AbstractID: 10138 Title: Setup Margins Requirements for Stereotactic Lung Radiotherapy
Setup Margins Requirements for Stereotactic Lung Radiotherapy
Introduction
The success of radiation therapy depends on the delivery of the prescription dose to the intended target
while minimizing dose to the surrounding normal tissue. Higher doses delivered in fewer fractions such as in
stereotactic body radiation therapy (SBRT) demand improved accuracy in target positioning. The uncertainties
associated with target positioning are taken into account in the CTV to PTV expansion by the use of margins. A
widespread approach is to use a standard setup margin for a patient population defined by their disease site. This
work presents an analysis of setup margins for stereotactic lung radiotherapy patients for one clinical practice.
Methods and Materials
Forty-five patients were treated with stereotactic lung radiotherapy using Cone Beam CT imaging (Varian
Medical Systems) OBI for daily target localization in routine clinical practice. At each treatment fraction at least
three CBCTs were acquired to assess the patient position. The first CBCT determined any necessary shifts required
after laser alignment (representing the amplitude of offsets for patient set up using only laser alignment), the second
to verify the position before treatment (corresponding to the residual offsets after correctional shifts) and the third to
estimate the intra-fraction motion. The CBCT images were matched with the planning CT using soft tissue contrast.
This provided the tumor offset relative to the simulation position for every fraction before and after the correctional
shifts. Setup margins were extracted from calculated systematic and random errors [1] and compared to the RTOG
0618 SBRT lung protocol requirements.
Results and Discussion
The setup error breakdown is shown in table 1. A large reduction in both systematic and random errors can
be observed when CBCT-IGRT is used. The margins required to give 95% of the prescribed dose to 90% of the
patient population (M 95%Dose 90%Pop) as well as the margins required to give 99% of the prescribed dose to
99% of the patient population are detailed below.
Table
1
SETUP ERROR BREAKDOWN
Systematic
Laser
Pre Tx
Post Tx
Random
Laser
Pre Tx
Post Tx
Vertical (mm)
5.1
0.9
1.3
5.9
0.7
1.5
Longitudinal (mm)
5.7
1.4
1.5
8.1
0.6
1.8
Lateral (mm)
7.6
1.1
0.7
8.3
0.6
1.1
MARGIN REQUIREMENTS
M 95%Dose 90%Pop
Laser
Pre Tx
Post Tx
M 99%Dose 99%Pop
Laser
Pre Tx
Post Tx
Vertical (mm)
17.0
2.8
4.3
22.9
3.7
5.8
Longitudinal (mm)
20.0
3.9
5.0
26.9
5.3
6.7
Lateral (mm)
24.8
3.2
2.6
33.4
4.3
3.5
These numbers provide a guideline for an evidence-based choice of setup margins when CBCT is used in
stereotactic lung radiotherapy. This demonstrates the accuracy of volumetric imaging for daily tumor positioning.
Conclusions
The amplitude of the 95%Dose-90%Population margin (3-5 mm depending on direction) is consistent with the
RTOG requirements of 5-10 mm. Larger margins might be required if higher dose or population coverages are
intended.
References
[1] M. van Herk et al., Int. J. Radiation Oncology Biol. Phys. 47, 1121 (2000)
AbstractID: 10138 Title: Setup Margins Requirements for Stereotactic Lung Radiotherapy
40
30
25
Laser alignment
30
# of fractions
# of fractions
35
25
20
15
10
Laser alignment
20
15
10
5
5
0
0
-40 -30 -20 -10
0
10
20
30
-40 -30 -20 -10
40
16
18
14
16
laser alignment
10
8
6
4
20
30
40
Laser alignment
12
10
8
6
4
2
2
0
0
-40 -30 -20 -10
0
10
20
30
40
0
10
20
30
40
50
60
3D magnitude of error (mm)
Lateral positioning error (mm)
120
120
Pre Tx CBCT
Post Tx CBCT
80
60
40
80
60
40
20
20
0
0
-15
-5
5
Pre Tx CBCT
Post Tx CBCT
100
# of fractions
100
# of fractions
10
14
12
# of fractions
# of fractions
Vertical positioning error (mm)
-15
15
-5
5
120
90
80
Pre Tx CBCT
Post Tx CBCT
70
# of fractions
100
80
60
40
60
Pre Tx CBCT
Post Tx CBCT
50
40
30
20
20
10
0
0
-15
-5
5
Lateral positioning error (mm)
15
Longitudinal positioning error (mm)
Vertical positioning error (mm)
# of fractions
0
Longitudinal positioning error (mm)
15
0
10
3D magnitude of error (mm)