Post on 02-Feb-2016
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Aspetti fisici della radioterapia moderna - II:Treatment planning, IMRT,
protoni
Marco Schwarzschwarz@atrep.it
Agenzia Provinciale per la ProtonterapiaTrento, Italy
23 Settembre 2010
Treatment Planningin 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
• Forced people to explicitely incorporate geometrical uncertainties into treatment planning
• Very appropriate tool for CRT: not too simple, not too complex.
CTV = Clinical Target Volume (visible + microscopic disease)
PTV = Planning Target Volume
‘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 doseprescription for all pointsabove a given probability ofpresence 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 rulesPTV - 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.
+ PTV expansion =
?
As if one should preferhomogeneous doses in the wrong PTV instead ofheterogenous doses in the right PTV
CTV
OAR
CTV
OAR
Treatment planning in IMRT
More degrees of freedom
More need to knowwhat you want
CRT IMRT
How to tell a machine what we want from it ?
Still struggling with TP in IMRT
Adapted from Das et al, JNCI 2007
Inst.1 Inst. 2 Inst. 3 Inst. 4 Inst. 5
In IMRT si hanno molti più gradi di libertà che in CRT, troppi In IMRT si hanno molti più gradi di libertà che in CRT, troppi per poter essere gestiti ‘a mano’.per poter essere gestiti ‘a mano’.
Gli scopi del trattamento devono essere espressi in un Gli scopi del trattamento devono essere espressi in un linguaggio comprensibile tanto dall’uomo quanto dalla linguaggio comprensibile tanto dall’uomo quanto dalla macchinamacchina
L’ottimizzazione in IMRT è la gestione via macchina di una serie L’ottimizzazione in IMRT è la gestione via macchina di una serie di obiettivi intrinsecamente in contraddizione.di obiettivi intrinsecamente in contraddizione.
Funzione di costo
• Traduzione quantitativa delle caratteristiche del Traduzione quantitativa delle caratteristiche del piano di trattamento in termini dipiano 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 functionThe objective cost function
1. Evaluator
Quantifies a relevant feature of the plan
DmeanDmin/DmaxDVHpoint
# segments
treatment time
plan robustness
…
2. Modifier
A function f of the difference between the actual (E) and the desired (E0) value of the evaluator
05
101520253035404550
0 5 10 15 20
E
C
3-step IMRT treatment planning
1. Fluence optimizationCost function minimizationUp to 10^4 ‘beamlets’Dose calc: fast but not very accurate
2. SegmentationMechanical and dosimetrical MLC parameters are includedDeterioration of the dose distribution
3. Final dose calculationNo 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 planTaking benefit of degeneracy --> More efficient delivery
Less computational burden =Possibility of using accurate dose algorithms
Patient specific QA
What do we talk about when we talk about
?
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 settingWhich aspect of the treatment chain is evaluated? The head modelIn most cases, field by field analysisSome techniques require whole treatment verification (e.g. VMAT)
Monte Carlo dose calculation (Tübingen)
Main advantages1)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
-evaluation
EPID portal dose (2D imager plane)
EPID dose (2D patient mid-plane)
back-projection
EPID treatment image(2D)
separate fields, 2D
Planning CT(3D)
Planning dose (2D patient mid-plane)
select mid-plane slice
Planning dose (3D)
Courtesy B. Mijnheer
In-vivo dosimetry(NKI)
Dose-based corrections protocols?
Planning CT + Planned dose
CBCT + In vivo dosimetry
Gamma analysis:dose errors
Vsanatomy 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
Bakai et al, PMB 2003
s=1-Dmax/Dpresc
2-step IMRT treatment planning
1. Fluence optimizationCost function minimizationUp to 10^4 ‘beamlets’Dose calc: fast but not very accurate
2. SegmentationMechanical and dosimetrical MLC parameters are includedDeterioration of the dose distribution
3. Final dose calculationNo 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 planTaking benefit of degeneracy --> More efficient delivery
Less computational burden =Possibility of using accurate dose algorithms
Author Mu S-
IMRT
MU
VMAT1
Mu
VMAT2
MU
CRT
Time S-
IMRT
Time
VMAT1
Time
Vmat2
Palma IJROBP 2008 789 492 454 295 9.6 3.7
Verbakel IJROBP
2009
1108 439 349
Cozzi R&O 2008 479 245 15 1.7
Vanetti R&O 2009 1126 463 584 15 1.3
Clivio R&O 2009 1531 468 545 9.4 1.1 2.6
Nicolini Rad On 2009 1398 796 11.5 3
Shaffer IJROBP 2009
(E-pub)
789 363 5.1 1.8
Zhang IJROBP 2009
(E-Pub)
642 290
Shaffer Clin Oncol
2009
1819 949 9.6 3.7
Cone Beam
Dose erogata in una singola/multipla rotazione del gantry
Durante la rotazione la fluenza è modulata:
- Variazione forma del campo(movimento
lamelle MLC) - Variazione dei pesi dei
campi (variazione di intensità)
Fan Beam
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
- Variazione dei pesi dei beamlets
Tecniche Conformal Arc, AMOA, IMAT, VMAT
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
Is it that bad ?It depends
Could we solve it by adding a margin ?
No(Not completely)
Intrafraction (‘interplay’) effects
Jiang et al, PMB 2003
1fr 30 fr 1fr 30 fr
sw
s&s10
s&s20
New treatment modalities
Radiation delivery technologies
HDR'Conventional' XRT
Tomotherapy IMXT
'Conventional' p+
Heavier ions(?)
Tomorrow's ideas
Where would we like to use p+ ?Where would we like to use p+ ?
In principle, for all patients
In practice, whenever dose sparing at all dose levels could make the difference
3DCRT TOMO
10% Dose
35%Dose
% Gy
IMPT
0% Dose
The Bragg peak
Protons vs photons – Version 2Protons vs photons – Version 2
Protons vs photons – version 3Protons vs photons – version 3
1.0 0.40.41.0 1.0 1.0
0.5 1.0 0.51.0
0.5
0.5
0.20.2 1.0
0.2
0.2
ProtoniFotoni
Protons vs photons – version 4Protons vs photons – version 4
XX
p+p+
+
3D modulation+
Steep dose fall off =
More degrees of freedom
IMRT IMPT
Protoni vs. Fotoni – caso pediatrico
G. Fava - ATreP
IMRT IMPT
Sezione assiale con aree di basse dosi
IMRT IMPT
Dosimetric effects of geometrical Dosimetric effects of geometrical uncertaintiesuncertainties
Noerrors
10mmsetup
5mmsetup
5mmsetup10 mmrespiration
M. Engelsman - MGH
(d)(c)
(b)(a)
X rays
protons
Our choices 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(?)