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Principal knowledge for commissioning of Linac using FFF Francisco J. Hern´ andez Flores franciscohernandez [email protected] MMP Student 2015 - 2016 May 26, 2016 Francisco J. Hern´ andez Flores (Arcispedale Santa Anna, Ferrara Italy) Radiation Therapy May 26, 2016 1 / 22

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Page 1: Flattening filter Free

Principal knowledge for commissioning of Linacusing FFF

Francisco J. Hernandez Flores

franciscohernandez [email protected]

MMP Student 2015 - 2016

May 26, 2016

Francisco J. Hernandez Flores (Arcispedale Santa Anna, Ferrara Italy)Radiation Therapy May 26, 2016 1 / 22

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1 Purpose2 Introduction3 Aspect of commissioning4 Profile normalization5 Dosimetric field size

6 Penumbra7 Slope8 The peak Position9 Conclusion

10 References

Francisco J. Hernandez Flores (Arcispedale Santa Anna, Ferrara Italy)Radiation Therapy May 26, 2016 2 / 22

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Purpose

To investigate dosimetric characteristics of a new linear accelerator designedto deliver flattened, as well as flattening filter-free (FFF), beams. To evaluate theaccuracy of beam modeling under physical conditions.[4]

To know the possible definitions and suggestions for some dosimetricparameters for use in quality assurance of FFF beams generated by medicallinacs in radiation therapy.

To compare the dosimetric accuracy of advanced dose calculation algorithmsfor flattened (FF) and unflattened (FFF) photon beams.[5]

We must to know the Dose calculation accuracy using Flattening filter free (FFF)in Advanced treatment techniques, such as IMRT, VMAT and SBRT.

Francisco J. Hernandez Flores (Arcispedale Santa Anna, Ferrara Italy)Radiation Therapy May 26, 2016 3 / 22

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Purpose

To investigate dosimetric characteristics of a new linear accelerator designedto deliver flattened, as well as flattening filter-free (FFF), beams. To evaluate theaccuracy of beam modeling under physical conditions.[4]

To know the possible definitions and suggestions for some dosimetricparameters for use in quality assurance of FFF beams generated by medicallinacs in radiation therapy.

To compare the dosimetric accuracy of advanced dose calculation algorithmsfor flattened (FF) and unflattened (FFF) photon beams.[5]

We must to know the Dose calculation accuracy using Flattening filter free (FFF)in Advanced treatment techniques, such as IMRT, VMAT and SBRT.

Francisco J. Hernandez Flores (Arcispedale Santa Anna, Ferrara Italy)Radiation Therapy May 26, 2016 3 / 22

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Purpose

To investigate dosimetric characteristics of a new linear accelerator designedto deliver flattened, as well as flattening filter-free (FFF), beams. To evaluate theaccuracy of beam modeling under physical conditions.[4]

To know the possible definitions and suggestions for some dosimetricparameters for use in quality assurance of FFF beams generated by medicallinacs in radiation therapy.

To compare the dosimetric accuracy of advanced dose calculation algorithmsfor flattened (FF) and unflattened (FFF) photon beams.[5]

We must to know the Dose calculation accuracy using Flattening filter free (FFF)in Advanced treatment techniques, such as IMRT, VMAT and SBRT.

Francisco J. Hernandez Flores (Arcispedale Santa Anna, Ferrara Italy)Radiation Therapy May 26, 2016 3 / 22

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Purpose

To investigate dosimetric characteristics of a new linear accelerator designedto deliver flattened, as well as flattening filter-free (FFF), beams. To evaluate theaccuracy of beam modeling under physical conditions.[4]

To know the possible definitions and suggestions for some dosimetricparameters for use in quality assurance of FFF beams generated by medicallinacs in radiation therapy.

To compare the dosimetric accuracy of advanced dose calculation algorithmsfor flattened (FF) and unflattened (FFF) photon beams.[5]

We must to know the Dose calculation accuracy using Flattening filter free (FFF)in Advanced treatment techniques, such as IMRT, VMAT and SBRT.

Francisco J. Hernandez Flores (Arcispedale Santa Anna, Ferrara Italy)Radiation Therapy May 26, 2016 3 / 22

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Introduction

In recent years, the clinical use of flattening filter free (FFF) beams is growing fast.Among the reasons, the very high dose rate achieved (up to four times the dose rateof the standard flattened (FF) beams plays a decisive role.

This allowed for stereotactic radiotherapy deliveries of very high dose per fraction (as20 to 25 Gy) in very short treatment times, comparable with the conventional fraction-ation time slots.

