ESTRO 36 Abstract Book

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S145 ESTRO 36 2017
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on MRI are potentially smaller than CT-based volumes,
which could lead to lower dose to organs at risk (OARs)
and, in turn, reduction of RT-induced toxicity. The
purpose of this study is to ascertain potential reduction in
target volume and OAR dose.
Material and Methods
23 breast cancer (cTis-3N0M0) patients from the MILANO
trial (NL50046.041.14) were scanned in supine position on
1.5 T, arms abducted, after SN biopsy and breast-
conserving surgery. MRI included a 3-dimensional (3D) T1-
weighted (T1w) spoiled gradient echo (T1-SPGR)
anatomical scan and two T2w fast spin echo (FSE)
techniques for LN detection, which were co-registered.
Axillary levels were delineated, using ESTRO guidelines
[Offersen et al. 2015, IJROBP], as well as OARs, including
the lungs, heart, chest wall (CW), brachial plexus (BP),
and humeral head (HH). LNs were identified by 4
observers, and delineated. Encompassing LN volumes – and
after 5 mm isotropic expansion of the LNs – were related
to axillary levels. In 5 patients (17–26 LNs), elective RT of
16 x 2.66 Gy = 42.56 Gy, delivered by 13 intensity-
modulated RT beams, was simulated on MRI for two
situations: (i) axillary levels I-IV, and (ii) all individual LN-
based targets (1 mm PTV margin). For this, pseudo-CT
scans were generated by bulk assignment of Hounsfield
units on MRI for water, lungs and air. OAR dose parameters
in both (i) and (ii) were compared.
Results
A median of 26 axillary LNs were delineated per patient.
Compared to the respective axillary levels, LN-based
target volumes, even after 5 mm isotropic expansion, are
considerably smaller [table 1]. Coverage of all targets was
excellent (V95% > 99%, V107% = 0; all PTVs) in (i) and (ii). For
elective RT on LN-based PTVs [figure 1], dose to all OARs
was substantially reduced compared to standard elective
RT: the average reduction of mean dose to lungs, heart,
and HH was 2.3 Gy, 2.2 Gy, and 13.3 Gy, respectively;
reduction of maximum dose to the BP and CW was 25.5 Gy
and 9.4 Gy.
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. Seregni1, C. Paganelli1, J. Kipritidis2, G. Baroni1,3, M.
Riboldi1
1Politecnico di Milano University, Dipartimento di
Elettronica- Informazione e Bioingegneria, Milano, Italy
2University of Sydney, Radiation Physics Laboratory-
Sydney Medical School, Sydney, Australia
3Centro 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 anatomy1. However,
out-of-plane motion (OOPM) cannot be measured and it
could affect the accuracy of 2D-2D registration