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Radiotherapy Year 3 > SRS > Flashcards

SRS Flashcards

1
Q

What is stereotactic radiotherapy characterised by?

A

Low number of fractions
High dose per fraction
Small volumes
Good immobilisation
Rigorous image guidance

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2
Q

What is the ICRU91 definition of stereotactic RT?

A

Stereotactic localization techniques combined with delivery of multiple small photon fields in a few high dose fractions leading to a highly conformal dose delivery with steep dose gradients

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3
Q

What are clinical indications of SRS?

A

Metastases
Benign tumours
Vascular lesions
Functional treatments

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4
Q

What are SRS/SRT/fSRT definitions?

A

SRS = single fraction
SRT = 2-5 fractions
fSRT = fractionated SRT = conventional fractionation (this would have reduced PTV margins due to improved accuracy of immobilisation, localisation and treatment delivery)

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5
Q

What is difference between frame - based v frameless?

A

Frame based: stereotactic system of external co ordinates used for localisation and positioning

v

Anatomy and IGRT in frameless

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6
Q

What is difference between invasive and non invasive frames?

A

Invasive: patient rigidly fixed to stereotactic system using invasive methods. Frame placed under local anaesthetic. Time pressure, same day treatment. Can still have sagging. High accuracy.

Non invasive: ‘patient friendly’ immobilisation which can be used for multiple fractions. Less time pressure

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7
Q

What is invasive frame based system suitable for?

A

Only really suitable for SRS because imaging, planing, QA and delivery must be done in a single day while patient stays in frame. Really need non invastive fixation for SRT

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8
Q

What platforms can be used for SRS delivery?

A

Gammaknife
Cyberknife
Linac (with additional technology)

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9
Q

What source does gammaknife use and how?

A

Co-60
Principle gamma energies of 1.17 and 1.33 MeV

Uses hundreds of small sources with many beamlets converging on the isocentre to produce spherical dose distributions with sharp fall off

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10
Q

How did early gammaknife units work?

A

Interchangeable helmets with small collimators for each source (4,8,14,18 mm)
201 sources
Produced spherical dose distribution at isocentre

Patients in rigid head frame and imaged in this frame

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11
Q

How are GK treatments shaped to tumour?

A

Sphere packing: varying number and position of isocentres, size of collimator, treatment time

Small spherical lesions can be treated with single shot

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12
Q

Gammaknife Perfexion

A

192 sources focussed on isocentre with circular collimators built in (4,8,16mm)
Sources in 8 sectors which can use different collimator sizes or be blocked to produce non spherical shots

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13
Q

Gammaknife icon

A

Comes with IGRT (CBCT)
Possibility of frameless techniques for fractionated treatments

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14
Q

What dose algorithms are used in GK?

A

Historically used TMR 10, factor based algorithm assuming patient head is water
Traditionally planned with MRI for targets
New convolution algorithm which takes density variations into account, uses CT
Need to alter prescription doses

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15
Q

How does cyberknife work?

A

Compact 6MV FFF linac mounted on robotic arm which can move around patient to treat from a range of angles, non-coplanar, non-isocentric directions. Stereotactic radiographic localisation of bony anatomy or implanted fiducials to track patient motion.

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16
Q

How are cyberknife beams shaped?

A

Newer models: MLC, 2 banks of 26 leaves
Previous models: iris variable collimator 5-60 mm. 12 fixed circular collimators

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17
Q

What imaging and tracking options are available with cyberknife?

A

Orthogonal x-ray imaging with automatic tracing of fiducials, landmarks, or tumour
(x-ray sources in ceiling and detectors in floor)
6D skull tracking
Xsight spine tracking
Xsight lung tacking
Reference images are DRR
Synchrony respiratory tracking: target tracked throughout delivery and compensates for motion. Uses set of LEDs on patient body

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18
Q

What is ZAP X?

A

Gyroscopic radiosurgery platform
Linac based delivery, 2.7MV linac mounted on gantry which rotates around isocentre. Fully self shielded

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19
Q

What are cone based linac techniques?

A

Early linac based SRS used cones of different diameters to collimate beam and treat with non-coplanar arcs.
Results in spherical dose distribution, ideal for small sphererical lesions

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20
Q

What are the disadvantages of cone based linac techniques?

A

Need multiple isocentres for larger or more irregular lesions, high dose inhomogeneity and long treatment times

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21
Q

What techniques are used in MLC based linac treatments?

A

Fixed conformal fields (forward planned)
Dynamic conformal arcs (forward or inverse, conforms to target as gantry rotates, good for small and uniform targets)
VMAT (inverse, vary shape, intensity, dose rate, gantry speed, better suited to larger or more irregular targets)

Can use small leaf MLC or tertiary collimator

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22
Q

What do we need to consider in collimator angle and MLC leaves in VMAT?

