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S473
ESTRO 36 2017
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Poster: Physics track: Adaptive radiotherapy for inter-
fraction motion management
PO-0875 Dosimetric effects of anatomical changes in
proton therapy of head and neck (H&N) cancer
G. Miori
1,2
, L. WIdesott
1
, F. Fracchiolla
1
, S. Lorentini
1
, P.
Farace
1
, R. Righetto
1
, C. Algranati
1
, M. Schwarz
1,3
1
Trento Hospital, Protontherapy, Trento, Italy
2
University of Rome Tor Vergata, Postgraduate School of
Medical Physics, Rome, Italy
3
INFN, TIFPA, Trento, Italy
Purpose or Objective
Anatomical changes in H&N patients can affect dose
distributions especially in proton therapy. A retrospective
analysis of H&N patients undergoing repeat CTs and
treated at our Proton Therapy Center was done to
evaluate dose changes and to identify a dosimetric index
for the need of replanning. Furthermore, TCP analysis was
performed to evaluate the magnitude of changes with
radiobiological parameters. Finally, non-adapted and
adapted plans were compared.
Material and Methods
All H&N patients treated in our center between October
2014 and September 2016 with at least one repeat CT
(eCT) were considered. 21 patients were identified: 18
patients had at least one eCT (1 to 6 eCTs), but did not
need replanning, and 3 patients needed replanning at
some stage of the treatment. The original plan was
recalculated on each eCT. Differences were calculated for
each treatment fraction, considering a stepwedge
interpolation on fractions where the eCT was missing. D1
variations (ΔD1) for cord, brainstem, optic chiasm and
optic nerves, and Dmax differences (ΔDmax) for lenses
were considered. Target coverage analysis was based on
differences in CTV V95 (ΔV95). ΔV95 values were included
in
Non-replanned
(
controls
) if they came from non-
replanned patients or from replanned patient calculations
on CT preceding the replanning CT (rCT). On the contrary,
ΔV95 were included in
Replanned
(
cases
) if they came
from replanned patients on the rCT and the following CTs.
The choice was made to consider the trend in target
coverage after the point identified for replanning. A cut-
off ΔV95 for the need of replanning was identified by the
maximum Youden’s index on the ROC analysis between
control
and
cases.
Next, TCP differences with respect to
the planning TCP (ΔTCP) were calculated. ΔTCP values
were divided in
Non-replanned
and
Replanned
as for DV95
analysis. Finally, a comparison between adapted and non-
adapted plans for the 3 replanned patients was done. All
statistics were made by t-Student tests.
Results
Patients show no significant variations in OARs doses
during the treatment (Table 1).
Target coverage analysis shows large differences between
Replanned
and
Non-replanned
(p<0.001). The maximum
Youden’s index identifies CTV ΔV95= -5% as an optimized
threshold level for replanning (sensitivity=87.5%;
specificity=100%). TCP analysis shows large variations
between
Replanned
and Non-replanned (p<0.001). ΔV95
and ΔTCP results are summarized in Table 1. ΔV95
comparison between non-adapted and adapted plans
shows significant CTV coverage improvements (Figure 1).
Conclusion
OARs doses were not affected by anatomical changes in all
H&N patients studied. On the contrary, there was a
significant difference in the effect of anatomical changes
for replanned and non-replanned patients, confirmed by
radiobiological changes. Therefore, ART reveal great
benefits in target coverage for patients that need
replanning which can be identified by a threshold
dosimetric index.
PO-0876 Treatment adaptation is mandatory for
intensity modulated proton therapy of advanced lung
cancer
L. Hoffmann
1
, M. Alber
2
, M. Jensen
3
, M. Holt
3
, D. Møller
1
1
Aarhus University Hospital, Department of Medical
Physics, Aarhus, Denmark
2
Heidelberg University Hospital, Department of
Radiation Oncology, Heidelberg, Germany
3
Aarhus University Hospital, Department of Radiation
Oncology, Aarhus, Denmark
Purpose or Objective
Large anatomical changes during radiotherapy are seen for
a large proportion of lung cancer patients. Precise delivery
of proton therapy is highly sensitive to these changes