160 / 1096
S147
ESTRO 36
_______________________________________________________________________________________________
Conclusion
LN-based target volumes on MRI are considerably smaller
than axillary levels, conventionally delineated on CT
according to the ESTRO guidelines. Addition of dedicated
MRI in regional RT planning leads to reduced OAR dose,
and a potential reduced RT-associated toxicity for breast
cancer patients
.
In the near future, this will be
investigated for more patients, and these results will be
available at ESTRO 36. Moreover, MR imaging of lymph
vessels is being investigated. Introduction of MRI-guided
regional RT, by direct visualization and delineation of
individual LNs and OARs, and future use on the MRL, may
reduce RT-induced toxicity.
PV-0282 Out-of-plane motion correction in orthogonal
cine-MRI registration
M. Seregni
1
, C. Paganelli
1
, J. Kipritidis
2
, G. Baroni
1,3
, M.
Riboldi
1
1
Politecnico di Milano University, Dipartimento di
Elettronica- Informazione e Bioingegneria, Milano, Italy
2
University of Sydney, Radiation Physics Laboratory-
Sydney Medical School, Sydney, Australia
3
Centro Nazionale di Adroterapia Oncologica,
Bioengineering Unit, Pavia, Italy
Purpose or Objective
Online motion monitoring in MRI-guided treatments
currently relies on the acquisition of 2D cine-MRI images
that are registered to the planning anatomy
1
. However,
out-of-plane motion (OOPM) cannot be measured and it
could affect the accuracy of 2D-2D registration
algorithms. This work investigates the feasibility of a-
priori estimation and correction of OOPM.
Material and Methods
Data from a thoraco-abdominal numeric MRI phantom
developed in-house were used
2
. A 10-phases 4DMRI,
simulating the planning dataset, was registered to the
exhale volume using 3D optical flow
3
, thus measuring in-
plane motion (IPM
3D
P
) and OOPM
P
along the three
orthogonal slices intersecting in the GTV. In addition,
IPM
2D
P
was obtained with 2D slice-to-slice optical flow
3
registration and the difference C = IPM
3D
P
− IPM
2D
P
represented the phase-specific a-priori correction.
A 36-frames volume sequence (duration 5.4s) represented
treatment data: sagittal/coronal/axial slices simulated
cine-MRI sequences, whereas 3D volumes served as
ground-truth. The diaphragm position measured on each
sagittal slice was used to identify the corresponding
breathing phase within the 4DMRI. Each axial and coronal
slice of the sequence was registered to the corresponding
exhale slices of the 4DMRI (IPM
2D
T
) and the phase-specific
correction was applied (IPM
COR
T
= IPM
2D
T
+ C). The average
end-point distances (EPD) against ground-truth IPM
(obtained through 3D registration) were measured with
and without correction. OOPM was estimated for each
frame as OOPM
P
measured in the corresponding 4DMRI
phase. Finally, the planning GTV was propagated from the
4DMRI exhale phase to each treatment frame using: (1)
IPM
2D
T
with OOPM = 0 and (2) IPM
COR
T
combined with
OOPM
P
. Dice indexes against ground-truth GTVs were
calculated for both scenarios. The sagittal slice, showing
OOPM < 1 mm, was excluded from the analysis.
Results
GTV motion amplitude was (4.0, 1.7, 0.2) mm (SI, AP, LR)
in the 4DMRI and (5.1, 1.2, 0.6) mm in treatment data.
Fig.1 reports EPDs and Dice indexes as a function of the
ground-truth OOPM. On average, the a-priori
correction/estimation approach resulted in EPD reduction
and in Dice index increase with respect to the scenario
without IPM correction and OOPM estimation (Tab.1).