Promises and Perils of
Proton Radiotherapy
py
Uncertainty Issues for non-moving Targets
M tij E
Martijn
Engelsman
l
The promises
The Promises:
Lower upstream dose
No downstream dose
2
The perils
The Peril:
Finite Range ! ! !
3
P t Mindset
Proton
Mi d t
CT-scan
4
Hounsfield units conversion
Electron Density
Protons
Proton stopping
pp g p
power
Relative stop
pping power
Photons
TPS
1.5
1
5
Plastics
Tissues
1.25
1
0.75
0.5
0.25
0
25
0
-1000
-750
-500
-250
0
250
500
CT Value
5
750
1000
1250
1500
3.5% plus 1 mm
Proton beam
3.5% plus 1 mm
6
CT artifacts
High-density streak artifacts
 Density override
Contrast / Onyx
y g
glue
 Multiple CT-scans
 Density override
7
Patient hardware
Correct for Titanium
8
CT reconstruction circle
Centered
FOV = 50 cm
Not centered
FOV = 50 cm
9
Not centered
FOV = 65 cm
Prostate example
•
•
Consider
C
id re-scanning
i ffor ttreatment
t
t planning
l
i
Consider prescribing a diet
10
Lung example
Arm position
•Limits beam directions
•Reproducible setup?
AP and PA fields
•Skim
Ski d
density
it surface
f
•With smearing  exit dose
11
P t Mindset
Proton
Mi d t
Target Delineation
12
Target delineation
Steenbakkers et al.
Radiother Oncol.
2005; 77:182-90
200
180
160
Dose (%)
140
120
100
80
60
40
Photon (10MV)
Proton SOBP (18cm)
20
0
0
5
10
15
Depth (cm)
•
•
Protons: Reduced proximal and zero distal dose
Treatment day
y field combination dependent
13
20
25
30
P t Mindset
Proton
Mi d t
Dose Calculation
14
Pencil beam vs Monte-Carlo
Monte Carlo
Treatment
Planning
system
Dose
difference
H. Paganetti
g
et al.
15
Patching
+
3 mm
2 mm
1 mm
0 mm
16
Patching: multiple match lines
17
P t Mindset
Proton
Mi d t
Patient Alignment
18
Hardware proximity
Airgap llarger th
Ai
then planned:
l
d
• Increased aperture projection
• Increased penumbra due to scatter in Range Compensator
Dosimetrist training / understanding
19
Smearing for setup errors
High-Density
Structure
Target
Volume
Beam
Critical
Structure
Range
Compensator
Body
Surface
A
Aperture
t
20
Smearing for setup errors
High-Density
Structure
Target
Volume
Beam
Critical
Structure
Range
Compensator
Body
Surface
A
Aperture
t
21
Setup verification
• Any “photon” approach can be a proton
approach e
approach,
e.g.
g
– Orthogonal X-rays + DRRs
– Ultrasound
– AlignRT
• Conebeam CT
– Full 3D patient alignment
– Observe density changes
22
Shape changes of patient and tumor
Mischa Hoogeman, Erasmus MC
23
Shape changes of patient and tumor
Before RT
After RT
E. M. Vasques Osorio et al.
IJROBP 70: 875-82
24
Other density changes
•
•
•
•
Patient weight gain / loss
Filling up of sinuses
(Sub-clinical) pneumonia
W t hair
Wet
h i / gell / hairspray
h i
Lei Dong, MDAH
25
“Catch” substantial density changes
•
•
•
•
•
•
Measure patient weight
Ali RT
AlignRT
Tightness of immobilization device
Physician follow-up
Repeat CT
CT-scanning
scanning
Conebeam CT
Currently: Unknown clinical importance
26
P t Mindset
Proton
Mi d t
Dose Delivery
27
Double whammy
Same number of MU for each SOBP
28
P t Mindset
Proton
Mi d t
Dose Evaluation
29
PTV in particle therapy
Tumor
PTV
Distal margin not needed for setup errors
30
PTV in particle therapy
Planned dose
Delivered dose
Please see Lei Dong’s
g p
presentation for PTV for e.g.
g lung
g tumors
31
Batched field delivery: Prostate
Left femoral head
9 Gy
32
Batched field delivery: Chordoma
Brainstem
33
Proton mindset necessary in:
•
•
•
•
•
•
CT-scan
T
Target
delineation
d li
i
Dose calculation
Patient alignment
Dose delivery
Dose evaluation
34
Ten year vision
• Acquire Proton CBCT prior to every fraction.
– low dose
– 3D / 4D proton stopping power information
– Accurate patient alignment
• Automatically draw (progress) target volume
and normal tissues
– How about biological information?
• On-line plan re-optimization and then delivery
– Pencil Beam Scanning is obviously required for this.
35
Conclusions
Think Density,
Densit Densit
Density, Densit
Density …
It’s high time to accurately assess the effects
of intra-treatment course density variations
36