FFF beams have been extensively investigated and characterized before their intro-duction in the clinical practice.[1]

Francisco J. Hernandez Flores (Arcispedale Santa Anna, Ferrara Italy)Radiation Therapy May 26, 2016 4 / 22

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Introduction

Flattening FilterConventional medical linear accelerators delivering photon beams are equippedwith a flattening filter (FF) in order to allow delivery of homogeneous dosedistributions with broad beams.

The differences between FFF and FF in terms of quality assurance is mainlyrelated to beam dosimetry, and not to mechanical characteristics of the linearaccelerator, for which the standard quality assurance procedures still hold.

Flattening Filter contribute to scattered, reduce dose rate, leakage from thetreatment head, beam hardening and also neutron fluence for high energy of Xray used in Linac.[2]

Francisco J. Hernandez Flores (Arcispedale Santa Anna, Ferrara Italy)Radiation Therapy May 26, 2016 5 / 22

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Flattening Filter free benefitsIncrease the dose rate and reduce treatment time small treatment time lesspatient movement.

Reduce leakage from the treatment head, they have 50% to 60% reducedcollimator and treatment head scatter.

reduced ”out of field” dose obserbed to be less than 10% at 2 cm for a 6MeVbeams FFF, up to 20% reduced neutron contamination for 18MeV Beams [2]

Flattening Filter free ProblemIon chamber and EPID saturation.

Inter leaf leakage, very high dose per pulse,

FFF can deposit dose of 1 Gy in 2.5 second inadvertent dose to criticalstructures can be dangerous in extremely short time so therapist and patientmust be educated (24 Gy/min)

Francisco J. Hernandez Flores (Arcispedale Santa Anna, Ferrara Italy)Radiation Therapy May 26, 2016 6 / 22

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Flattening Filter free benefitsIncrease the dose rate and reduce treatment time small treatment time lesspatient movement.

Reduce leakage from the treatment head, they have 50% to 60% reducedcollimator and treatment head scatter.

reduced ”out of field” dose obserbed to be less than 10% at 2 cm for a 6MeVbeams FFF, up to 20% reduced neutron contamination for 18MeV Beams [2]

Flattening Filter free ProblemIon chamber and EPID saturation.

Inter leaf leakage, very high dose per pulse,

FFF can deposit dose of 1 Gy in 2.5 second inadvertent dose to criticalstructures can be dangerous in extremely short time so therapist and patientmust be educated (24 Gy/min)

Francisco J. Hernandez Flores (Arcispedale Santa Anna, Ferrara Italy)Radiation Therapy May 26, 2016 6 / 22

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Quality Assurance of FFF beams

Flattening Filter Free (FFF)FFF beams are used in the linac without FF in place of carousel. FFF delivered withconventional medical linear accelerator have the conical flattening filter removed andreplaced by a thin foil.[2]

This foil is introduced for two reason:

For safety. It will stop the electron beam reaching the patient if the targetcollapses.

Producing enough signal in the ion chamber by producing electrons.[1]

The main advantages of removing the flattening filter are an increased doserate, reduced scatter, reduced leakage and reduced out-of-field doses.[1]

Francisco J. Hernandez Flores (Arcispedale Santa Anna, Ferrara Italy)Radiation Therapy May 26, 2016 7 / 22

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Spectrum of two energy using FF and FFF

(a) spectrum of 6 MeV (b) spectrum of 10 MeV

Figure: Photon spectra for 6 MV FFF and 6 MV (left), and 10 MV FFF and 10 MV(Right).

Francisco J. Hernandez Flores (Arcispedale Santa Anna, Ferrara Italy)Radiation Therapy May 26, 2016 8 / 22

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Profile normalization

The inflection point

Ponisch et al [3] suggested the use of the inflection point at the field edge torenormalize a FFF beam to the same dose level of a FF beam. From thisrenormalized profile it is then possible to evaluate penumbra and the field size.

The correct evaluation of the inflection point position is critical, being locatedby definition at the point of the highest dose gradient. [1]

The re-normalization valueThe use of a renormalization factor, compared to the inflection point procedure,allows for a location of the normalization point in a less critical position, at theprofile shoulder, where the FF and the corresponding FFF profiles start to differ,and it is located at the second maximum point of the third derivative of theprofile.

Renorm Factor =a + b ∗ FS + c ∗depth1+d ∗ FS + e ∗depth

(1)

Francisco J. Hernandez Flores (Arcispedale Santa Anna, Ferrara Italy)Radiation Therapy May 26, 2016 9 / 22

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Profile normalization

Based on the fact that FFF beams deliver higher dose to the central axis, FFF and FFbeams should be mutually renormalized to superimpose the profile fall-off. Two meth-ods can be followed: the inflection point or the renormalization value. Both methodshold only for symmetric beams.

