Agenzia Provinciale per la Protonterapia
Trento, Italy
Aspetti fisici della
radioterapia moderna - II:
Treatment planning, IMRT,
protoni
Marco Schwarz
[email protected]
23 Settembre 2010
Treatment Planning
in 3D CRT
3D CRT
Target defined in soft tissues on CT images
Higher target/OAR doses than in 2D CRT
3D Treatment planning
Safety margins must be considered while designing
treatment field
• ICRU 50(1993) and ICRU 62(1999) set the standard for
dose planning and dose reporting reference volumes.
•
•
•
•
PTV concept: pros
CTV = Clinical Target Volume (visible +
microscopic disease)
PTV = Planning Target Volume
• Forced people to explicitely incorporate geometrical
uncertainties into treatment planning
• Very appropriate tool for CRT: not too simple, not too
complex.
‘Margin recipes’
Analytical solution for spherical targets (van Herk 2000)
Derived/verified with simulations for real cases (e.g. Stroom 1999,
Van Herk 2002) as a function of population-based data on
geometrical uncertainties
Different 'recipes'
according to the
desired probability level
PTV planning= same dose
prescription for all points
above a given probability of
presence for target cell
PTV: cons
• Use of accurately defined margins still
quite rare
• Dose homogeneity in the PTV became
a must more for technical than for
clinical reasons
• N.B. IGRT mostly aims at reducing PTV
margins without radically changing
PTV-based RT techniques
• Most important: the PTV concept works
only if three assumptions are valid:
PTV - playing by the rules
The PTV is a tool for dose planning and dose reporting.
There are three underlying assumptions:
1. The dose distribution is invariant for (small) translations
and rotations
2. The margins are chosen appropriately as a function of the
geometrical uncertainties one wants to compensate for
3. The dose distribution in the PTV is as homogeneous as
possible.
Condition1 is granted using photons, 2 and 3 must be
ensured using correct planning practices.
CTV
+
PTV expansion
=
OAR
CTV
?
OAR
As if one should prefer
homogeneous doses in the wrong PTV instead of
heterogenous doses in the right PTV
Treatment planning in IMRT
More degrees
of freedom
More need
to know
what you want
CRT
IMRT
How to tell a machine what we want from
it ?
Still struggling with TP in IMRT
Inst.1
Inst. 2
Inst. 3
Inst. 4
Inst. 5
Adapted from Das et al, JNCI 2007
In IMRT si hanno molti più gradi di libertà che in CRT, troppi
per poter essere gestiti ‘a mano’.
Gli scopi del trattamento devono essere espressi in un
linguaggio comprensibile tanto dall’uomo quanto dalla
macchina
L’ottimizzazione in IMRT è la gestione via macchina di una serie
di obiettivi intrinsecamente in contraddizione.
Funzione di costo
• Traduzione quantitativa delle caratteristiche del
piano di trattamento in termini di
– Obiettivi di dose (e.g. Dmin, Dmax)
– Intenti del trattamento (e.g. controllare la dose vs.
massimizzarla/minimizzarla)
– Trattamenti precedenti
– Informazioni biologico/funzionali
– Informazioni geometriche (e.g. errori di set-up)
– Parametri di erogazione
– …
The objective cost function
1. Evaluator
2. Modifier
Quantifies a relevant
feature of the plan
A function f of the difference between
the actual (E) and the desired (E0)
value of the evaluator
Dmean
Dmin/Dmax
DVHpoint
C
# segments
treatment time
plan robustness
50
45
40
35
30
25
20
15
10
5
0
0
…
5
10
E
15
20
3-step IMRT treatment planning
1. Fluence optimization
Cost function minimization
Up to 10^4 ‘beamlets’
Dose calc: fast but not very accurate
2. Segmentation
Mechanical and dosimetrical MLC
parameters are included
Deterioration of the dose distribution
3. Final dose calculation
No reoptimization
Dose calculations: slower, but more
accurate than in step 1.
Aperture based treatment planning
1. Initial fluence optimization
2. Initial Segmentation
3. Tuning of a deliverable plan
Taking benefit of degeneracy
--> More efficient delivery
Less computational burden =
Possibility of using accurate
dose algorithms
What do we talk about when we talk about
Patient specific QA ?
Don’t forget the big picture
Huq et al, IJROBP 2008 71(1) Supp.
2-D dosimetry + gamma analysis
What is patient specific in this approach? The beam setting
Which aspect of the treatment chain is evaluated? The head model
In most cases, field by field analysis
Some techniques require whole treatment verification (e.g. VMAT)
Monte Carlo dose calculation (Tübingen)
Main advantages
1)It solves the main dosimetric problem of IMRT dose calculation
algorithms (source model)
2)Combined with hardware QA, it allows to come back to separate hw
e sw QA, as in CRT
In-vivo dosimetry(NKI)
Planning CT
(3D)
EPID treatment image
(2D)
Planning dose
(3D)
EPID portal dose
(2D imager plane)
select mid-plane slice
back-projection
Planning dose
(2D patient mid-plane)
EPID dose
(2D patient mid-plane)
-evaluation
Courtesy B. Mijnheer
separate fields, 2D
Dose-based corrections protocols?
