SBRT

Question 1

Features of SBRT planning

Answer 1

▪ Very high doses/#
▪ Inhomogeneous Dose inside PTV
▪ Sharp Dose Fall Off outside PTV
▪ 2 or more arcs are needed to create conformal dose distributions (sometime
non-coplanar beams)
▪ Lower prescription Isodose levels

Question 2

Ideal beam energy for SBRT

Answer 2

6MV
- reduced penumbra
- rapid dose fall off

Question 3

What beam model is typically used for SBRT

Answer 3

FFF
-> increased dose rate
-> reduced penumbra

Question 4

What ICRU report is followed for SBRT

Answer 4

ICRU 91

Question 5

ICRU 91 goal for dose prescription

Answer 5

‘Absorbed dose is prescribed to the isodose surface DV that covers an optimal percentage volume of the PTV while optimally restricting
the dose to the PRV.

Question 6

What does ‘optimal coverage’ mean for an SBRT plan?

Answer 6

‘Optimal in this context means the best possible coverage of the PTV according to the clinical situation (e.g., brain metastases, spine)’

Question 7

How does dose prescription vary between the brain, spine and lung (provide examples)

Answer 7

Brain – the prescription could be close to 100 %
of the PTV

Spine- The spine treatment achieves the
prescription dose to only 85 % of the PTV,
D85%, because of the nearby cord (OAR).

Lung -The lung metastasis receives prescription
dose to 98 % of the PTV, D98 %

Question 8

What is RTOG 0813 and RTOG 0915

Answer 8

Lung protocols that acts as criteria for PTV coverage, high dose spillage and dose fall off

Question 9

What is the maximum dose in SBRT plan

Answer 9

If PD = 100%
- maximum dose must be at lease 111.11% but not more than 166.67%

Question 10

Where should the max dose be located within an SBRT plan

Answer 10

GTV -> if included within the plan

PTV -> if GTV is not included within the plan

Generally, max dose is prescribed as a %, not in Gy

Some departments allow up to 170% for certain targets

Question 11

ICRU 91- Level 2 dose reporting

Answer 11

PTV median absorbed dose (D50%)
SRT near-minimum dose (Dnear-max)

Question 12

What is the purpose of D2cm

Answer 12

A mechanism for evaluating dose fall-off geometrically

D2cm contour can also be used like a ring structure to control dosimetry

Question 13

What is the gradient index (R50)

Answer 13

R50 is the ratio of 50% prescription isodose volume to the PTV volume

GI = PTVhalf/PIV

Question 14

Why is GI better than CI?

Answer 14

R50 is a function of the size of the PTV’s
- It is smaller for large PTV’s
- it differentiates plans with similar conformity but with different gradients

Question 15

What algorithm is commonly used for SBRT

Answer 15

Acuros XB
- Faster and more accurate than AAA

Question 16

Why are higher max doses allowed?

Answer 16

higher max -> faster dose fall off outside the target, so if your max is higher the R50 and D2cm should be better

Some departments allow up to 170% for certain targets

Question 17

Planning approaches for cases with two or more lesions that require separate plans

Answer 17

Bias Dose Plan

2 Prescription Approach

Question 18

Target margin

Answer 18

Distance from PTV to MLC
Keep it tight to allow steep dose gradient
Leads to small penumbra