Baylor University Medical Center Proceedings July 2017 - 276

Clinical experience with machine log file software
for volumetric-modulated arc therapy techniques
Luis Alberto Vazquez-Quino, PhD, Claudia Ivette Huerta-Hernandez, PhD, and Dharanipathy Rangaraj, PhD

Mobius FX, an add-on software module from Mobius Medical
Systems™ for intensity-modulated radiation therapy (IMRT) quality
assurance (QA), uses linac treatment logs to efficiently calculate and
verify the 3D dose delivered to patients. An advantage of the Mobius
FX module is that it does not require device positioning. In this study,
we compared the Mobius FX with another QA option, ArcCheck, as
well as with the treatment planning system (TPS) using 30 volumetricmodulated arc therapy (VMAT) plans planned and delivered on a Varian
TrueBeam linac. The plans, which involved 6 and 10 MV and consisted
of 2 to 3 arcs per plan, were selected to provide a clinically relevant
sample. The average gamma value for all plans between Mobius FX
and the TPS was 99.96% for the criterion of 3%-3 mm and 98.80%
for the criterion of 2%-2 mm. Very similar results were found when
comparing Mobius FX and the TPS dose calculations with those acquired by traditional methods (i.e., ArcCheck). As the gamma criterion
of the analysis was narrowed, discrepancies between Mobius FX and
traditional methods appeared. Profile analysis showed the production
of comparable results when using the Mobius FX method or traditional
QA methods. In conclusion, the Mobius FX method for pretreatment
of patient-specific QA is capable of producing results similar to those
obtained by traditional methods.

ith the introduction of intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc
therapy (VMAT) techniques, the complexity of the
treatment plan in radiation oncology has increased
along with the need for patient-speciﬁc pretreatment quality
assurance (QA) measurements. The need for fast and reliable
methods has created a wide variety of available solutions for
IMRT QA, including ion chamber, diode array, electronic portal
imaging device (EPID), ﬁlm, and radio-chromic ﬁlm measurements, which have been extensively studied and are widely
accepted (1-15). The Mobius FX solution (Mobius Medical
Systems, Houston, TX) for IMRT QA uses linac treatment logs
to calculate and verify the 3D dose delivered. This approach is
attractive since it does not require ion chamber, diode array,
EPID, ﬁlm, or any external measurements and thus should be
faster since no device needs to be positioned. In this study, we
compared Mobius FX with ArcCheck (Sun Nuclear Corporation, Melbourne, FL), the standard QA tool used for IMRT

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that measures the actual dose or ﬂuence by means of a physical
method. ArcCheck is a 4D measurement array that consists of
1386 SunPoint® diode detectors (0.019 mm3) arranged on a
helical detector grid. This arrangement is specially designed for
rotational deliveries. ArcCheck has proven to be a very reliable
method for IMRT and VMAT patient-speciﬁc pretreatment
QA (5, 6).
METHODS
In this study, 30 VMAT plans were planned and delivered
using a Varian TrueBeam linac (Varian Medical Systems, Palo
Alto, CA). The plans were randomly selected and included
diﬀerent treatment sites: lung, pelvis, brain, and head and neck.
The plans included in the research were planned with 6 and
10 MV and consisted of 2 to 3 arcs per plan.
All VMAT plans were delivered and measured at the same
time with ArcCheck (Figure 1) and Mobius to ensure consistency in the data. The linac log ﬁles were acquired simultaneously, taking advantage of the Mobius FX method allowing
both measurements to be made at the same time. The ArcCheck
was calibrated at the beginning of the measurements to ensure
dose calibration uniformity using an open ﬁeld and a known
number of monitor units for a known dose. The Eclipse dose
calculations were exported to Mobius before the start of the
measurements so that Mobius FX could record and analyze the
delivery occurring during the ArcCheck measurements.
Mobius FX uses the Mobius 3D dose calculation algorithm
based on log ﬁles recorded from the linac during the IMRT QA,
and it goes one step further by calculating the dose delivered to
the patient for each fraction (Figure 2a). In addition, it provides
a detailed report that includes isodose, gamma analysis, dose
volume histogram (Figure 2b), and root mean square (RMS)
analysis for each fraction of the patient treatment. Mobius 3D
uses a collapse cone algorithm that has proven to be a highly
accurate dose calculation engine (16, 17).
From the Radiation Oncology Department, Baylor Scott & White Health and Texas
A&M Health Science Center College of Medicine, Temple, Texas.
Corresponding author: Luis Alberto Vazquez-Quino, PhD, DABR, Department
of Radiation Oncology Physics, Baylor Scott & White Health, Texas A&M Health
Science Center College of Medicine, 2401 S. 31st Street, Temple, TX 76508
(e-mail: Luis.Vazquez@BSWHealth.org).
Proc (Bayl Univ Med Cent) 2017;30(3):276-279

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