(a) Renormalization Point (b) Infection Point

Figure: Renormalization point obtained through the profile third derivative [1]Francisco J. Hernandez Flores (Arcispedale Santa Anna, Ferrara Italy)Radiation Therapy May 26, 2016 10 / 22

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Dosimetric field size

Once the FFF beams are renormalized as above, the concept of dosimetric field sizeas the distance between the 50% dose levels can be used for FFF beams, as for FFbeams [generally the full width half maximum (FWHM) is used for standard FF beamsnormalized to 100% at central beam axis].

Alternatively, as suggested by Pnisch et al.[3] the distance between the left and rightinflection points could be used

Francisco J. Hernandez Flores (Arcispedale Santa Anna, Ferrara Italy)Radiation Therapy May 26, 2016 11 / 22

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Penumbra

For conventional flattened-beam profiles, the definition of the penumbra is based onthe 80-20% dose values. This is not applicable in the unflattened case. Therefore, thepenumbra of the unflattened profile were derived from the spatial distance betweenthe positions where the doses were 20 and 80 % of the normalized dose Dn

Dn =DuDf∗DCAX (2)

Figure: Normalization of an unflattened profile of a measured 6-MV photonbeam. [3]

Francisco J. Hernandez Flores (Arcispedale Santa Anna, Ferrara Italy)Radiation Therapy May 26, 2016 12 / 22

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Flattened region and field regionThe flatness is based on the flattened region definition, and should be applied to a”field region” in a way that it could be used for both beam modalities. Once renormal-ized as above, the ”field region” can be defined as the region within a certain definedpercentage of the field. The percentage could be the same for all field sizes, or it canbe changed.

Flatness and UnflatnessUnflatness is the parameter relative to FFF beams corresponding to flatness for FFbeams. Unflatness can be defined as the ratio between the dose level at the beamcentral axis and the dose level at a predefined distance from the central axis as afunction of field size, or at the edge of the field region.

Unflatness =Dosecentral axis

Doseoff axis(3)

Francisco J. Hernandez Flores (Arcispedale Santa Anna, Ferrara Italy)Radiation Therapy May 26, 2016 13 / 22

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Slope

The peak shape of the FFF profile can be defined by the slope parameter describingthe left and right inclinations of the profiles. Because the FFF profile depends on theenergy, with different shapes in terms of concavity or convexity of the slopes.

Slope =(x1 − x2) ∗ (y1 − y2)

(x1 − x2)2(4)

Figure: slope of profile FFF

Francisco J. Hernandez Flores (Arcispedale Santa Anna, Ferrara Italy)Radiation Therapy May 26, 2016 14 / 22

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The peak Position

The peak of the FFF profile is the indication of the forward direction of the beam.

Intuitively this peak should be located on the beam central axis. The peakposition parameter is defined as the off-axis position of the intersection point ofthe left and right slopes, as follows:

peakposition =IL − IR

SR −Sl(5)

where IL and IR are the left and right intercepts and SL and SR are the left andright slopes,

I = y2 − x2 ∗(x1 − x2) ∗ (y1 − y2)

(x1 − x2)2(6)

Francisco J. Hernandez Flores (Arcispedale Santa Anna, Ferrara Italy)Radiation Therapy May 26, 2016 15 / 22

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Symmetry

Symmetry, as a parameter checking the equality level between left and right sides of aprofile, can be defined as usual for standard FF beams, with the only difference that theevaluation area should be within the field region for FFF beams instead of the flattenedregion commonly used in FF beams.[1]

The maximum dose ratio :

(Dx

D−x

)max

(7)

The maximum Variation : (Dx −D−x)max (8)

where Dx and D−x are the doses at x and -x positions (symmetric relative to centralaxis).

The area ratio :

∣∣∣∣∣ LeftIntegral −RightIntegralLeftIntegral +RightIntegral

∣∣∣∣∣ (9)

where LeftIntegral and RightIntegral are the areas bounded by the profile on the leftand right of the beam central axis.

Francisco J. Hernandez Flores (Arcispedale Santa Anna, Ferrara Italy)Radiation Therapy May 26, 2016 16 / 22

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Energy spectrum and quality index

FFF beams present an energy spectrum significantly different from FF beamssince the thick conical shaped attenuator is removed.[1]

Despite the differences in the FFF spectrum with respect to the corresponding FFbeam, there is no reason to change quality index definitions that can be.[1]

QI = 1.2661 ∗ D20cmD10cm

−0.0595. (10)

Francisco J. Hernandez Flores (Arcispedale Santa Anna, Ferrara Italy)Radiation Therapy May 26, 2016 17 / 22

