MRI vs CBCT image guidance when treating lymph nodes in patients with locally advanced (LA)-NSCLC
Brown, Sean Dubec, Michael Chuter, Robert Eccles, Cynthia L Hales, Rosie Parker, J. Rodgers, John Whiteside, L. Van Herk, Marcel
Brown, Sean
Dubec, Michael
Chuter, Robert
Eccles, Cynthia L
Hales, Rosie
Parker, J.
Rodgers, John
Whiteside, L.
Van Herk, Marcel
Citations
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Abstract
Purpose or Objective
CBCT is routinely used to guide curative radiotherapy (RT)
for lung cancer. This is limited by poor soft tissue
discrimination. Consequently anatomical surrogates for
lymph nodes (LNs) are used to facilitate registration and
generous PTV margins are needed. MRI can improve soft
tissue discrimination, and may allow a more accurate setup.
The aim of this study was to compare MRI to CBCT for
the identification and localisation of LNs in patients
undergoing curative RT for LA-NSCLC.
Material and Methods
Four experienced RTTs rigidly registered the outlined LNs
on planning CT (pCT) with the CBCT and MR images of 10
patients with LA-NSCLC. MR and CBCT images were
acquired 2 weeks into RT. The MR sequences used were:
T2 turbo spin echo (TSE) non-fat saturated (Seq1) and T1
DIXON water (Seq2). Observers were asked to localise each
LN on the study image and register this to the LN contour
on the pCT (study images had been offset in advance by
5cm in the X, Y and Z planes). The resulting X, Y, Z
translations were recorded. Observers then rated their
confidence in performing the registration and the quality
of the image in permitting visualisation of the LN, with a
view to contouring (Fig 1). The process was repeated for
each individual LN using all study images. For each LN,
translations from all observers were reviewed for each
plane and the standard deviation (σ) was calculated. The
σ from all planes were used to calculate the vector value
(σV). The mean σV for each image modality was compared
to assess inter-observer variation.
Results 24 LNs distributed between hilar (station 10-11) and
mediastinal regions (1-7) were assessed. Fig 2 shows the
distribution of individual LN σV values for each imaging
modality. The mean σV for CBCT was not significantly
different than either MR Seq1 (0.22 vs 0.27, p= 0.09) or
Seq2 (0.22 vs 0.20, p= 0.47). However the mean σV for Seq
1 was significantly greater than Seq2 (0.27 vs 0.20, p=
0.04). Compared to CBCT the mean confidence scores for
Seq1 (2.5 vs 1.8, p<0.001) and Seq2 (2.7 vs 1.8, p<0.001)
were both significantly better. Seq2 had a significantly
greater mean score than Seq1 (p=0.041). Regarding image quality, compared to CBCT, both Seq1 (2.66 vs 1.75,
p<0.001) and Seq2 (2.74 vs 1.75, p<0.001) had significantly
greater scores. No difference was seen between Seq1 and
Seq2 (p=0.5).
Conclusion
Observers scored the image quality of MR higher than CBCT
and had significantly greater confidence using MR to
perform direct LN registrations. T1 DIXON outperformed
the T2 TSE MR sequence in providing more accurate
registrations. The T2 TSE shows fat and LNs as bright
contrast. This might make it difficult to differentiate the
edges of mediastinal LNs, whereas the T1 DIXON provides
a fat suppressed image which may aid LN identification in
this scenario. However no difference was seen between MR
and CBCT.. This work suggests MR may be useful in
increasing the accuracy for set-up correction during RT for
LA NSCLC. Further evaluation of its potential role is
needed.
Description
Date
2020
Publisher
Collections
Keywords
Type
Meetings and Proceedings
Citation
Brown S, Dubec M, Chuter R, Eccles C, Hales R, Parker J, et al. PD-0673: MRI vs CBCT image guidance when treating lymph nodes in patients with locally advanced (LA)-NSCLC. Radiotherapy and Oncology . 2020 Nov;152:S374.