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energy of the Cobalt source compared to those of clinical
X-ray beam lead to increase of the surface dose.
A previous study (SIMBAD 01) investigated the feasibility
of comparative planning among different radiotherapy
technologies (i.e: IMRT vs VMAT vs MRI-
60
Co) optimizing for
target coverage and organs at risk sparing. Hardware and
software implementation on MRIdian will be completed at
our institution within February 2017. Experimental
measurements are planned when the system will become
clinically operative. Aim of the present analysis is to
quantify the dose delivered to subcutaneous tissues in the
SIMBAD 01 planning conditions (without specific
optimization for subcutaneous tissues).
Material and Methods
Ten patients affected by locally advanced rectal cancer
(LARC) were included in this study. For each patient a
VMAT RapidArc, a 5-beams sliding window IMRT and one
MRIdian treatment plan were performed.
All treatment plans were calculated according to the
Quality Assurance protocols adopted in our Institution: the
PTV1 was represented by tumor and corresponding
mesorectum; the PTV2 by mesorectum in toto and pelvic
nodes. Isotropic 0.7 cm margins were added to PTVs. The
total prescribed dose for PTV1 was 55 Gy and 45 Gy for
PTV2 through Simultaneous Integrated Boost.
All plans were optimized for PTV coverage and sparing of
bowel bag and bladder.PTV coverage was evaluated by
calculating the V95 and V105 values. For bowel bag V45
and for bladder the mean dose was considered.
The presence of magnetic field has been taken into
account during MRIdian MRI-
60
Co planning procedures..
The evaluation of subcutaneous skin dose was obtained
calculating the median dose in a 5 mm wide ring dummy
structure contoured 3 to 8 mm far from body surface.
The first 3 mm from body surface were not taken into
account to avoid inconsistencies related to uncertainties
in dose calculation at air-body interface due to different
dose calculation algorithms (AAA Collapse Cone for VMAT
and IMRT treatments, Montecarlo for MRIdian TPS).
Results
All plans optimized for this study satisfied the constraints
on PTV coverage and organs at risk sparing. Dosimetric
values obtained in this planning comparison are listed in
table 1.
The median skin dose resulted higher using the MRIdian
system (1.97 Gy against 1 Gy for IMRT and 0.98 Gy for
VMAT)
Conclusion
A subcutaneous skin dose increase is observed with the
employment of MRI-
60
Co RT when compared to LINAC
treatment planning, even if still in clinically acceptable
constraints. Considering and contouring such a structure
seems useful. Specific study will be performed to define
how reducing dose to subcutaneous tissues by optimizing
for
such
structures.
EP-1267 In silico Evaluation of the impact of Magnetic
Field on dose distribution using of MRIdian MRI- 60Co
D. Cusumano
1
, L. Boldrini
2
, L. Azario
3
, S. Teodoli
1
, M.
Balducci
4
, G.C. Mattiucci
4
, S. Chiesa
2
, G. Chiloiro
2
, N.
Dinapoli
2
, M.A. Gambacorta
4
, C. Masciocchi
5
, D. Piccari
6
,
M. Rapisarda
6
, M.V. Antonelli
6
, M. Ferro
2
, E. Placidi
1
, A.
Piermattei
3
, F. Cellini
2
, V. Valentini
4
1
Fondazione Policlinico A. Gemelli, UOC Fisica Sanitaria -
Gemelli ART - Radiation Oncology, Rome, Italy
2
Fondazione Policlinico A. Gemelli, Gemelli ART -
Radiation Oncology, Rome, Italy
3
Università Cattolica del Sacro Cuore, Rome, Italy
4
Università Cattolica del Sacro Cuore, Gemelli ART -
Radiation Oncology, Rome, Italy
5
Fondazione Policlinico A. Gemelli, KBO Labs - Gemelli
ART - Radiation Oncology, Rome, Italy
6
Fondazione Policlinico A. Gemelli, TSRM - Gemelli ART -
Radiation Oncology, Rome, Italy
Purpose or Objective
The MRIdian MRI
60
Co radiotherapy system (ViewRay,
Oakwood, Ohio) combines an open split-solenoid MRI
scanner equipped for parallel imaging and three
60
Co
gamma-ray sources.
The quantification of dose distribution perturbations due
to the presence of 0.35 T magnetic field represents an
issue.
The MRIdian TPS is equipped by two Montecarlo based
algorithms to calculate the dose distribution: the first one
has faster calculation time; and does not account for the
presence of magnetic field (B
off
). The second one has
slower calculation time; and takes into account for the
presence of magnetic field (B
on
). Hardware and software
implementation on MRIdian will be completed at our
institution within February 2017. Experimental
measurements are planned when the system will become
clinically operative.
Aim of this study was to compare the two algorithms in
order to evaluate which could be more accurate in an
in
silico
treatment planning study designed for locally
advanced rectal cancer (LARC).
Material and Methods
This study includes 7 cases of patients affected by LARC.
For each patient of the study two plans were developed.
Same priority values for the optimization were applied.
The calculation of dose distribution was performed using
the two different algorithms object of this study (B
off
and
B
on
).
Plans were performed in IMRT modality, adopting same
beams geometry consisting in one pseudo-arc composed
by three beam triplets.
The treatment plans were optimized according to usual
Quality Assurance protocols adopted in our Institution for
Linac IMRT treatments: the PTV1 was represented by
tumor and corresponding mesorectum; the PTV2 by
mesorectum in toto and pelvic nodes. Isotropic 0.7 cm
margins were added to PTVs. The total prescribed dose for
PTV1 was 55 Gy and 45 Gy for PTV2 through Simultaneous
Integrated Boost.
All plans were optimized for PTV coverage and sparing of
bowel bag and bladder. For PTVs coverage V95 and V105
were considered. For bowel bag V45 and for bladder the
mean dose were considered, respectively. Plans were
normalized at target median.
Results
Table 1 summarizes the median values for PTV coverage
and organs at risk sparing obtained in the two cases. No
significant differences have been reported between the
two algorithms.