ESTR

O 2

01

7

Poster presented at:

Flattening filter free beam profile analysis

using two different normalization methods

Nicolini G.1, Fogliata A.2, Vanetti E.1, Reggiori G.2, Stravato A.2, Mancosu P. 2, Scorsetti M.2,3, Cozzi L.2,3

1 Radiqa Developments, Medical Physics Team, Bellinzona, Switzerland

2 Humanitas Research Hospital, Radiotherapy and Radiosurgery Dept., Milan-Rozzano, Italy

3 Humanitas University, Dept. of Biomedical Sciences, Milan-Rozzano, Italy

Results Figure R1 shows the cax normalization value applied with RF method, IP method, and perturbed RF study for both beam energies; in

average, the two methods show an agreement within the 2%, with a tendency of a greater N with IP respect RF method for 10MV. In any case, some

outliers are present, with a discrepancy that reaches the 10%; this is expected, since the IP method suffers of the uncertainty of IP position

determination in the practice, as pointed out by the author himself (Pönish 2006). Table summarizes the comparison between the two methods.

As an example , unflatness for a 20x20cm2 field with 6FFF was 1.248 at dmax and 1.317 at 30cm depth with the RF, while within 1% with the IP

method.

For further information: Giorgia Nicolini, giorgia.nicolini@radiqa.ch

IntroductionFlattening filter free (FFF) beams present a profile peaked on the beam central axis (cax), unsuitable for flatness and symmetry description that

usually characterize standard beam profiles (FF). Definitions of unflatness and slope have been recently proposed, requiring a preliminary suitable FFF

profile normalization. Two main normalization processes as far published are: the inflection point IP (Pönish 2006, Med Phys 33:1738), and the

renormalization factor RF (Fogliata 2012, Med Phys 39:6455). In both formalisms the FFF dose fall-off at the field edge is superimposed with the

corresponding FF profile. The present study aims to compare FFF specific profile parameters using the two normalization procedures, for 6 and 10MV

FFF beams of a Varian TrueBeam.

ConclusionThe two normalization methods are both suitable for subsequent FFF profile description. Unflatness parameter resulted similar when computed using

the two different normalization formalisms with no significant differences. Slope values are more sensitive to normalization value, and therefore some

outliers were observed due to uncertainty of IP position in the practice. The RF procedure, with the published fitting parameters is easier to use and

more robust respect to measurements sampling and detector size.

Material and MethodsDosimetric data from a Varian TrueBeam with 6 and 10 MV nominal accelerating potentials, FF and

FFF modes have been collected at SSD =100cm and 5 depths.

The cax normalization value N was evaluated for the IP method as:

N = Dcax ∙ Du

Df

where Dcax and Df are the doses on cax and at the IP

of the penumbra region for the corresponding FF beam, Du is the dose at the IP of the FFF beam.

The N value for the RF method, based on the third derivative approach, was evaluated by using

the fit dependent on the field size FS and depth:

𝑁 =𝑎 + 𝑏 ∙ 𝐹𝑆 + 𝑐 ∙ 𝑑𝑒𝑝𝑡ℎ

1 + 𝑑 ∙ 𝐹𝑆 + 𝑒 ∙ 𝑑𝑒𝑝𝑡ℎ

where a, b, c, d and e are the fitting parameters,

here taken from the published data (Fogliata 2016, Med Phys 1:205).

Figure R1: Normalization values N

Once the FFF beams have been renormalized

as above, the main profile parameters of FFF

photon beams were computed: field size,

penumbra, unflatness, slope, and peak-

position parameters.

To systematically investigate the impact of the

N value variation on profile parameters, they

were recomputed with a RF value modified of

+ 1,2,3,5,7,10% (perturbed RF).

Table R1: Ratio between values derived with IP and RF methods: some statistics.

Analysis with perturbed RF values, shows that with a variation up to 10% of N value, the peak position

remains within 0.05 mm, the unflattens within 0.5% and 1% for 6FFF and 10FFF beams respectively, while

the slope has a variation almost of the same amount of N itself. On the whole range of measurements, field

size difference is within 1 mm if N variation is within 5%, while the range is [-1.7,1.4 ] mm with N -/+10%.

Penumbra values are the most sensitive parameters to N value variation.

6FFF: unflatness 6FFF: slope 10FFF: unflatness 10FFF: slope

6FFF

10FFF

EP-1469Giorgia Nicolini DOI: 10.3252/pso.eu.ESTRO36.2017

Physics track: Dose measurement and dose calculation