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Radiotherapy Year 3 > SABR > Flashcards

SABR Flashcards

1
Q

What does SABR stand for and refer to?

A

Stereotactic Ablative Body Radiotherapy
Precise irradiation of an image-defined extra-cranial lesion with the use of high radiation dose in a small number of fractions

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2
Q

What does stereotactic mean

A

Referring to precision positioning in 3D space

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3
Q

What does ablation mean

A

A procedure in which a tissue or body part is removed or destroyed by surgery, radiation, or photocoagulation

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4
Q

What is SRT?

A

Stereotactic radiosurgery
Term ICRU uses to encompass SRS, SBRT, SABR

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5
Q

What is SABR characterised by?

A

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

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6
Q

How does a SABR plan differ to a 3DCRT plan?

A

Small volumes
High conformity
Inhomogeneous dose
Steep dose fall-off

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7
Q

How does SABR DVH curve differ to standard DVH curve?

A

SABR plans deliver a higher dose to the ITV because of the non-homogeneous dose distribution.
Also delivers to higher dose because the doses are prescribed to 95% and higher doses are acceptable.

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8
Q

What are the potential uses of SABR?

A

Primary cancers (particularly ones considered radio-resistant or where surgery is not possible)
Metastatic disease
Locally recurrent disease

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9
Q

Use of SABR for primary RCC

A

Can’t give radical treatment with cRT without causing long term damage, SABR could be better. Surgical resection is SOC but this is not always an option.
Generally accpeted that EQD2 of > 72Gy is needed

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10
Q

What is pseudo progression?

A

Imaging straight after SABR, can see initial increase in size due to treatment induced inflammation and oedema, and can be confused with progression. Should image after 3 months.

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11
Q

What is included in SABR consortium guidelines?

A

General QA standards
Site specific sections including:
lit review
patient selection
RT guidance (on delineation, prescription, OAR constraints)

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12
Q

Use of SABR for metastatic renal cancer

A

Standard of care is systemic therapy, IO and targeted therapies like TKI showing improval
SABR promising as option for mRCC in place of or in combination with SACT

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13
Q

What is the abscopal effect?

A

When a local therapy such as RT shrinks not only the targeted tumour but also untreated tumours elsewhere in the body. Understanding of the biological mechanisms are incomplete.

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14
Q

SABR for recurrence

A

SACT is SOC for recurrent renal cancer. Local therapy may help delay disease progression but most RT has been delivered as IORT. Little evidence currently, but could play a role in salvage of recurrence with low toxicity.

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15
Q

What are the immobilisation considerations in SABR?

A

Need to balance stability and reproducibility with patient comfort. Doses are higher so treatment length can be longer and may need more rigorous imaging before and during treatment.

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16
Q

What does the SABR consortium say to specifically consider in dose calculation in SABR?

A

Build up effect and skin folds

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17
Q

What are the four main methods of motion management?

A

Incorporate motion into plans (4DCT)
Reduce or control motion (abdo compression, ABC, DIBH)
Gating
Track motion

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18
Q

What are pre-treatment imaging consideration of SABR?

A

Should contain whole OARs where dose limits are based on organ
Fine slice CTs give better delineation for smaller targets
Contrast may be important for delineation but need to consider timing and HU

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19
Q

How do we improve inter-observer variation?

A

Consistent windowing
Standard contouring guidelines
Additional imaging info such as PET
Training
Peer review/independent checks
Audit

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20
Q

What will determine target definitions?

A

Motion management strategy - 4DCT will have large ITV, gated will have smaller volumes etc

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21
Q

What considerations must be made in OAR contouring in SABR?

A

Amount of organ delineated has significant impact on dose statistics - for consistency should use published guidelines

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22
Q

How can we plan SABR?

A

Using either static conformal beams or VMAT

23
Q

What are the benefits and disadvantages of VMAT/inverse planning?

A

Benefit is quick treatment planning and delivery, may allow greater conformity and OAR sparing. Can consider if you want traditional peaked distribution or more homogeneous plan.
Disadvantage is greater area of patient receiving a low dose

24
Q

Why do you want to avoid very homogeneous plans in SABR in general?

A

Increased modulation - interplay effect.
Dosimetric consequences of treating moving targets with dynamic techniques, can under or overdose target

25
Q

Why is interplay more of a concern in SABR?

A

There are less fractions than in conventional radiotherapy and so the any effects won’t blur out over the entire treatment

26
Q

Where is dose prescription information?

A

RCR has published guidance on dose fractionation, which includes SABR regimes

27
Q

Why is uniform dose less of a concern in SABR?

A

Aiming for ablative dose

28
Q

What are SABR dose distributions characterised by?

