Proton treatment delivery. Scanning in practice. Courtesy IBA. Physics of Proton
Therapy. I. basic interactions. – energy loss. – scattering. – nuclear interactions.
Proton Therapy Treatment Planning: Challenges and Solutions
Proton treatment delivery Passive scattering in practice Aperture Range modulator wheel
Jatinder R Palta PhD Daniel Yeung PhD Roelf Slopsema MS Department of Radiation Oncology University of Florida
Patient
Range compensator conforms dose to distal edge target volume; the aperture shapes field in lateral direction.
Brass aperture
Lucite range compensator
Physics of Proton Therapy
Scanning in practice Tumor divided into iso-Energy slices
Vacuum Chamber
Scatterer
Target
Proton treatment delivery
Fast
Compensator
I. – – –
Y
Already irradiated slices
Slow
Intensity Modulated Beam
basic interactions
Z
energy loss scattering nuclear interactions
X
IC’s A
IC’s B Slice being Irradiated with raster scan pattern
Pair of Quads Isocenter
Scanning Magnets
Bragg Peak
II.
clinical beams – –
percentage depth dose lateral penumbra
Courtesy IBA
Proton interactions: Energy loss
Bragg peak
Primarily protons lose energy in coulomb interactions with the outer-shell electrons of the target atoms.
• excitation and ionization of atoms • loss per interaction small → ‘continuously slowing down’ • range secondary
e+
p+
15 g/cm2,
Analytical model of proton BP (up to ~ 200 MeV) • accounts for energy spread • empirical model of nuclear fragmentation (data fitting) • numeric depth dose calculation of fitted BP • assumption – range straggling ‘constant’ with depth
R=19.8 g/cm2
Range straggling
depth
Error in Bortfeld model
Bortfeld Model
Eclipse Model
SOBP Calculated versus Measurement
Beam Profile Calculated versus Measurement R=15.1 M=10.4
d=10 cm
Proton beam dose calculation in water is generally very accurate! d=5 mm
20%-80% Lateral Penumbra [cm]
Option B5 - R=15.1 g/cm^2, M=10.4 g/cm^2, Air gap = 11.7cm, SSD=220.1, aperture: 15cmx15cm
1.3
Eclipse convolved - @9.9 cm Eclipse convolved - @0.5 cm Eclipse convolved - @14.1 cm
Measurement (av) - @9.9 cm Measurement (av) - @0.5 cm Measurement (av) - @14.1 cm
1.2 1.1 1
0.9 0.8 0.7 0.6 0.5
d=15 cm UFPTI Data
0.4 0.3 0.00
5.00
10.00
15.00
20.00
Air Gap [cm]
25.00
30.00
UFPTI Data
35.00
Challenges in Proton Therapy Planning 1. Uncertainties: à à à à à
2. Evaluation of proton plans à
How to evaluate a proton plan in the presence of various uncertainties?
3
Protons on water I - dependence w
The peak spread increases with energy
2
1
0
PTV? Error bars of dose distributions?
P122 Iw67 P122 Iw75 P122 Iw80 P183 Iw67 P183 Iw75 P183 Iw80 P230 Iw67 P230 Iw75 P230 Iw80
2
à
4
Intrinsic basic physics uncertainty (I-values) CT numbers (stopping powers; range), Dose calculation errors due to complex inhomogeneities, Intra-fractional organ motion, Inter-fractional changes in anatomy and motion patterns, Mis-registration of tissue compensators (passively scattered proton beams, Uncertainties in immobilization devices and patient support devices
dE/dz (MeV/g cm ) per incident particle
à
Intrinsic basic physics uncertainty (I-values)
0
5
10
15
20
25
30
35
2
depth in water (g/cm ) P Andreo, Phys Med Biol, 2009
122 MeV Protons on water: I - dependence w
4
average I-values of various soft tissues
2
w
P122 I = 75eV w
P122 I = 80eV w
3
2
2
dE/dz (MeV/g cm ) per incident particle
0.3 g/cm P122 I = 67eV
1
Peak spread is .7 g/cm2 for 230 MeV protons 0 9.5
10.0
10.5
11.0
11.5
MUSCLE SKELETAL (ICRP) LUNG (ICRP) BLOOD (ICRP) WATER LIQUID TESTES (ICRP) TISSUE SOFT (ICRU-33 4 comp) HEART ADULT (BLOOD-FILLED) MUSCLE STRIATED (ICRU) HEART ADULT (HEALTHY) GI-TRACT (INTESTINE) HEART ADULT (FATTY) EYE LENS (ICRP) BRAIN (ICRP) UREA SKIN (ICRP) TISSUE SOFT (ICRP) TISSUE SOFT (ICRU-44 MALE) TISSUE SOFT (ICRU-44 FEMALE) BREAST (WHOLE)-50/50 BREAST (WHOLE)-33/67 ADIPOSE TISSUE (ICRP)
2
depth in water (g/cm )
Intrinsic basic physics uncertainty makes the argument of “sub-millimeter precision” an issue, which deserves careful consideration
62
74
76
HU-Stopping Power Calibration Curve
Peak spread assuming 10% uncertainty in I-values
2
Realative Stopping Power
1.8
P 164 muscle skeletal ICRP P 164 tissue soft ICRU-44 F
1.4 1.2 1 0.8
Large phantom, Average position, 140kV, 400mAs, Big bore
0.6
Body phantom, Average position, 140kV, 400mAs, Big bore Full head phantom, Average position, 140kV, 400mAs, Big bore Empty head phantom, Average position, 140kV, 400mAs, Big bore
0.4
0 -1000
P 164 muscle skeletal ICRP P 164 tissue soft ICRU-44 F
17.5
1.6
0.2
P 164 water (I=75)
0.0 17.0
72
2
1.5
0.5
70
P Andreo, Phys Med Biol, 2009
0.7 g/cm
2.0
1.0
68
P Andreo, Phys Med Biol, 2009
2
dE/dz (MeV cm /g) per incident particle
2.5
66
I-value (eV)
164 MeV protons on various tissues (+/- 10% change in I-values) 3.0
64
18.0
-500
0
500
CT #
18.5
19.0
19.5
2
depth (g/cm )
P Andreo, Phys Med Biol, 2009
1000
1500
2000
S. Flampouri, UFPTI
Range uncertainty due to CT calibration is generally taken as 3.5%. However, it can be minimized with better calibration techniques.