Planning CT +
Planned dose
CBCT +
In vivo dosimetry
Gamma analysis:
dose errors
Vs
anatomy changes
McDermott, R&O2008
Delivery
Delivery
Tecniche ad arco. Perché?
• Aumento numero di campi >>
aumento gradi di libertà
• Migliore conformazione della dose
• In caso di target concavi migliore
risparmio degli OAR
• Erogazione più veloce e riduzione
movimenti intra-fraction
• Molti parlano inoltre di migliore
efficienza e riduzione MU, ma
l’affermazione è discutibile
From De Neve, in “Image-guided
IMRT”, Springer Ed. 2007
Time/efficiency
Treatment complexity vs monitor unit
s=1-Dmax/Dpresc
Bakai et al, PMB 2003
2-step IMRT treatment planning
1. Fluence optimization
Cost function minimization
Up to 10^4 ‘beamlets’
Dose calc: fast but not very accurate
2. Segmentation
Mechanical and dosimetrical MLC
parameters are included
Deterioration of the dose distribution
3. Final dose calculation
No reoptimization
Dose calculations: slower, but more
accurate than in step 1.
Aperture based treatment planning
1. Initial fluence optimization
2. Initial Segmentation
3. Tuning of a deliverable plan
Taking benefit of degeneracy
--> More efficient delivery
Less computational burden =
Possibility of using accurate
dose algorithms
Author
Palma IJROBP 2008
Mu SIMRT
MU
VMAT1
Mu
VMAT2
789
492
454
Verbakel IJROBP
2009
1108
439
349
Cozzi R&O 2008
479
245
Vanetti R&O 2009
1126
463
Clivio R&O 2009
1531
468
Nicolini Rad On 2009
1398
Shaffer IJROBP 2009
(E-pub)
Zhang IJROBP 2009
(E-Pub)
Shaffer Clin Oncol
2009
MU
CRT
295
Time SIMRT
Time
VMAT1
9.6
3.7
15
1.7
584
15
1.3
545
9.4
1.1
796
11.5
3
789
363
5.1
1.8
642
290
1819
949
9.6
3.7
Time
Vmat2
2.6
Cone Beam
Fan Beam
Dose erogata in una
singola/multipla rotazione
del gantry
Dose erogata grazie ad un fan
beam che ruota
continuamente in
concomitanza alla
traslazione del lettino
Durante la rotazione la
fluenza è modulata:
- Variazione forma del
campo(movimento
lamelle MLC)
- Variazione dei pesi dei
campi (variazione di
intensità)
Tecniche Conformal Arc,
AMOA, IMAT, VMAT
Durante la rotazione la fluenza
è modulata:
- Variazione forma del
campo
- Variazione dei pesi dei
beamlets
Tomoterapia
seriale/elicoidale
IMRT
(Angoli fissi)
IMAT
(Archi
multipli)
VMAT
Single arc
Tomoterapia
Il gantry ruota per 360° creando 51 proiezioni
Modulazione ottenuta variando il tempo di On/Off
per ogni lamella
Velocità di rotazione del gantry e tempo di
trattamento dipendono da:
dose di prescrizione, lunghezza target, dose rate
Single/Few Arc(s) vs TOMO
Prostata
HT e IMAT: distribuzioni comparabili;
IMAT: erogazione più veloce
IMAT: riduzione dose integrale
Canale Anale
HT: migliore qualità piani; migliore
copertura e omogeneità target; migliore
risparmio genitali
H&N
HT: migliore qualità piani gradiente di
dose più elevati
Solid line: IMAT
Interplay effects
Bortfeld et al, PMB 2002
Could we solve it by adding
a margin ?
No
(Not completely)
Is it that bad ?
It depends
Intrafraction (‘interplay’) effects
1fr
30 fr
1fr
sw
s&s10
s&s20
Jiang et al, PMB 2003
30 fr
New treatment modalities
Radiation delivery technologies
Heavier ions(?)
HDR
'Conventional' XRT
Tomotherapy
IMXT
Tomorrow's ideas
'Conventional' p+
Where would we like to use p+ ?
3DCRT
IMPT
TOMO
%
35%D
ose
Gy
10%
Dose
0%
Dose
In principle, for all patients
In practice, whenever dose sparing at all dose levels could
make the difference
The Bragg peak
Protons vs photons – Version 2
Protons vs photons – version 3
1.0
1.0
1.0
0.4
Fotoni
1.0
0.5
0.4
Protoni
0.2
0.5
0.5
1.0
0.5
0.2
1.0
0.2
0.2
Protons vs photons – version 4
X
3D modulation
+
Steep dose fall off
More degrees of freedom
p+
+
=
Protoni vs. Fotoni – caso pediatrico
IMRT
IMPT
G. Fava - ATreP
IMRT
IMPT
IMRT
IMPT
Sezione assiale con aree di basse dosi
Dosimetric effects of geometrical
uncertainties
No
errors
5mm
setup
(a)
(b)
10mm
setup
(c)
(d)
5mm
setup
10 mm
respiration
M. Engelsman - MGH
X rays
protons
Our choices
PT center as the first module of a new public regional
hospital
Emphasis on availability and clinical usability
No significant local development on PT technology
Delivery mode: PBS only
Interest in patient set up outside the treatment room
First treatments: first half of 2013(?)