

Common issues

A 
Optimal distance = 30-50cm
Issues
- arm position
- patient height
- patient BMI
- indexing on vacbags
26
Q

Organ motion

A
  1. Skeletal/muscular (i.e voluntary)
  2. Respiratory motion - evaluate with 4DCT, manage with compression, breath hold or gating
  3. Cardiac motion
  4. Peristalsis - managed with compression
  5. Bladder and bowel - filling and empty (enemas, medications and catheterisation)
27
Q

Motion management strategies

A
  • breath hold - DIBH/EEBH
  • elekta bodyfix
  • compression belt
  • compression plate
  • gating
28
Q

Sources of positional error

A
  1. resolution of imaging (affects size and appearance of structures)
  2. accuracy of image registration
  3. target delination
  4. mechanical isocentre
  5. isocentre of radiation/treatment isocentre
  6. resolution of couch positioning
  7. resolution of infrared camera for movement verification
29
Q

Interplay effect and VMAT lung

A

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
30
Q

Requirements for IGRT

A
  • 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

CSB094 (2 decks)

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