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Surface DoseDue to different electron contamination and lower photon energy spectrum, surfacedoses of FFF are expected to be different from FF beams.18 The surface dose param-eter Ds is defined here as the relative dose at d = 0.5 mm with respect to the doseat dmax .Due to different electron contamination and lower photon energy spectrum,surface doses of FFF are expected to be different from FF beams. The surface doseparameter Ds is defined here as the relative dose at d = 0.5 mm with respect to thedose at dmax.[1]

Output FactorThe head scatter component of a FFF beam relative to the corresponding FFbeam is markedly different. Variation in output factors is then less pronouncedfor FFF beams due to the head scatter component.[1]

Output factor definitions are kept identical for both FFF and FF beams.[1]

In both setup conditions the output factors of FFF fields are less spread, inparticular for in air evaluation, confirming the lower head scatter component forsuch fields.[1]

Francisco J. Hernandez Flores (Arcispedale Santa Anna, Ferrara Italy)Radiation Therapy May 26, 2016 18 / 22

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Dose RateIn FFF beams the dose rate increase in two to four times higher than standardbeams.

The common check on dose rate dependence has to be performed on the entiredose rate range, keeping the consolidated experience in use for FF beams.

For FFF beams particular attention has to be paid in the dosimetry systemchoice: the collection efficiency of ionization chambers, the possible saturationare just examples to consider for correct measurements.

Absolute dose calibrationAbsolute calibration of the beam output shall follow dedicated protocols.

There is no reason to change the reference conditions for calibration, but thereis a need for a re-evaluation of the beam quality factor (kQ ) values for FFFbeams in relation to beam quality indices, as they are not listed as clinical usedbeams.

To note that the recombination factor ks changes slightly between FFF and FFbeams, but this difference is systematic.

Francisco J. Hernandez Flores (Arcispedale Santa Anna, Ferrara Italy)Radiation Therapy May 26, 2016 19 / 22

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Dose RateIn FFF beams the dose rate increase in two to four times higher than standardbeams.

The common check on dose rate dependence has to be performed on the entiredose rate range, keeping the consolidated experience in use for FF beams.

For FFF beams particular attention has to be paid in the dosimetry systemchoice: the collection efficiency of ionization chambers, the possible saturationare just examples to consider for correct measurements.

Absolute dose calibrationAbsolute calibration of the beam output shall follow dedicated protocols.

There is no reason to change the reference conditions for calibration, but thereis a need for a re-evaluation of the beam quality factor (kQ ) values for FFFbeams in relation to beam quality indices, as they are not listed as clinical usedbeams.

To note that the recombination factor ks changes slightly between FFF and FFbeams, but this difference is systematic.

Francisco J. Hernandez Flores (Arcispedale Santa Anna, Ferrara Italy)Radiation Therapy May 26, 2016 19 / 22

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Conclusion

Removing the flattening filter improved the characteristics of the accelerator in termsof smaller penumbras especially for the 18-MV mode, reduced MLC leakage, and lessvariation in the total scatter factors.[3]

Although the FFF beams provide much high dose rate at the treatment target, the ionrecombination effect of the Farmer, PinPoint, and plane-parallel chamber in the FFFphotons is not significantly different from the flattened photons. These ion chambersare suitable in the quality assurance and exposure measurement for the FFF beamsregarding their negligible ion recombination and sufficient collection efficiency.

We have presented ideas regarding the quality controls (QC) that have to be consid-ered during the establishment of a quality assurance program (QA) when introducingFFF beams into a clinical setting.

Francisco J. Hernandez Flores (Arcispedale Santa Anna, Ferrara Italy)Radiation Therapy May 26, 2016 20 / 22

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References

A.Fogliata et al, Definition of parameters for quality assurance of flatteningfilter free (FFF) photon beams in radiation therapy, Oncology Institute ofSouthern Switzerland, 3 October 2012

Dhuruvan Viswanathan, flattening Filter Free LINAC, RPD 331 Hales , 17 August2014

Falk Pnisch, Properties of unflattened photon beams shaped by a multileafcollimator, Houston Texas 77030, 08 April 2006

JAN HRBACEK, Commissioning of the photon beams of flattening filter-free linearaccelerator and the accuracy of beam modeling using an anisotropic analyticalalgorithm, Department of Radiation Oncology, University Hospital Zurich, Zurich,Switzerland, Elsevier, 2011.

Gabriele Kragl et al, Radiation therapy with unflattened photon beams:Dosimetric accuracy of advanced dose calculation algorithms,Article fromDepartment of Radiotherapy Medical University of Vienna, 23 July 2011

Francisco J. Hernandez Flores (Arcispedale Santa Anna, Ferrara Italy)Radiation Therapy May 26, 2016 21 / 22

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Many thanks for your kind attentio

n

Francisco J. Hernandez Flores (Arcispedale Santa Anna, Ferrara Italy)Radiation Therapy May 26, 2016 22 / 22