SBRT

Question 1

SBRT meaning

Answer 1

Stereotactic Body Radiation therapy
Does not involve any stereotactic treatment to the brain

Question 2

Why SBRT

Answer 2

Dose escalation - targets in close proximity to OAR
Extra cranial
Around 1-5#
More than 8Gy per fraction
Highly conformal with steep gradients

Question 3

SABR

Answer 3

Stereotactic ablative body Radiotherapy
Used for ablation - not many OARs surrounding
Extra cranial

Question 4

SRS

Answer 4

Single fraction
For brain
Gamma knife is an example of SRS

Question 5

SRT

Answer 5

Stereotactic radiotherapy
Intra cranial
For larger lesions that are not suitable for SRS
Fractionated typically 2-5#

Question 6

Radiobiology in SBRT fractionation

Answer 6

Anti-tumour effects can not be predicted by classic radiobiology
Tumours may not be hypoxia therefore no benefit from reoxygenation
Hypo-fractionated alters micro environment which leads to more death of tumour cells
Dead tumour cells released quantities of antigens which stimulated antitumour immunity

Question 7

The abscopal effect

Answer 7

Distant tumour regression after localised irradiation

Question 8

What can we treat

Answer 8

• inoperable tumours
• oligometastic state
• <5cm max dimension
• non-malignant conditions
• for ablation e.g. lung, liver
• for dose escalation e.g. spine, prostate

Question 9

Patient contraindications

Answer 9

• prior RT
• unable to lie flat
• cannot receive chemo 1-4 weeks pre or post SBRT
• severe connective tissue disease
• claustrophobia
• mental status prohibitive of patient compliance

Question 10

Average intensity profile versus maximum intensity profile

Answer 10

Maximum - sharper image
Average - more fuzzy, easier to match in treatment

Question 11

Image fusion issues

Answer 11

Image artefacts - from motion, metal implants
Image distortion - for PET and MR delineation
Registration errors - can be the largest source of error in the entire treatment process

Question 12

SBRT techniques

Answer 12

Mohawk, flipper, seatbelt

Question 13

Dosimetry for SBRT

Answer 13

Always highly conformal
Inhomogenous dose distributions
170% max dose
Dose painting techniques

Question 14

Stereo dose distribution

Answer 14

Heterogenous
Fall off outside PTV 60-80%
PTV covered by 100% isodose

Question 15

Prescription

Answer 15

Prescribe at 80% iso line
Prescribe at an acceptable maximum point dose dose (e.g. 125%)

Question 16

Plan evaluation priority

Answer 16

Check OAR goals - different OAR prescribed to conventional RT
Check PTV cover
Dose fall off beyond target

Question 17

R50

Answer 17

Ratio of the volume covered by the isodose representing 50% of the prescription dose to the volume of the PTV

Function of the size of the PTV - smaller for large PTVs
This is volumetric not geographical

Question 18

Gradient index

Answer 18

Ratio of the volume of half the prescription isodose to the volume of the prescription isodose
Differentiates plans with similar conformity but with different gradients

Question 19

D2cm

Answer 19

The dose at any point at 2cm from the PTV is recorded and is expected to meet set criteria
Geographical evaluation

Question 20

Elekta hexapod

Answer 20

Patient immobilisation equipment must fit inside it
High sensitivity optical tracking

Difficulties
- must be positioned 30-50cm distance of iso
- arm position
- patient height
- patient BMI
- Indexing on vac bags

Question 21

When to use rotational corrections

Answer 21

Spine
Pancreas
For intercranial lesions, particularly if located at the base of the skull
For lesions abutting, overlapping with or within 2cm of critical normal tissue structures
For treating multiple lesions
Anything that is not round

Question 22

Motion management strategies

Answer 22

Breath hold
Elekta body fix
Compression belt and plate
Gating

Question 23

Sources of positional error

Answer 23

Resolution of imaging
Accuracy of image fusion
Accuracy of target delineation
Accuracy of mechanical iso
Accuracy of radiation/treatment iso
Resolution of couch positioning

Question 24

Sources of error - patient factors

Answer 24

Position
Immobilisation
Organ motion - respiration, cardiac function

Question 25

Level of accuracy

Answer 25

Dependent on sources of error and how they are managed Dependent on clinical protocol (fractionation, margins, pathology)

Question 26

TPS technique

Answer 26

FFF - fast treatment Collimator angle choice VMAT - not ideal for single fraction due to leaf interplay

Question 27

MLC leaf interplay

Answer 27

Interaction between the movement of the MLC segments in VMAT or IMRT and the motion of a tumour with the respiration cycle Best practice to plan on AV IP data set for dosimetry more representative of the total resp cycle

Question 28

Spine treatment

Answer 28

For oligometastatic disease Very steep dose gradients around SC Most commonly 1 vertebrae but can be multiple PRV 1.5-3mm Larger cord structures or the cal sac will impact PTV cover

Question 29

Lung patient conditions

Answer 29

Inoperable Central (fractionate) >5cm diameter No tissue diagnosis Salvage after prior RT T3 tumour with chest wall invasion Synchronous or multi-focal tumours

Question 30

Lung techniques

Answer 30

VMAT most common - except for single fraction regimes due to leaf interplay Use FFF and DIBH

Question 31

Target delineation for lungs

Answer 31

ITV to be marked to encompass full range of tumour excursion If using DIBH - do 2 CTs to check variability in breath holds

Question 32

When an OAR dose goal conflicts with covering iso lines, what happens

Answer 32

The dose to OAR and covering isodose should run like parallel or concentric

Question 33

Liver SBRT

Answer 33

Common site of Mets For patients unsuitable for surgery Can treat up to 5 lesions if Mets Treats up to 3 lesions if HCC Must have 800cc of uninvolved liver

Question 34

liver dose

Answer 34

48gy in 3 40-50Gy in 5 Lower dose used where there is underlying liver dysfunction

Question 35

Plan techniques for liver

Answer 35

Shorter arc lengths to avoid entry dose through liver Partial arcs - slightly higher than mid range dose but lower low dose Usually co-planar VMAT to avoid increasing low dose wash in liver

Question 36

Prostate SBRT

Answer 36

Mono therapy As a boost Requires fiduciary markers for accurate localisation Space OAR needed

Question 37

Pancreas SBRT

Answer 37

Fidcucials useful Use of oral contrast to distend duodenum Critical structure. - duodenum Very similar to liver protocol

Question 38

Desired accuracy

Answer 38

1mm 1 degrees

Question 39

Patient positioning considerations

Answer 39

Arm position Comfortable and reproducible Allow good access to target Minimise dose to normal tissues Minimise restriction on beam angle choice Avoid creating build up with equipment Consider head position, position of tubing for body fix, loc bars Check whether patient comprehends and understands instructions Pain control and management Morbidity/co-morbidity

Question 40

What moves?

Answer 40

1. Skeletal/muscular (i.e. the patient) – mitigate through stabilisation 2. Respiratory motion – evaluate with 4DCT, manage with compression, breath hold or gating 3. Cardiac motion – remains 4. Peristalsis – manage with compression 5. Bladder and bowel filling and emptying – manage with protocols, enemas, medications, catheterisation