SABR treatment planning Flashcards
What is SABR?
Stereotactic ablative radiotherapy High dose/fraction Low number of fractions Small volumes Good immobilisation Rigorous image guidance Steep dose gradients
What are the treatment options for lung cancers?
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%
Is conventional radiobiology applicable for SABR?
We don’t know, some argue LQ model has limitations with such high treatment times and new biology could have a role
What do the UK SABR consortium guidelines include?
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
What are the patient selection criteria for lung SABR?
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
How are the target volumes defined in SABR?
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
Who checks the VOIs?
2 consultant clinical oncologists, checked by a consultant radiologist
How can inter-observer variations be improved?
Consistent windowing Standard contouring guidelines Additional imaging info Training Peer review Audit
What are the fractionation regimes and when are they used? What are the BEDs?
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
How long should the inter-fraction interval be?
Between 40hrs and 4 days
What are the characteristics of SABR dose distributions?
Highly conformal dose distributions
Peaked dose distribution with high max doses
Sharp fall off of dose to maximise sparing of OARs
What are the UK recommendations for target dose constraints?
95% PTV gets 100% prescribed dose
99% PTV gets 90% prescribed dose
Dmax is between 110-140%
How are the target dose constraints achieved in conventional and VMAT planning?
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
What are the standard treatment techniques for SABR?
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
Which data sets should be used for the dose calculation?
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
If planning on the AvIP, where does the dose go?
The dose follows the tumour
Why is interplay a bigger issue for SABR?
It won’t blur out over time if sub-field and beamlets under- or over-dose parts of the target
What dose calculation algorithm should be used?
Type B or Monte-Carlo
What beam energy should be used?
6MV
What does adaptive RT include?
Real time imaging to track the position of tumours
Why could adaptive RT be useful in SABR?
The morphology and position of the tumour can change during treatment due to tumour response and weight loss
What follow up can be done for SABR?
CT scans at regular intervals looking for disease and fibrosis - can look worse before getting better
What other sites are being trialed for SABR?
Pancreatic Primary lung + mets NSCLC Advanced biliary track Oligomets from breast/lung/prostate Early prostate Metastaic meanoma with pembrolizumab Advanced NSCLC oligoprogressive disease
What requirements were placed on the centres offering CtE commissioning for treatment of oligometastatic disease, pelvis and spine, and hepatocellular carcinoma?
Process documents
Outlining QA
Planning QA
Dosimetry audit
What are the aims of SABR for oligometastises?
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
What sites is SABR of oligomets commissioned for?
Lung Liver Adrenal Lymph nodes Bone Spine
What are the patient criteria for oligomets?
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
Are the doses constant for oligomet therapy?
No - vary between sites, can be lowered if mandatory OAR constraints can’t be met
Why may 2 isocentres be used?
2 PTVs that can move in relation to each other
How can tumours in the liver be visualised?
Contrast
