ÖGMP Workshop – ÖGRO 2009 IDOS Symposium 2010

Flattening filter free photon beams: Dosimetric characterisation, beam quality and peripheral doses

Kragl Ga, af Wetterstedt Sb, Knäusl Ba, Baier Fa, Albrich Da, Lutz Sd, Dalaryd Mc,

McCavana Pb, Wiezorek Td, Knöös Tc, McClean Bb, Georg Da

a Radiotherapy Department, Division Medical Radiation Physics, Medical University of Vienna, Austria

b Radiotherapy Department, St Luke‘s Hospital, Dublin, Ireland

c Radiation Physics, Lund University and University Hospital, Lund, Sweden

d Department of Radiotherapy, University Hospital Jena, Germany

IDOS Symposium 2010

Motivation

Why do we want to use unflattened beams?

• unflattened beams can be used for IMRT and SBRT

• increased dose rate – reduction of delivery time Fu et al, PMB (2004)

• SBRT – high fractional doses (DIBH, gating)

• reduced scatter, leaf transmission and radiation head leakage Kragl et al, R&O (2009)

• reduced variation of beam quality across the beam Georg et al, MP (2010)

• reduced peripheral exposure Kry et al, PMB (2010)

• compared to conventional RT, IMRT has been found to increase PD Kry et al, PMB (2010)

IDOS Symposium 2010

Material and methods – Linear accelerators

• reduces electron contamination

• beam stopper

• absorbs very low energies

Courtesy B McClean.

Photon fluences Φγ, per initial electron, at the upstream surface of the water phantom as a function of photon energy Eγ. (FS 40x40cm2). Ti

tt e

t a

l, M

P 3

3 (

20

06

)

Electron fluences Φe, per initial electron, at the up-stream surface of the water phantom as a function of electron energy Ee. (FS 40x40cm2).

IDOS Symposium 2010

Material and methods – Linear accelerators

• reduces electron contamination

• beam stopper

• absorbs very low energies

Courtesy B McClean.

Photon fluences Φγ, per initial electron, at the upstream surface of the water phantom as a function of photon energy Eγ. (FS 40x40cm2). Ti

tt e

t a

l, M

P 3

3 (

20

06

)

Electron fluences Φe, per initial electron, at the up-stream surface of the water phantom as a function of electron energy Ee. (FS 40x40cm2).

IDOS Symposium 2010

Material and methods – Linear accelerators

Available beam qualities

Medical University of Vienna (MUW)

• 6 MV flattened: 6F* (QI = 0.686)

• 6 MV unflattened: 6U* (QI = 0.684)

• 10 MV flattened: 10F (QI = 0.734)

• 10 MV unflattened: 10U (QI = 0.714)

Saint Luke’s Hospital, Dublin (SLH)

• 6 MV flattened: 6F (QI = 0.681)

• 6 MV unflattened: 6U (QI = 0.664)

* TPR20/10 matched for 10 x 10 cm2 field size in reference conditions

• reduces electron contamination

• beam stopper

• absorbs very low energies

Courtesy B McClean.

IDOS Symposium 2010

Results – PDDs

• unflattened beams softer

• dose fall-off with depth more

pronounced

• Average dmax

• same for the 10 MV beam (~23 mm)

• differs up to 2mm for the 6MV

beam (FF: ~15 mm vs. FFF: ~17 mm)

Smaller field size variation of dmax for unflattened beams

IDOS Symposium 2010

Results – Lateral beam profiles

• for the determination of

penumbral widths – the profiles

were normalized to the inflection

points of the FF and FFF beams in

the penumbral region

Pönisch et al, MP (2006)

• penumbral widths agreed within

0.7 mm for all examined energies

and field sizes (6 and 10 MV,

5x5cm2 to 20x20cm2)

IDOS Symposium 2010

Results – Head scatter factors

• very small head scatter variation

• ~ 2% increase from 10 cm to 40 cm

• collimator exchange effect (CEE) reduced

• 60% smaller variation for unflattened beams for both energies

• simpler head scatter modeling for IMRT dose calculation

IDOS Symposium 2010

Material and methods – HVL (narrow beam geometry)

Georg et al, MP (2010)

IDOS Symposium 2010

Results – Off-axis energy variation

• off-axis softening needs to be modelled for

accurate dose calculations

• errors of 5% if not taken into account

for flattened beams Olofsson et al, MP 2006

• much less pronounced effect for unflattened

beams

• differences between unflattened for relative

HVL data was <1%

• generic expression

HVL(0°) /HVL(θ) = 1 + 0.000324 θ +

0.001005 θ² - 0.000062 θ³ (R2 = 0.971)

Georg et al, MP (2010)

IDOS Symposium 2010

Results – Leaf transmission & Leakage radiation

• RT patients are exposed to secondary radiation

• scatter from within patient and collimator

transmission are dominant sources near the

treatment field

• discussed for IMRT, pediatrics, SRT/SBRT

Cashmore, MP (2008)

• at around 30 cm distance

from the field edge:

→ leakage radiation

becomes major source

of PD

Stovall et al, MP (1995)

IDOS Symposium 2010

Material and methods

Courtesy M Dalaryd.

• treatment plans were created with Oncentra

Masterplan (Nucletron)

a) 7-field plan (6, 10 MV)

b) SBRT plan (6, 10 MV)

c) prostate IMRT (10 MV)

d) head&neck IMRT (10 MV)

• dosimetric measurements

• TLD-700 (Harshaw)

• radiochromic films (GafChromic EBT)

• Farmer type ion chamber (PTW)

• MC simulations

• mean energies of photons in the keV

range

• energy correction according to Nunn et

al, MP (2008)

IDOS Symposium 2010

Results – SBRT Cases (6 MV vs. 10 MV)

• DVHs nearly identical

• MU reduced by 3.5% for unflattened beams

• average dose reduction at 22 cm distance from the field edge

• 23% for 6 MV and 31% for 10 MV unflattened beams

IDOS Symposium 2010

Results – Prostate IMRT Case (10 MV)

• number of segments and MUs comparable for FF and

FFF beams

• dose reduction at 18 cm distance from the field edge

• 16.1% with TLD-700

• 16.2% with Farmer chamber

• 16.6% with EBT films

IDOS Symposium 2010

Conclusions & Outlook

• comprehensive dosimetric description of unflattened photon beams

• off-axis energy variation is less than half the one for flattened beams – ignoring the effect of

off-axis energy variation for dose calculations in unflattened beams can be clinically justified

• reduced peripheral doses for advanced treatment techniques - patients might benefit from

decreased exposure of normal tissue

• treatment planning studies ongoing

• due to the smaller variation of head scatter, off-axis energy distribution, electron

contamination and leaf transmission dose calculation accuracy is expected to increase Cashmore, PMB (2008)

IDOS Symposium 2010

Acknowledgements

• General Hospital of Vienna / MUW

• Saint Lukes Hospital Dublin

• University Lund and Skåne University Hospital

• University Hospital Jena

• Elekta Oncology Systems

• Neil Dodd

• Kevin Brown