SABR treatment planning

Question 1

What is SABR?

Answer 1

Stereotactic ablative radiotherapy
High dose/fraction
Low number of fractions
Small volumes
Good immobilisation
Rigorous image guidance
Steep dose gradients

Question 2

What are the treatment options for lung cancers?

Answer 2

Surgical resection - 5y OS 60-70% - not everyone is suitable
Conventional RT - 5yr OS 10-30% - dose can’t be escalated without toxcicity
SABR with BED>100Gy - 5yr OS 40%

Question 3

Is conventional radiobiology applicable for SABR?

Answer 3

We don’t know, some argue LQ model has limitations with such high treatment times and new biology could have a role

Question 4

What do the UK SABR consortium guidelines include?

Answer 4

QA standards
Literature review
Patient selection
Immobilisation and image acquisition
Voluming and treatment planning - recommendations of OAR, dose prescription and OAR constraints
Treatment delivery
Clinical follow-up

Question 5

What are the patient selection criteria for lung SABR?

Answer 5

Confirmed NSCLC - positive histology, PET, or growth in sequential CTs
Clinical stage T1, T2(<5cm), T3(<5cm)
Not suitable for surgery - co-morbidities, inoperable, or patient choice
WHO performance status of 0-2
Lesion outside ‘no fly zone’ - 2cm from proximal bronchial tree
18+
Respiratory motion <1cm

Question 6

How are the target volumes defined in SABR?

Answer 6

GTV - radiologically visible tumour in lung - contoured on lung windows, using PET info
ITV - tumour volume obtained from 4D scan - usually MIP with OARs done on AvIP
PTV - ITV + 5mm - can be different depending on immobilisation

Question 7

Who checks the VOIs?

Answer 7

2 consultant clinical oncologists, checked by a consultant radiologist

Question 8

How can inter-observer variations be improved?

Answer 8

Consistent windowing
Standard contouring guidelines
Additional imaging info
Training
Peer review
Audit

Question 9

What are the fractionation regimes and when are they used? What are the BEDs?

Answer 9

Standard - 54Gy/3# - 154Gy
Conservative - 55Gy/5# - when PTV contacts chest wall - 115Gy
Very conservative - 60Gy/8# - when OAR doses can’t be met - 108Gy

Question 10

How long should the inter-fraction interval be?

Answer 10

Between 40hrs and 4 days

Question 11

What are the characteristics of SABR dose distributions?

Answer 11

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

Question 12

What are the UK recommendations for target dose constraints?

Answer 12

95% PTV gets 100% prescribed dose
99% PTV gets 90% prescribed dose
Dmax is between 110-140%

Question 13

How are the target dose constraints achieved in conventional and VMAT planning?

Answer 13

Conventional - small MLC margins, prescribe to encompassing isodose usually 80%
VMAT - prescribe dose to isodose covering 95% of volume and allow higher max doses than conventional IMRT plan

Question 14

What are the standard treatment techniques for SABR?

Answer 14

Use conformal or VMAT - VMAT quicker and more conformal but has greater area receiving low dose - can have interplay
Non-coplanar beams to spare OARs
FFF beam to speed up delivery
Can put isocentre at centre of patient to simplify imaging

Question 15

Which data sets should be used for the dose calculation?

Answer 15

Doesn’t have to be MIP, can be AvIP (especially if treating in free breathing)
Can plan on a representative phase of 4D, or max inhale/exhale

Question 16

If planning on the AvIP, where does the dose go?

Answer 16

The dose follows the tumour

Question 17

Why is interplay a bigger issue for SABR?

Answer 17

It won’t blur out over time if sub-field and beamlets under- or over-dose parts of the target

Question 18

What dose calculation algorithm should be used?

Answer 18

Type B or Monte-Carlo

Question 19

What beam energy should be used?

Answer 19

6MV

Question 20

What does adaptive RT include?

Answer 20

Real time imaging to track the position of tumours

Question 21

Why could adaptive RT be useful in SABR?

Answer 21

The morphology and position of the tumour can change during treatment due to tumour response and weight loss

Question 22

What follow up can be done for SABR?

Answer 22

CT scans at regular intervals looking for disease and fibrosis - can look worse before getting better

Question 23

What other sites are being trialed for SABR?

Answer 23

Pancreatic
Primary lung + mets
NSCLC
Advanced biliary track
Oligomets from breast/lung/prostate
Early prostate 
Metastaic meanoma with pembrolizumab
Advanced NSCLC oligoprogressive disease

Question 24

What requirements were placed on the centres offering CtE commissioning for treatment of oligometastatic disease, pelvis and spine, and hepatocellular carcinoma?

Answer 24

Process documents
Outlining QA
Planning QA
Dosimetry audit

Question 25

What are the aims of SABR for oligometastises?

Answer 25

Achieve local control - prevent clinical squelae of disease progression at that site
Improve disease free survival - defer/delay systemic therapy and maximise QoL
Improve overall survival

Question 26

What sites is SABR of oligomets commissioned for?

Answer 26

Lung
Liver
Adrenal
Lymph nodes
Bone
Spine

Question 27

What are the patient criteria for oligomets?

Answer 27

Metastatic Carcinoma with proven primary
1-3 sites of metastatic disease (max 2 sites for spine)
Max size of lesion 6cm (5cm for lung or liver)
Disease free interval >6m
Life expectancy >6m
Performance status ≤2
Discussed at SABR MDT and with disease site specific CCO
Consent to follow up + data collection for ≥2y

Question 28

Are the doses constant for oligomet therapy?

Answer 28

No - vary between sites, can be lowered if mandatory OAR constraints can’t be met

Question 29

Why may 2 isocentres be used?

Answer 29

2 PTVs that can move in relation to each other

Question 30

How can tumours in the liver be visualised?

Answer 30

Contrast