A

Want to reduce bridging dose between mets. Minimise sharing of leaves

Smaller leaves in centre of MLC improve conformity, although difference will be smaller for VMAT than DCA

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23
Q

What are advantages of FFF use?

A

Increases dose rate, particularly useful in high fraction doses, can have significantly shorter treatments
Reduce leakage and peripheral dose
For small fields, beam pretty flat with FFF

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24
Q

What are advantages of GK?

A

Very conformal treatments
Sharpest dose gradients
Lowest reported uncertainties
No couch or gantry movements
Can treat very small lesions

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25
Q

What are disadvantages of GK?

A

Long treatment times
Radioactive sources need to be replaced and security implications
Intracranial targets only
Have to use multiple isocentres for multiple lesions

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26
Q

What are advantages of CK?

A

Non-isocentric – very conformal treatments
Excellent motion management
Can treat extracranial targets

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27
Q

What are disadvantages of CK?

A

Long treatment times
Can’t treat through couch

28
Q

What are advantages of stereotactic linac?

A

Can be used for other techniques
Fast treatment times, especially with FFF
Lots of imaging options
Some motion management possible
Can treat extracranial targets
Can treat multiple lesions with single isocentre

29
Q

What are disadvantages of stereotactic linacs?

A

Limited non-coplanar treatments
Minimum treatment size limited by MLC leaf width and dosimetric accuracy

30
Q

What are specific challenges faced by SRS?

A

Short treatment courses and high doses (less chance of identifying and correcting error)
Enhanced reliance on imaging
Technical challenges associated with commissioning
Special resources for staff and training
Workflow differences

31
Q

What are technical challenges associated with small field dosimetry?

A

Complex dosimetry with many sources of error
(complicated by loss of LCPE, source occlusion resulting in penumbra overlap and reduction in output, strong dependence on size and construction of detector)
Crucial to use correct dosimetry equipment to avoid miscalibration

32
Q

How can significant dosimetry errors be avoided?

A

Use multiple layers of redundancy (more than one different type of detector)
Peer review of data, compare to others
Ask for manufacturers reference data
Independent end to end review

33
Q

What enhanced QA must be done?

A

Starshot tests to check rotational accuracy of gantry, collimator, couch
Winston-Lutz test tests accuracy of radiation isocentre

34
Q

How is Winston Lutz test done?

A

Set up ball bearing using lasers or light field and irradiate with small field, projection of BB should be centred in field at all angles
Ideally < 0.5 mm

35
Q

What other uncertainties are in the SRS pathway?

A

Contouring of tumour/OAR, image registration
Prescription dose and isodose line, conversion to BED
Dose calculation, treatment planning
Patinet setup, tumour setup, immobilisation

36
Q

What is the purpose of E2E testing and how it done?

A

Estimate overall combined accuracy for given treatment process.
Usually done by following whole patient pathway with phantom, start simple and get more complex

37
Q

What are particular challenges of PSQA in SRS?

A

Traditionally done with ion chamber and film
Mets can be tiny, small field dosimetry is tricky
Multiple lesions: unless you can move ion chamber in phantom, need new QA plan for each met
Film measurement is time consuming
If using array like Delta4 need to consider size of detector and resolution. Might need to collapse couch angles (can’t irradiate electronics so deliver as if fields are coplanar)

38
Q

How is localisation carried out?

A

CT localisation which detects fiducial rods (or immobilisaiton base plate in case of brainlab)
Need full skull scan to use vertex fields
Angiography localisation for AVMs brings 2D and 3D data into same coordinate system

39
Q

Why is multi modality imaging used?

A

CT is distortion free baseline, other modalities fused to CT. Also used for dose calculation
Better contrast etc in MR
May want to use angiography for info it gives

40
Q

What are future delineation options?

A

Fiber tracking: diffusion tensor imaging MRI, selects fiber bundles. Converts to 3D objects for planning
BOLD
Contrast clearence analysis: MRI based method for high resolution depiction of contrast clearence and accumulation, shows high and low vascular activity

41
Q

What are some typical dose prescriptions used at NCCC?

A

There is no consensus and wide variation

NCC:
< 7cc, 21 Gy single fraction
7-13 cc, 18 Gy
13-20 cc, 15 Gy (or SRT, 24/3)

For mets in brainstem, 25/5

42
Q

How is dose typically prescribed?

A

To isodose surface (typically 40-90%) which encompasses target volume, GK 50%, linac 80%

18Gy to 50% -> 100% = 36Gy

43
Q

What are some plan quality metrics

A

Paddick Conformity index
Gradient index
Normal brain V12Gy (cc)
Bridging dose

44
Q

Where can we get OAR dose limits?

