ESTRO 36 Abstract Book

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S780 ESTRO 36
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Purpose or Objective
VMAT treatments in radiotherapy are verified usually by
an experimental method, such as, measurement in
discrete points with an ionization chamber, radiochromic
film, EPID or other dosimetric systems. In this study we did
an analysis of the error detection capability in treatment
plans of aS1200 EPID of TrueBeam 2.0 linac together with
the Epiqa v4.0.12 dosimetric software from EPIdos. To do
this we have introduced forced errors in the treatment
plans of 5 different VMAT patients in order to test if the
dosimetric system can detect different forced errors
introduced in the treatment plans.
Material and Methods
5 different patients have been selected, a
hypofractionated prostate+vesicles treatment, a
hypofractionated prostate+vesicles+lymph nodes a head
and neck patient, a spine SBRT and finally a treatment of
the paraaortic lymph nodes. Treatment plans were
designed using 1 or 2 complete arcs. We have introduced
forced errors in the Pinnacle treatment plans using excel
home made macros. A total of 160 different plans have
been irradiated with the TrueBeam 2.0 linac and dose has
been measured with the EPID. The errors introduced were:
i) collimator angle modified 1º, 2º and 3º, ii) X-jaws
modified +2 mm, +5 mm, -2 mm and +5 mm, iii) Y-jaws
modified 5 and -5 mm, iv) gantry modified 2º in 4 control
points (CP), 1º in 8 CP, 1º in all the control points and
finally 2º in all the control points, v) in other plans total
UM, that is, total dose, was modified by 1, 2, 3, 5 and 10%,
vi) UM for 4 individual CP were modified by 10%, 10% for 8
CP and 20% for 8 CP, vii) the position of all the leafs were
modified +0.5, -0.5, +1 and -1 mm, viii) in other plans leafs
were modified in each CP in a random way with a
maximum displacement of 1, 2 and 5 mm and finally ix)
leafs were modified in a random way with a maximum
displacement of 2, 5 and 10 mm but using the same
displacement for a particular leaf in all the CP. We have
compared the dose obtained with the EPID with that
calculated by the Pinnacle TPS collapsing the VMAT plans
and using the Epiqa software. The gamma 3%/3mm,
2%/2mm and 2%/1 mm have been obtained. We have
looked also for visual differences between the dose
obtained with the EPID and that obtained with the
Pinnacle TPS.
Results
The analysis of the data shows that some errors can be
detected, such as the collimator errors, some X and Y jaws
errors, leafs with a systematic displacement, plans with a
difference in total dose of 3, 5 and 10% error. Some errors
in plans were very hard to detect or undetectable such as
that plans with different UM in some control points,
different gantry positions, total dose difference of 1 or 2%,
and random errors in the leaves in each control point.
Conclusion
The aS1200 EPID of TrueBeam 2.0 Linac plus the Epiqa
software is capable to detect errors in the irradiation of
treatment plans although some other errors are
undetectable by this system. This makes EPID and
interesting dosimetric equipment for the QA of VMAT
plans.
EP-1461 Scintillator dosimetry reveals lung tumor size
dependency of 6 MV AAA dose calculations
W. Ottosson1, P. Sibolt 1,2, C.F. Behrens1, C.E. Andersen2
1 Herlev Hospital, Radiotherapy Research Unit-
Department of Oncology, Herlev, Denmark
2 Technical University of Denmark, Radiation Physics-
Center for Nuclear Technologies, Roskilde, Denmark
Purpose or Objective
Radiotherapy for lung cancer generally has a poor
prognosis. Motion during imaging and treatment is a major
challenge, but also other factors may contribute to the
poor prognosis. One such factor is the ability of current
treatment planning systems to accurately compute
absorbed dose to tumors in the thorax region where large
heterogeneities are present. The current study was
designed to experimentally address the question: What is
the agreement between actual delivered dose and
computed dose using the Anisotropic-Analytical-Algorithm
(AAA) in Eclipse treatment planning system for a thoracic-
like geometry with tumors of different sizes? This is an
important question given the widespread use of AAA and
the changes in tumor sizes both over the course of
treatment, and from patient-to-patient.
Material and Methods
An in-house developed thoracic-like phantom, enabling
measurements of radiotherapy under well-defined
conditions, was used. The phantom has a body of PMMA
and can be filled with inserts of various materials,
including simulated spherical lung tumors made of PMMA
(ranging from 1-8 cm in diameter) which are embedded in
low-density balsa wood that simulates lung-tissue. 14
different phantom setups underwent CT scanning,
structure delineation, and treatment planning. 56
isocentric treatments of different complexity and
phantom configurations were calculated using AAA.
Treatment techniques investigated included single
conventional field technique, four-field conventional box
technique, five-field intensity-modulated radiotherapy
and dual-arc volumetric-modulated arc technique. To
perform accurate dosimetry under these non-reference
conditions, point measurements were carried out using
water-equivalent, organic plastic scintillator detectors
(PSDs), positioned in the center of the PMMA tumors. Dose
differences between measurements and AAA calculations
were calculated.
Results
Considerable tumor-size dependence was observed. For
tumor sizes ≤ 2 cm, the dose deviations between AAA
calculations and PSD measurements were 7.4±1.8%
(median ± 1SD). For larger tumor sizes (3-8 cm in
diameter) corresponding dose deviations were 4.2±1.4%.
For the most homogeneous setup, the dose deviations
were insignificant (0.3±0.6%). The results were essentially
independent of treatment technique.
Conclusion
This study suggests a systematic tumor-size dependent
dose calculation error for treatment planning on small
tumor sizes in heterogeneous setups. This may originate
from imperfections in the AAA algorithm. The largest dose
deviations were observed for the smallest tumor sizes.
Although, it is well known that AAA has issues in
heterogeneous regions, we are not aware of any previous
experimental study demonstrating a similar systematic
tumor-size effect. The effect is large enough to
potentially have implications for lung cancer treatment
planning. Monte Carlo simulations are currently being
conducted in order to verify these findings.
EP-1462 The impact on VMAT optimization using Type
C vs B algorithms for patients with temporary gas
pockets
B. Smulders1, J. Thomsen1, P. Munck Af Rosenschöld1
1Rigshospitalet, Department of Oncology, Copenhagen,
Denmark
Purpose or Objective
In our clinic, we have introduced a type C (i.e. Monte
Carlo-like) dose calculation algorithm and dose to medium
as standard practice. Previous work has shown difference
between type B and type C calculation algorithms for dose
calculation in high and very low density areas. However,
little attention has been given to study the robustness of
the treatment plans optimized using type C algorithms
during the course of radiotherapy. In particular, the initial