493 / 1096
S478
ESTRO 36
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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
which may result in under-dosage of target. We
investigated the applicability of a decision suppo rt system
developed for photon therapy in a proton therapy setting.
Material and Methods
Twenty-three consecutive NSCLC patients stage 1B to IV
treated with adaptive photon therapy were
retrospectively planned using intensity modulated proton
therapy. The adaptive protocol was based on geometrical
measures of target positioning and large anatomical
changes as e.g. atelectasis, as observed on daily CBCT
scans. Two surveillance CT-scans were acquired during the
treatment course. The consequences of anatomical
changes were evaluated by recalculation of the proton
plans on the surveillance scans. The CTV receiving 95% of
the prescribed dose was analyzed. Proton treatment plans
were scaled to prescribed doses of 70, 74 or 78Gy, to
investigate if full CTV coverage at 95% of 66Gy = 62.7Gy
could be maintained by increasing the prescribed dose.
Results
Fourteen (61%) patients needed adaptations when treated
with protons, given that 95% of the CTV must be covered
by 95% of the dose. In comparison, no patients needed
adaptation when treated with photons using this criterion.
Figure 1 shows CTV coverage for all patients. For proton
therapy, the adaptive protocol was found to identify
patients with large target under-dosage (six patients,
group A). Additionally, under-dosage was observed for
another eight patients (group B) with non-rigid changes up
to 15mm in the positioning of the bones. The median
decrease in coverage for all patients was 92.8% [48.1-
100%]. Robust optimization reduces the decrease in target
coverage, but does not eliminate the under-dosage, see
Fig.2.All patients in group B would be treated sufficiently
when prescribing 74Gy with all CTVs receiving 95% of
66Gy. For patients in group A, only two patients would be
treated sufficiently with a 78Gy prescription. A geometric
decision support protocol as the present is thus mandatory
in order to maintain target coverage of the patients in
group A. When increasing the prescribed dose, the
maximum dose to important normal tissue such as the
oesophagus, trachea, bronchi, and heart increases and
may thus be the dose limiting factor.
Conclusion
Large anatomical changes can be corrected for by an
adaptive protocol. Non-rigid positioning erro rs are not
identified by the geometrical criteria used for photo ns
but can be compensated by an increase in the prescribed
dose keeping in mind that this requires additional
attention to organs at risk. Robust optimisation reduces,
but does not eliminate the risk of under-dosage. Daily
imaging and treatment adaptation for a high fraction of
patients is mandatory in proton therapy for loco-regional
lung cancer.