A

Highly conformal dose distributions
Peaked dose distribution with high max doses
Sharp dose fall off to maximise sparing of surrounding OARs

29
Q

How does SABR consortium guidance recommend normalising dose?

A

95% of PTV receives at least 100% of dose.
For lung 99% of PTV should recieve 90% of prescription dose
Dmax should be between 110-140%

30
Q

How have conformation metrics changed?

A

R100% = Vol(100%)/Vol(PTV) –> prescription dose spillage = Vol(100%)/PTV100%
Same at 50% for modified gradient index

31
Q

How should SABR treatments be verified?

A

Using IGRT on-line imaging and correction before treatment. Imaging modality used based on location, soft tissue CBCT is standard, 4D may be necessary where movement is issue

32
Q

Lung cancer statistics

A

Third most common type of cancer
Leading cause of cancer death
NSCLC is most common type 80%, of which 20% have best chance of cure
Most common in elderly and smokers

33
Q

What is gold standard of treatment of early stage NSCLC

A

Surgical resection

34
Q

Why might patients not have surgery?

A

Co-morbidities
Technically inoperable
Unwilling to undergo surgery

35
Q

Why is conventional radiotherapy poorer than surgery?

A

Dose can’t be escalated without unacceptable toxicity. Not all patients are fit enough for cRT.

36
Q

Why is it considered that SABR shows better results than cRT?

A

Much higher BED, >100Gy by extreme hypofractionation, 54 in 3

37
Q

Why is it difficult to prove that SABR is comparable to surgery?

A

Used in very different cohorts, operable patients are more likely to see better results in the end (general fitness, no other comorbidities)

38
Q

What is inclusion criteria for lung SABR?

A

MDT diagnosis of NSCLC (positive histology, PET scan)
Stage T1M0N0 or T2(<5cm)M0N0, or chest wall invasion only T3(<5cm)M0N0
Not suitable for surgery (for any reason)
WHO performance status 0-2
Peripheral lesions outside IASLC central zone
Age 18+

39
Q

What is the central zone and why do we want to avoid it

A

2cm away from bronchial tree, heart, major vessels, oesophagus, spinal cord, brachial plexus
High toxicity has been reported when treating centrally located tumors

40
Q

What was the no-fly zone?

A

Around brachial tree

41
Q

What is the ultra central zone?

A

Within 1cm of proximal bronchial tree
Do not treat

42
Q

What are exclusion criteria for lung SABR?

A

Tumour no definable on planning CT
Previous RT within planned treatment volume
Pregnancy or lactation
Inability to give informed consent or comply with treatment requirements

43
Q

What are relative contra indication of lung SABR?

A

Target motion due to respiration > 1cm, potentially use motion management to deal with it
Presence of pulmonary fibrosis, need to consent for increased risk of significant toxicity

44
Q

What are most common volumes used in lung SABR in UK?

A

GTV is radiologically visibly tumour contoured on lung windows. Motion adapted GTV = ITV

ITV is tumour volume obtained using 4DCT

PTV is ITV plus margin (usually isotropic 5mm) but margins depend on immobilisation and verification at centre

45
Q

What are options for generating the ITV adn OARs?

A

Draw directly on MIP accounting for tumour positions
Draw on max inspiration and expiration scans
Contour on all individual phases of 4DCT and combine

OARs usually drawn on AvIP

46
Q

How are dose distributions calculated for lung SABR?

A

Don’t need to be calculated on the data set used to generate treatment volumes.
Dose distributions can be generated on AvIP/3DCT even if targets generated with 4DCT

47
Q

What are SABR fractionations?

A

54/3 when not abutting the chest wall - BED 154 Gy - standard
55 in 5 when abutting chest wall - BED 115 Gy - conservative
60 in 8 when within central zone but outside ultra central zone - BED 108 Gy - very conservative

48
Q

What is metastatic disease?

A

Cancer that has spread from the primary site to other parts of the body, usually lymph nodes, bone, spine, liver.

49
Q

What is oligometastatic disease?

A

An intermediate stage between localised primary disease and widespread metastatic disease. Limited number of mets.

50
Q

What is oligoprogressive disease?

A

A clinical concept describing progression at only a few sites of metastatis in patients with otherwise controlled widespread disease

51
Q

What are the aims of treating oligometastases?

A

Achieve local control at metastatic site and prevent disease progression
Improve disease free survival, defer or delay systemic therapy and maximise QoL
Improve OS

52
Q

What is ventricular tachycardia?

A

An unusually fast heart rhythm arising from improper electrical activity in the ventricles. ICDs can be used to pace or restore normal heart rhythm

53
Q

How does cardiac SABR work?

A

Option for patients who have no other options. Need to map electrical activity onto planning scan for treatment. Good response rate

Radiotherapy Year 3 (8 decks)

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