SBRT/SABR Flashcards
What is SBRT used for?
For dose escalation Extracranial (spine/prostate) 1-5#, up to 8 >8Gy High precision, highly conformal, intra fraction motion management essential
What is SABR used for?
For ablation
Extra cranial (Liver, lung, renal)
>8Gy
High precision, highly conformal, intra fraction motion management essential
What is SRS used for?
Single fraction
12 -90 Gy +
192-201 Co sources
GK, CK, Linac
What is Stereotactic Radiotherapy?
Cranial
Larger lesions not suited for SRS
2-5 fractions
Lower BEDs than SRS
What is the dose fractionation for conventional RT?
1.8-2.4 per day, 15-40 fractions over 3-8 weeks cell repair repopulation after RT Re-distribution in cell cycle re oxygenation radio sensitivity
What is the dose fractionation SBRT?
> 8Gy per day
1-5 fractions
1-2 weeks
-Less tissue irradiated due to IGRT and dose delivery
-Anti-tumour effectss may not be due to radiobiology
-Tumours may not be hypoxic
-NSCLC results are impressive
What is the Abscopal effect?
reaction of cells within an organism that has not been directly exposed and is shown by tumour progression of non irradiated tumours
Who do we treat?
Primary and secondary disease (oligometastatic state)
< 5cm max dimension
Non-malignant conditions (SC, AVM, meningioma)
Applied to tumours considered radioresistant( renal cell, melanoma)
For ablation (lung, liver)
Dose escalation (spine, prostate)
What is oligometastasis?
intermediate state between purely localised state and widespread metastasis
What is the patient performance criteria?
Performance status 0-2
Life expectancy >6 mths (3mths for liver)
Low metastatic burden (<5 mets, <5cm in dimension)
What are the contraindications for SBRT?
Prior RT
- Unable to lie flat for prolonged period
- Cannot receive chemo 1-4 weeks pre or post SBRT
- Severe connective tissue disease or scleroderma
- Claustrophobia
- Mental status prohibitive of patient compliance
Imaging types?
delineate targets and critical normal tissues
MRI, PET, CT, 4DCT, inhale, exhale, FB, contrast, MIP, Ave IP
What is Av IP (Average Intensity Profile)
A re-constructed data set which shows the average value from each of the 4D bins for each voxel • Averages motion effects • Use for dose calculation
What is the Maximum Intensity Profile (MIP)
A re-constructed data set which shows the maximum value from each of the 4D bins for each voxel
• Shows maximum range of motion of a tumour
What are the limitations of Image fusion?
Imaging artefacts- motion, metal implants
-Image distortion- for PET and MR
-Tumour delineation
Registration errors-largest in treatment process
What are the positioning considerations for SBRT?
- Comfort and reproducibility
- Enable delivery of technique (access to target, minimise dose to tissue, restrictions on beam angle, arm, head position, accomodate equipment)
What are the patient considerations SBRT?
Comprehension/understanding
Pain control/management
Mobility/ comorbidities
What are test runs good for?
- Small rotational corrections can require large translational moves
- Check breath hold reproducibility
- Check tumour excursion
- Check visibility of lesion and surronding anatomy on CBCT
Dosimetry considerations?
Technique depends on site
Always conformal
Inhomogenous dose distributions (prostate an exception)
Increased number of beams and non coplanar beams to create sharp isotopic dose fall off
Small or no beam margins
3DCRT/IMRT/VMAT/Conformal ARC/FFF
Beam setup for conventional RT?
PTV covered by 95% isodose
• Dose range 95 – 105% • Falloff outside PTV 95% - 0 • Up to 10mm margin depending on number of fields
• Homogeneous distributio
Beam setup for Stereo?
PTV covered by 100% isodose
• Acceptable max dose is prescribed, covering isodose is a % of this max dose
• Little to no margin on PTV • Fall off outside PTV 60 - 80% - 0
• Heterogeneous distribution
What are the two SBRT prescriptions?
Option 1 – Prescribe 28 Gy at 80% isodose – Maximum = 100% = 35Gy
Option 2 – Prescribe 28 Gy with an acceptable maximum of 125% – Maximum = 28 x 125% = 35Gy – Then 35 Gy = 100% and 28 Gy = 80%
What is the isocentre placement?
Centre of PTV
If near a critical structure of centre place the isocentre at critical structure.
- Can use one isocentre for multiple lesions but has to be within 5cm and does it have same translational shifts
- Must be same dose/fractionation
What is the R50/ Gradient Index
he volume covered by the isodose representing 50%/ half of the prescription dose to the volume of the PTV
What is D2cm?
The dose at any point 2cm from the PTV is recorded and is expected to meet set criteria
Delivery of SBRT
GK, CK, Linac <5mm MLC
kV imaging
Ability to make rotational corrections
IGRT-kV, CBCT, 4DCBCT (organ motion)
Considerations for Hexapod?
Optimal frame to isocentre distance 30 – 50 cm
• Difficulties
Arm position
Patient height
Patient BMI
Indexing on vac bags
• Vectors for isocentre correction are 1 m long
When do we use rotational corrections?
Spine
• For intracranial lesions, particularly if located at the base of the skull
• For lesions abutting, overlapping with or within 2 cm of critical normal tissue structures • For treating multiple lesions
When don’t we use rotational corrections?
Round targets that can be corrected for with translations only e.g. kidney
• Lungs (often round and - causes gantry collisions!)