HU‐Stopping Power Conversion Uncertainties Results in Range Uncertainties
Impact of CT Hounsfield number uncertainties on dose distributions +3.5%
-3.5%
Dong/MDACC
S Flampouri, UFPTI, 2007
CT Artifacts and Hounsfield Numbers
0% uncertainty Individualized patient determination of tissue composition along the complete beam path, rather than CT Hounsfield numbers alone, would probably be required even to reach “sub-centimeter precision”
Proton Range Uncertainties
10% range error
“It is imperative that body-tissue compositions are not given the standing of physical constants and their reported variability is always taken into account” (ICRU-44, 1989).
The advantage of protons is that they stop.
The disadvantage of protons is that we don’t always know where… Lomax: PTCOG47
Proton Range Uncertainty in the Presence of heterogeneities Soft tissue
Soft tissue
ΔRhom
Bon e
Impact of Organ Motion on Proton Dose Distributions Free breathing treatment
ΔRinho m
Tsunashima/MDACC
Lomax : AAPM SS 2003
Dong: ASTRO 2008
Plan DVH Evaluation (PRV) What you see is not what you always get..
Impact of Tumor Shrinkage on Proton Dose Distribution Dose recalculated Original Proton Plan
on the new anatomy
Volume
Proton DVH
Photon DVH
Dose Bucci et al. ASTRO Abstract, 2007
Free breathing Treatment
Practical Solutions Gated treated on exhale
L Dong: MDAH
Improving CT number accuracy and reducing metal artifacts with Orthovoltage CT imaging H2O
Brain Al Cortical bone
Lung1
(a)
Al
(b)
Lung2 125 kVp CT
Air 125 kVp
320 kVp CT
Brain
Cortical bone
LDPE
125 kVp CT
LDPE 320 kVp
320 kVp CT
Conventional 125 kVp CT Orthovoltage CT at 320 kVp (c) Yang et al. Med Phys 35 (5):1932-1941, 2008
(d) Yang et al. Med Phys 35 (5):1932-1941, 2008
Match and Patch Fields ¾ used to avoid OARs adjacent to target
Match Fields match fields abutting each other penumbra matching penumbra Inf - LAT
Sup - RAO
¾ partition target into segments (submatchline
targets) ¾ sub-targets treated with ‘sub-beams’ ¾ angle sub-beams to avoid OARs ¾combined with other fields for dose uniformity
Lacrimal Gland Carcinoma
Patch Fields thru beam txt partial target residual txt with patch lateral penumbra (t-beam) ‘matched’ with distal falloff (p-beam) LPO beam (inferior) patched with SPO (superior)
(PTV 50.4) partition into sup + inf targets brainstem
sup inf
LPO-Inf (spare optics & BS)
SPO-patch
PTV50.4: 5 fields with match & patch Target
50%
Patch Field Selection
Fields
PTV
LAO
PTV(Inf)
LPO‐Inf
PTV(Sup)
LAO‐match
Gantry 205°
optimal geometric coverage (G = 230°) avoid inhomogeneity along path (G = 205 °)
G=205° G=230° Patch line
Gantry 230°
Patch line
Gantry 205°
SPO‐patch
Patch Field – Beam Angle Selection Gantry 230°
Patch Field Angle Selection
LPO
Distal Blocking
Distal Blocking
added compensator
selective pullback of range to spare OARs pullback achieved with added compensator potential pitfalls ¾setup error or motion may nullify sparing ¾‘simple’ distal blocking may compromise target coverage assess robustness of approach
Target
OAR
} range pullback
Things to remember!
Distal Blocking •
Proton beams stop - no exit dose –
underdose
• Whole BS
distal block
Partial BS
PTV
• BS
Proton beams are more sensitive to –
CT Hounsfield number/Stopping Power accuracy
–
Organ motion
–
Anatomy changes
Proton plans are difficult to evaluate –
Partial BS
•
Although we don’t know exactly where they stop
“What you see is not what is delivered”
Protons demonstrate excellent low dose sparing
RPO Field
Things to remember! • Inter/Intra-fractional variations have far more significant consequences in patients treated with proton therapy
Final thought What you see is not what you always get…. IMXT Plan
IMPT Plan
(with Confidence-weighted Isodose Distribution)
– Approaches and data to deal with this issue is still lacking • Minimize it and develop strategies to deal with the residual motion
• Empirical approaches used in defining margins for range uncertainties, smearing, and smoothing are questionable – No real data exist to support any of these approaches
• Repeat imaging and reevaluation based on deformable registration may be necessary – In some cases repeat planning may be clinically beneficial
Lomax PSI
Jin UF
180 160 120 80 40 cGy
There is no easy way to show what patient will get in proton therapy
