Dosimetry in profile RT

1. Master in Medical Physics ICTP 2015-2016
Measure profile in Photon beam
Francisco J.Hernández Flores∗
International Centre for Theoretical Physics
franciscohernandez_f2010@hotmail.com
August 31, 2015
Abstract
In this task we measured the beam profile these are measured at multiple points on a
plane perpendicular to the central beam axis. The measurement was performed in a water
phantom using a cylindrical ionisation chamber. A beam profile can be one dimensional
(along one axis) or two dimensional (measuring in the x and y axes)using cylindrical
ionization chamber inside de water for scanner the data in different depth moving the
chamber in direction of x or y, the chamber is handled by mephisto program.
I. Introduction
The measure of beam profile is The variation of dose occurring on a line perpendicular
to the central beam axis at a certain depth is known as the beam profile. It represents
how dose is altered at points away from the central beam axis. There are typically three
parts:
• The central region which is usually flat and includes doses over 80% of the central
beam axis.
• The penumbra region where dose falls off rapidly at the beam edge, between a
dose of 20-80% of the central beam axis [2]
• The penumbra region where dose is minimal (under 20% of the central beam
dose)
Other distinctive features are the lateral horns, which are only present in photon
beams (and more pronounced for 18 MV photons). The lateral horns are caused by
the flattening filter, which aims for a flat dose at a particular depth. To create this, at
depths above this point the beam has horns of higher dose [1]
II. Material and method
We used Linear accelerator, water phantom 3D, two cylindrical ionization chamber one
of them for reference and the other is used for measured, mephisto program.
I) Align the water phantom the center of the phantom with the reticle of the linac.
II) Fill the water phantom.
∗Radiotherapy Practical 3
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2. Master in Medical Physics ICTP 2015-2016
III) Put the both chamber, in corresponding position.
IV) The chamber ionization is positioned at centre of the field and it was placed in
effective point of measurement, after verify the limit of movement the chamber in
direction the axis ± X and ± Y.
V) Start to measure of the profile.
During the measure teacher was explain which is the procedure of calculate the flatness,
symmetry and penumbra and which is the effect of the flat filter in hight dose and how
disappears the horns with the depth.
III. Analysis and discussion of results
The results of profile for the beam 10x10 cm is show in graph 1 for red line was measure
at 5 cm depth, green line at 10 and blue line at 20 cm depth. how can see in this curves
the behaviour of flatness improve with the depth, This is attributed to an increase in
scatter to primary dose ratio with increasing depth and decreasing incident photon
energy off axis
Figure 1: Beam profile for beam 10x10 cm to different depth
IV. Conclusion
• For radiotherapy treatments with photon beams it is very important to get the
beam profile as with this we realize the behaviour of the dosage as we move to
the edges of the field and know how to influence the dose when calculated out of
central axis.
• The flatness of photon beams is extremely sensitive to change in energy of the
incident beam. A small change in the penetrative quality of a photon beam results
in very large change in beam flatness.
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3. Master in Medical Physics ICTP 2015-2016
References
[1] Mara Severgnini ,Lecture of Oncology 2, ICTP, 2015.
[2] Rossela Vidimari,Lecture of Exercise Radiation Oncology 1, ICTP, 2015.
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