RHÖN-KLINIKUM AG
Benefit of Reducing the Distance
between Patient and Nozzle
and
Usage of a Range Shifter
Uli Weber1,2, Gerhard Kraft3 and Klemens Zink2,1
1 Universitätsklinikum
Gießen und Marburg, RHÖN KLINIKUM AG, Germany
Hochschule Mittelhessen (THM) - University of Applied Sciences , Germany
3 GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt, Germany
2 Technische
PTCOG 53, Shanghai, Friday, June 13th, 2014
RHÖN-KLINIKUM AG
Simulation of Scattering at the nozzle for a proton beam
140 cm
Vacuum
Window
100 cm
32 cm
40 cm
Vacuum
Beamline
Proton beam
MWPC1 3 x IC
target
shifted
Isocenter
MWPC2
Nozzle
• Nozzle is based on GSI pilot project development
• Very similar to the nozzles of Heidelberg, Marburg and Shanghai
(Siemens AG)
Relevant Scattering Targets:
- plastic vacuum window (Polyester/ Kevlar combination)
- 3 gas-filled ionization chambers
with metalized polyester mesh (electrodes) , thin polyester windows
- 2 gas-filled MultiWire chambers, thin polyester windows
- Detector gas Argon CO2 (80% / 20%) and Air
Simulation method
• Based on method of
B. Gottschalk et. al. , NIM-B 74 (1993) 467-490,
(Highland approximation)
•
Ion-optical beam parameters fitted
to measured values of the envelope in air,
TPS base data for the Marburg facility (2011)
Proton Pencil Beam Optimization by reduced nozzle-to-target distance
RHÖN-KLINIKUM AG
RHÖN-KLINIKUM AG
Treatment Planning Study for reduced Nozzle-to-Target Distance
Methods:
•
Syngo® PT Planning VA11 (Siemens AG)
Syngo TPS uses a realistic data base for the scattered beam envelopes
•
Proton plans for geometrical target volumes: spheres and cylinders of 2 or 3 cm radius in
different depths of 7 – 20 cm with 140 cm nozzle-to-target distance vs. 100 cm
•
Sharpest available proton beam spot (from TPS base data library)
•
3 mm lateral scan spot grid / 2mm range steps
•
Study repeated for small, medium and large lateral scan spot margins (ca. 6 / 8 / 10 mm)
•
Scan spot margins slightly adjusted for best possible identical DVHs in the target volume
(for the shifted and non-shifted case)
note: Scan Spot Margin = ”Virtual Target Expansion” (in Syngo TPS)
RHÖN-KLINIKUM AG
Lateral dose profiles for a Sphere plan (d=4cm) in 7 cm depth ;
140 cm vs. 100 cm nozzle-to-target distance
80.0
lateral dose profile
not shifted
shifted
60.0
Dose [GyE]
140 cm nozzle-to-target distance
90%
25 % steeper
gradient
for the
shifted case
40.0
20.0
10%
0.0
0
100 cm nozzle-to-target distance
20
x [mm]
40
60
RHÖN-KLINIKUM AG
Lateral dose Distribution for a cylinder (d=6 cm) at 8 cm depth;
140 cm vs. 100 cm nozzle-to-target distance
D95-10% = 14.4 mm
140 cm
D95-10% = 10.1 mm
100 cm (shifted)
RHÖN-KLINIKUM AG
Reduction (∆) of the integral dose in the normal tissue
nozzle-to-target distance 140 cm vs. 100 cm
140 cm
High
penetration
Sphere 4 cm
depth
(≈ 20 cm )
Cylinder 6 cm
Low
penetration
Sphere 4 cm
depth
(≈ 7 cm)
Cylinder 6 cm
100 cm
∆
„Shell“
∆
∆
RHÖN-KLINIKUM AG
Benefit of a Range-Shifter for Proton Beams
Range-Shifter with
2 cm PMMA, Siemens AG
A „Range Shifter“ is a pre-absorber plate in front of the patient
•
Range Shifter are basically needed for CTV‘s close to the skin
(minimum energy of the accelerator)
•
Range Shifter can be used to reduce the lateral scattering,
because a higher energy from the accelerator can selected
→ reduced multiple scattering at the nozzle
Isocenter
1400 mm
1100 mm
Vacuum
Beamline
60 MeV protons
80 MeV protons
CTV
Range
Shifter
nozzle
(2cm PMMA)
Target
RHÖN-KLINIKUM AG
Methods for the Range Shifter Study
•Proton Scattering Calculation based on the method of B. Gottschalk et. al. ,
NIM-B 74 (1993) 467-490, (Highland approximation)
•Investigation of geometrical treatment plans for protons beams for spheres
with 20, 50 and 75 cm diameter at various penetration depths
In many cases, there are
restrictions for the
positioning
of the Range Shifter
(eg. by the treatment table)
•Investigation of 20 clinical treatment plans for protons
(Prostate, head and lung cases)
•Averaging of the beam width in the target over all scanning spots (weighteddistance
by the number of protons) → mean FWHM value
min
•Analysis of the relation between
FWHMmean and the distance between Range-Shifter and skin
beam
Range
Shifter
treatment
table
RHÖN-KLINIKUM AG
3cm
RangeShifter
2cm
RangeShifter
no RangeShifter
Benefit for
distance = 0
RHÖN-KLINIKUM AG
Conclusions
•Reduced nozzle-to-target distance has positive dosimetric effects for tumors at low
penetration depth
•Especially, for tumors at depth < 15 cm, e.g. intracranial tumors
→ typically 15 - 25 % reduction of the integral dose in the surrounding tissue
•Benefit depends from the individual design of the nozzle (thickness of windows and
distance from the iso-centre)
•Drawbacks
• higher requirements on QA
• imaging in the treatment position is often not possible
•Range Shifters have beneficial effects for tumors at low depth and when the distance
from the skin is not so far (see empirical rule)
Acknowledgements
to the Siemens AG, Healthcare Sector, Erlangen, Germany
for providing the treatment planning system Syngo® PT Planning VA11
to Dr. Urszula Jelen for providing the treatment plans for the Range Shifter study
RHÖN-KLINIKUM AG
Thank you for your attention!
Particle Therapy Centre in Marburg
RHÖN-KLINIKUM AG
Similar study from Marburg for clinical proton plans (intracranial
tumors)
“ The decrease in mean dose (Dmean)
was 12.5% to the whole brain,
16.2% to the brainstem … ”
U. Jelen, U. Weber et. al.
Dosimetric impact of reduced nozzle-to-isocenter
distance in intensity-modulated proton therapy
of intracranial tumors ;
Radiation Oncology 2013, 8:218
http://www.ro-journal.com/content/8/1/218
free online