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(?)