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ESTRO 36
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Results
In initial plans, the mean calculated dose in parotid glands
ranged from 24.1 to 26.2 Gy. There is significant variation
in parotid contouring. The degree of variation varied from
patient to patient, with maximum differences up to 23%
in mean dose to parotid glands. Maximum differences in
mean dose to parotid glands due to the uncorrected setup-
shifts was up to 12%.
Conclusion
Intraobserver variability in contouring of OARs and daily
variations in patient setup are significant contributors to
uncertainty in radiotherapy treatment planning, and
consequently in delivered dose. Our analysis indicates that
the not-precise contouring can lead to larger difference
between delivered and calculated dose.
EP-1647 Validation of a set up procedure for
IMRT/VMAT breast treatment using in vivo dosimetry
with EPID
S. Kang
1
, J. Li
1
, P. WANG
1
, X. Liao
1
, M. Xiao
1
, B. Tang
1
, X.
Xin
1
, L.C. Orlandini
1
1
Sichuan Cancer Hospital, Radiation Oncology, Chengdu,
China
Purpose or Objective
In vivo dosimetry (IVD) is an important tool able to verify
the accuracy of the treatment delivered and its
reproducibility. The change of a consolidated existing
procedure can be performed if an indisputable evidence
of improvement may be proved and the feasibility in a
clinical workflow is guarantee. In this work IVD performed
with electronic portal imaging device (EPID) was used to
evaluate VMAT and IMRT breast treatment performed with
a new set up and immobilization procedure.
Material and
Methods
IVD with EPID was performed over 32 patients that
underwent an IMRT or VMAT breast plus supraclavicular
treatment. Half of the patients followed the standard set
up procedure (SP) of the department, consisting of a
thermoplastic mask covering the district to be treated,
patient marks over the mask, bolus applied over the mask;
the others followed a new procedure (NP) and were
immobilized with a breast board and a knee support,
patient marks over the skin and bolus applied over the
skin. The accuracy of the treatment was evaluated with a
commercial software (SOFDISO, Best Medical Italy) that
provided two indexes: the ratio R between the
reconstructed (D
iso
) and planned (D
tps
) isocenter dose
(R=D
iso
/D
tps
) which can represent the accuracy of the dose
delivered, and a Pγ% obtained performing a gamma
analysis between the first EPID image and the next ones
acquired immobilized. Three consecutives tests were
scheduled during the first week of treatment and
successively two IVD test per week. The MLC log files of
the treatments delivered where analysed with a
commercial software and compared with the planned
treatment in order to discriminate the deviation coming
from the patient (anatomy and set up), from the deviation
coming from the linac.
Results
Only the IVD test coming from a delivery with the
machines log file in tolerance were considered. The
results of 545 IVD tests obtained over 32 patients were
reported in Table 1. Every treatment performed with IMRT
and VMAT resulted in 100% of the patients with R and Pγ
indexes in tolerance as for SP as for NP. The percentage
of Pγ index in tolerance as for VMAT as for IMRT increased
with NP. A 10% of, off tolerance tests persisted. The IVD
tests off tolerance were reported in the acceptable
threshold before the next fraction.
Discussion:
the new
immobilization procedure enabled a direct localization of
the patient skin and of the bolus positioned over it. The
use of the mould mask, positioned over the patient’s lead
to a non-direct evaluation of the patient rotation and
accommodation inside it. The beam can lack of
reproducibility if considering its path: air gap between the
bolus, the mask and the patient skin not considered in the
treatment planning. This aspect is moreover important for
IMRT treatment where for some beam entry this situation
can be more evident.
Conclusion
IVD in is a powerful tool that can be helpful in the
validation of new set up and immobilization procedures.
EP-1648 Thermoplastic mask dependency with
interfractional uncertainties for head and neck VMAT
treatments
E.M. Ambroa Rey
1
, R. Gómez Pardos
1
, D. Navarro
Giménez
1
, A. Ramirez Muñoz
1
, M. Colomer Truyols
1
1
Consorci Sanitari de Terrassa, Medical Physics Unit-
Radiation Oncology Department, Terrassa, Spain
Purpose or Objective
Volumetric-modulated arc therapy (VMAT) techniques
have the ability to deliver a highly conformal dose
distribution to the target and high dose gradient at the
interface between the tumor and the normal tissues,
decreasing the irradiated volume and sparing OARs.
However inaccurate alignment of the radiation beam with
the patient can lead to critical organs to receive an
unwanted high dose or the tumor to receive a reduced
dose producing a loss in tumor control. Radiation therapy
for head-and-neck (H&N) cancer requires a reliable
immobilization
for
an
accurate
treatment.
The purpose of this study is to establish the interfractional
setup error for VMAT H&N patients, using a kilovoltage
cone beam CT (CBCT) and a robotic treatment couch
(HexaPOD) for accurate patient positioning in six degrees
of freedom and analyze the differences between two
types of thermoplastic masks (Head mask (H) and Head
and Shoulder (HS) mask).