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S146
ESTRO 36
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radiation oncologist and a resident radiation oncologist on
all repeat CT scans and consensus was reached. The
treatment plan of application one was projected on the
repeat CT scans to simulate the other applications.
Projected treatment plans were categorized as clinically
acceptable or unacceptable. Additionally, new treatment
plans were derived from the repeat CT scans by an
experienced treatment planner. A conformity index,
taking into account CTV coverage and dose to organs at
risk, was used to quantify conformity of both the
projected and the repeated treatment plans. Dose
distributions were scaled to a prescription dose of 7 Gy.
Using the Wilcoxon signed rank test, the conformity index
and cumulative CTV D98 of the projected and repeated
treatment plans were compared.
Results
Fourteen out of 22 projections were clinically
unacceptable. In 8 of those 14 projections, replanning was
of added value. In the remaining 6 unacceptable cases,
replanning was of limited value as first an intervention
would have been necessary to remove air and/or faeces.
The figure shows a repeat CT with an unacceptable
projection and corresponding replanning. The table
summarizes the conformity index and cumulative CTV D98
of the non-adaptive and the adaptive approach.
Parameters are presented both for all cases and for all
cases excluding those that needed an intervention. Repeat
CT-based adaptive HDR-BT resulted in a significantly
higher conformity.
Conclusion
Repeat CT-based adaptive HDR-BT resulted in a more
conformal treatment and should be standard practice in
radical treatment with HDR-BT in rectal cancer patients.
Poster Viewing : Session 6: Imaging
PV-0281 Lymph node MRI in regional breast
radiotherapy leads to smaller target volumes and lower
OAR dose
T. Van Heijst
1
, H.J.G.D. Van den Bongard
1
, N. Hoekstra
1
,
M.E.P. Philippens
1
, D. Eschbach
1
, J.J.W. Lagendijk
1
, B.
Van Asselen
1
1
UMC Utrecht, Radiotherapy, Utrecht, The Netherlands
Purpose or Objective
Elective axillary regional radiotherapy (RT) in breast
cancer patients is performed with RT-planning CT scans,
using delineation guidelines based on anatomical
boundaries. In contrast to CT, MRI can directly image
axillary lymph nodes (LNs) in RT position [van Heijst
et al.
2016,
BJR
]. Our MRI linac (MRL) system is designed to be
able to treat those LNs precisely. LN-based target volumes
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 (
V
95%
> 99%,
V
107%
= 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.