Plan verification

1. Master in Medical Physics ICTP 2015-2016
Radiotherapy treatment verification with Octavius phantom
Francisco J.Hernández Flores∗
International Centre for Theoretical Physics
franciscohernandez_f2010@hotmail.com
August 31, 2015
Abstract
The present task is mainly focused on the verification of the calculated dose. During a
radiotherapy treatment, a dosimetric verification or a geometric localization can be done,
in order to assess the quality of the treatment. The dosimetric verification is generally
performed measuring the dose at some points inside or outside the patient, and comparing
it to the dose at the same points calculated and predicted by the treatment planning system.
This was evaluated with phantom Octavius 4D. Dose distributions were calculated for
volumetric modulated arc therapy (VMAT) treatment plans with Monaco the results of
gamma analysis was 98% passing criteria.
I. Introduction
Volumetric Modulated Arc Therapy (VMAT) is a new intensity-modulated radiation
therapy (IMRT) technique that improves critical structures and healthy tissue sparing,
in dramatically shorter treatment times all without comprising target coverage and
patient safety [2]
when advanced techniques used in radiotherapy and to finalize a plan of VMAT
in the TPS is mandatory dosimetric verification before clinical delivery using diodes
or ionization chambers, in our case we used the Octavius phantom to check the
effectiveness of the plan dose delivered to the patient and compare the passes of the
plan take into account the gamma factor and DTA.
OCTAVIUS 4D is a new 4D dosimetry system designed to verify IMRT treatment
plans based on true independent measurements. It rotates with the gantry, measuring
the dose inside the entire phantom volume, always per pendicular to the incident
beam. As a consequence, OCTAVIUS 4D requires no angular corrections or detector
calibrations to compensate for the directional response of its detectors which makes it
perfectly suited for rotational delivery techniques and easy-to-use in clinical routine. [1]
II. Theory
Distance To Agreement (DTA) is the distance between a measured data point and the
nearest point in the calculated dose distribution that exhibits the same dose [2].
The gamma function evaluates the difference between measured and calculated
dose distribution using both dose and spatial coordinates [3].
Γ(rm, rc) =
r2(rm, rc)
∆d2
m
+
δ2(rm, rc)
∆D2
m
(1)
∗Radiotherapy Practical 2
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2. Master in Medical Physics ICTP 2015-2016
with δ(rm, rc) = Dm − Dc; r(rm, rc) = |rm, rc|
γ(rm) = min{Γ(rm, rc)}∀{rc} (2)
γ(rm) = min
r2(rm, rc)
∆d2
m
+
δ2(rm, rc)
∆D2
m
∀rc (3)
γ(rm) function, the minimum generalized Γ(rm, rc) function in the set of evaluated
points. ∆dm and ∆Dm the acceptance criteria for distance to dose r(rm, rc) is the distance
between the measured point rm and a point rc chosen in calculated dose distribution δ
is the corresponding dose difference. As a general rule, acceptance criteria of 3% dose
difference and 3 mm distance to agreement. The dose-difference criterion is ∆Dm , and
the DTA criterion is ∆dm.
III. Material and method
Verification of Treatment planning with dose measure using Treatment Planning sistem
Monaco, red mosaiq, phantom Octavius 4D (see fig. 1) with respective software,
motorized cylindrical phantom specially designed for rotational delivery techniques,
includes control unit and wireless inclinometer.
The plan used in this practice was calculated, teacher only explain how she made
the plan because the practice is only verification plan, she told us how is the procedure
for align the phantom and connected all the tools, the software use the equation 3 to
calculate the gamma factor for the quantitative evaluation of dose distributions.
Figure 1: Phantom Octavius 4D
IV. Analysis and discussion of results
The rsult obtained during the verification planning treatment was 98% passing criteria
based in clinical standard for photon beam. This plan was analyzed using the two
parameter DTA (limit 3%), (limit 3 mm), and index with the 3% dose tolerance and
3 mm distance to agreement in relation to the treatment planning system. The range
of acceptant pass of the gamma criterion depend anatomical site, Results confirmed a
good agreement between the two distribution with high and conformed dose to the
target and low dose to the organ at risk.
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3. Master in Medical Physics ICTP 2015-2016
V. Conclusion
• Clinical quality assurance practice based on linac treatment log files for verifi-
cation of delivered 3D dose distributions in the patients geometries represents
a paradigm shift from dose measurements in a phantom. This approach offers
independent verification of VMAT dose distributions encompassing treatment
planning beam modeling differences as well as the linac uncertainties during
treatment delivery of the plan.
• Verification of a planned dose distribution is a complex and time consuming
procedure because all described parameters DTA and γ-index are very important
and using only one of them is not enough to accept a treatment plan.
References
[1] Elekta Manual of the phantom Octavius 4D,Viena 2000
[2] Daniel A. Low, William B. Harms, et al A technique for the quantitative evaluation
of dose distributions, Division of Radiation Oncology, 510 South Kingshighway Blvd.
[3] Michele Avanzo, Lecture of Oncology 3, ICTP, 2015.
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