A

UK SABR consortium has guidance on OAR limits, previous limits can’t be used because they are not applicable to hypo fractionated treatments

45
Q

How small are PTV margins typically?

A

0 mm for GK/CK, 1mm forlinac

45
Q

What are the treatment aims and what impacts them?

A

High conformity and rapid dose fall off

Influenced by number of fields and placement or using fall off as objective function. Might relax dose homogeneity requirements to allow increased dose conformity and dose fall off.

46
Q

How can plans be more efficient?

A

Less couch angles
For multiple mets, single isocentre techniques are more efficient than a separate isocentre for each one

47
Q

How does hypofractionation with dose escalation impact radiobiology?

A

Limitations of LQ model in SRS might suggest existence of new cell death mechanisms related to stem cells, vascular damage, bystander effects adn immune mediated effects
Because traditional understanding of radiobiology (cell killing due to DNA damage) can’t explain results of hypofractionated treatments

48
Q

How precise should target verification and localisation be in SRS?

A

Sub mm precision

49
Q

What are the four tiers of SRS/SRT?

A

1: neuro oncology
2: skull base
3: vascular (AVM)
4: other rare indications (trigeminal neuralgia)

Tiers 3 and 4 are only delivered at two centres

50
Q

What are some typical brain met fractionations?

A

18-24 Gy in 1#
24-27 Gy in 3#
In brainstem, 25 in 5

Depends on size and location

51
Q

What limits are put on brain mets?

A

Some centres have limits on number of mets (more common in linac than GK)
Volume < 20 cc (complications like radionecrosis likelihood increases with brain volume irradiated)

52
Q

What is the rationale for treating mets with SRS?

A

Gives similar local control than whole brain RT but with reduced neurotoxicity
No difference in overall survival for patients with 5-10 mets and 2-4: safe to treat more than 4 mets

53
Q

What are issues with multiple isocentre multiple target techniques?

A

Long treatments, even with FFF, treatment time scales with number of mets
Time consuming to check, plan, QA

54
Q

What is the advantage of single isocentre multiple target techniques?

A

Treat simultaneously in single plan: much quicker
May reduce intra fraction motion due to shorter treatment times

55
Q

What are planning considerations for SIMT?

A

Treating off axis means rotational set up errors are critical (translational have same impact on SIMT and MIMT, rotational much worse for SIMT) - errors get worse further off axis
Minimising normal brain dose
Minimise bridging dose between lesions
How are independent checks performed?

56
Q

What is a typical fractionation for a meningioma?

A

14-15 Gy in 1#
Benign rain tumour

57
Q

Typical dose for acoustic neuroma

A

12-13Gy in 1#
benign tumour

58
Q

Typical dose for arteriovenous malformation

A

15-19Gy in 1#
Congenital abnormality, entanglement of blood vessels, goal is complete removal or obliteration

59
Q

Typical dose for trigeminal neuralgia

A

70-90Gy 1#

60
Q

Why are MLC linac methods preferable to cone based methods?

A

MLC methods will have higher dose inhomogeneity and longer treatment times in lesions that are not regular

Cones typically limited to use in very small lesions

61
Q

Give four indications for SRS. Why is SRS used in these and what are some typical dose fractionations?

A

AVMs. Microsurgical resection usually used but cannot be used in some regions of the brain. 15-19 Gy in 1#

Benign tumour. Effective alternative to surgery with high control rates and hearing preservation, low risk of damage to nerves 12-13 Gy in 1#

Trigeminal neuralgia. If medication fails, least invasive of alternative procedures: pain relief with minimal side effects. 70-90Gy in 1#.

Mets. Similarly effective to WBRT but reduces toxicity. Fractionation depends on size/number.

62
Q

What is the patient pathway?

A

Immobilisation, localisation (multi modality imaging and image fusion), delineation, treatment planning, setup and verification, treatment

63
Q

When might you want to use SRT or fSRT?

A

SRT: instead of SRS if you want to reduce toxicity risk from OAR

fSRT: if you just want to use the minimised margins that you can use with stereotactic techniques

64
Q

What is frame used for in frame based techniques?

A

Frame used for localisation, set up, immobilisation and during treatment

65
Q

What are advantages of frame based vs frameless?

A

Frame based
+ very high accuracy
− Invasive
− time pressure, only suitable for SRS
− can still sag

Frameless
+ not invasive
+ less time pressure
− not as accurate as frame based (but approaching it)

66
Q

Why is SRS an important option?

A

An alternative to surgery for patients who are inoperable or have targets that are hard to reach or close to critical organs.

Radiotherapy Year 3 (12 decks)

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