What are movement issues?
Skeletal/muscular (i.e. the patient) – mitigate through stabilisation
- Respiratory motion – evaluate with 4DCT, manage with compression, breath hold or gating
- Cardiac motion – remains
- Peristalsis – manage with compression 5. Bladder and bowel filling and emptying – manage with protocols, enemas, medications, catheterisation
What does respiratory dose affect?
Affects tumour sites in Lung, ribs, Abdomen (liver, kidney) and Pelvis
• Respiratory motion can differ from day to day • Tumours grow and shrink • Lung dose is of high importance, more so if patients have/had chemotherapy
• Primary tool for evaluation is 4DCT
What are the sources of Positional errors?
Resolution of imaging
Accuracy of image fusion
Accuracy of target delineation
• Accuracy of mechanical isocentre (1mm radius at PAH) • Accuracy of radiation/treatment isocentre (0.3mm/0.1°)
• Resolution of couch positioning
• Resolution of infrared camera for movement verification (within 0.7mm of MV iso)
Sources of errors -Patient factors?
Position
• Immobilisation – choice of equipment
• Organ motion – Respiration – Cardiac function – Peristaltic activity – Organ filling/emptying
• Correct use of immobilisation equipment
What does the level of accuracy depend on?
Dependant on sources of error and how they are managed
• Dependant on clinical protocol – Fractionation – Margins
– Pathology (anticipated response)
– Patient condition
– Target size
– Proximity to critical normal tissue
Who are the patients reccommended for lung SBRT/SABR
Inoperable patients • Central tumours (fractionate!) • > 5cm diameter • No tissue diagnosis • T3 tumour with chest wall invasion • Salvage after prior RT Synchronous or mutli-focal tumours • Post pneumonectomy with new primary in remaining lung
Dose for SBRT lung
Depends on location of lesion • GTV/ITV ≥ 1.5cm from ribs - 54Gy in 3
• GTV/ITV ≤ 1.5 cm from ribs – 48 Gyin 4
• Lesions approaching central zone or larger lesions – 50Gy in 5
• Small favourable tumours – 26 – 33 Gyin 1 (SRS)
• Covering isodose59 – 80% (max 125% - 170%)
• Inter fraction interval > 40hrs
Target delineation goals for lung
D100 ITV ≥ PD
• D95 PTV ≥ PD
• D99 PTV ≥ 90% PD
• ITV to be marked to encompass full range of tumour excursion
• If using DIBH – do 2 CTs to check variability between breath hold
What are CT acquisition recommendations?
Maximum 2mm slices Scan limits for DVH Scan limits for NCP 10-15cm sup/inf more 4DCT check for artefacts Contrast Multiple scans DIBH-coaching
4DCT considerations?
-create ITV with full range of tumour motion
-check 4D for errors or mismatches in tumour trajectory
RO- approval needed
-Use symmetry scan (4DCBCT) to verify ITV
-Artefacts common with 4DCT
Beam choice for treatment planning?
3DCRT (> 9 fields) or Conformal Arc (≥ 3 arcs)
Delivery without FFF ~ 450 MU per minute (Elekta AxesseBeam Modulator) FFF ~ 1400 MU per minute for 6 MV
VMAT/IMRT – not ideal – due to leaf interplay –
conformal arc ideal – speed with rotational delivery but no leaf interplay
What is MLC Leaf Interplay?
Is the interaction between the movement of the MLC shapes for segments in an IMRT beam or VMAT arc and the motion of a tumour with the respiration cycle
Planning considerations for 3DCRT?
NCP beams No opposing beams No overlapping Min 5 NCP beams at 3 couch angles 8-10 beams Need significant separation to minimise overlap Clearance Dose fractionation Future treatment
What are the IGRT considerations
Pre treatment CBCT
RO present
Large clip box
Pre-fraction CBCT translations > 1mm are actioned*
Pre-fraction CBCT rotations < 3˚ are accepted
IGRT challenges?
Rotations > 3˚, < 5˚ may be treated or re-positioned in consultation with RO – Rotations > 5˚ are re-positioned
Spine SBRT/SABR considerations?
Oligometastatic state
Steep dose gradients around cord
1-2 vertebrae
(18-20 #1. SC 11-12) (24-28 #2 SC 14-15) (24-30 #3 SC 17)
Immobilisation for Spine C1 to T4?
Vac loc bag arms down knee bolster hexapod frame Mask <1mm 1 degree
Immobilisation for Spine (T5-Sacrum)
T shaped bags (support for arms)
Full length bags (pelvis/legs)
Beam choice for Spine
VMAT is technique of choice
IMRT-9 fields
Collimate to shield cord
SBRT/SABR for liver?
primary secondary disease
up to 5 lesions if mets
up to 3 for HCC
End exhale breath hold
Dose for SBRT/SABR Liver?
48 in 3 or 60 in 6
BED 110-120 Gy
Lower doses 24-54 Gy in 3-6
Patient Immobilisation for Liver?
Vacbag
Abdominal compression if tumour excursion >1-1.5cm
CT acquisiton for Liver
2mm slices Planning CT free breathig CT contrast (arterial H-CC) Mets- Venous phase 4DCT Oral contrast
Prostate SBRT/SABR
35-40 Gy in 5
As a boost 46 in 23 [ls 19-21 in 2
Uses fiducial markers for localisation
