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ESTRO 36 2017
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Conclusion
CBCT images for a head and neck VMAT treatment provide
accurate dose calculation in adaptive radiotherapy,
making them suitable for the assessment of possible
changes over the original treatment planning for all the
calibration curves analyzed.
EP-1669 Assessment of the clinical value of off-line
adaptive strategies for tomotherapy treatments
D. Dumont
1
, X. Geets
2
, M. Coevoet
2
, E. Sterpin
1
1
Université catholique de Louvain, MIRO, woluwe-saint-
lambert, Belgium
2
Cliniques Universitaires Saint-Luc, radiotherapy,
woluwe-saint-lambert, Belgium
Purpose or Objective
This study assessed the clinical potential of offline
adaptive strategies based on the dose computed on daily
MVCTs (Tomotherapy). We defined clinical indicators that
were subsequently used to identify the percentage of
plans that should have been adapted due to significant
dose deviations to TVs or OARs. Only the consistency of
the initial plan throughout the treatment was addressed.
Thus, dose was reported to constants TVs and deformed
OARs.
Material and Methods
Cumulative doses were calculated from daily MVCT for 41
lung, 50 prostate and 21 H&N patients, using research
versions of off-line adaptive solutions from Accuray and
21
st
century Oncology. All deformed contours were
checked by an experienced radiation oncologist, while all
dose calculations were crosschecked using our in-house
Monte Carlo model (TomoPen). The clinical indicators
were the DVH metrics used during the treatment planning
for each considered OAR (e.g. D
2
, V
5
) and TVs (e.g D
50
).
Dose constraints were also defined according to the tumor
site (e.g. D
mean
Parotid < 30 Gy). Two levels of warning
were considered:
•
red flag: a 10% deviation of the clinical indicator
relative to the planned value (e.g. for the
parotid ΔD
mean
(cumulated)>10% D
mean
(planned))
AND
a violation of a dose constraint (e.g. for the
parotid D
mean
(cumulated) >30 Gy)
•
orange flag: a 10% deviation of the clinical
indicator relative to the
dose constraint
(e.g.
for the parotid ΔD
mean
(cumulated) >3 Gy).
Both adaptive software evaluated the dose to TVs using
deformed PTVs. This approach is questionable because the
PTV corresponds to a geometrical (not anatomical) safety
margin. Therefore, we reported the dose on rigidly
registered PTVs.
Results
Deformed contours were judged acceptable for all H&N
and lung cases. However, registrations failed for most
pelvic cases, for which large anatomical deformations
occurred (see figure 1). Consequently, pelvic cases were
excluded.
Dose calculation of both analytical engines were in good
agreement with TomoPen (around 1.5% mean difference
on PTV D
50
).
Results are reported in Table 1. For TVs, only 6 flags (out
of 62 patients) were reported for the rigidly registered
PTV, which was considered as the only relevant volume.
The flags reported for lung cases were irrelevant because
of the blurring of the tumor density leading to large dose
calculation deviations. For the H&N case, the red flag was
rejected after analysis (wrong doses in part of the PTV out
of the external contour). For the OARs, one H&N was
flagged (true flag) with an increase of 11% of the mean
parotid dose that exceeded the dose constraint (30 Gy).
Conclusion
Considering a constant PTV, the impact of treatment
adaptation on the quality of delivered plans is minor for
the included patients. The conclusion might be different
for pelvic cases due to the larger anatomical
deformations. Conclusions might also differ for an adapted
PTV, but such strategy must address clinical
considerations before implementation.
EP-1670 Couch shifts in NAL protocols: ¿Which is the
optimal threshold?
A. Camarasa
1
, V. Hernández
1
1
Hospital Universitari Sant Joan de Reus, Servei de
Protecció Radiològica i Física Mèdica, Reus, Spain
Purpose or Objective
The NAL protocols applied to patient positioning in
treatments evaluated by CBCT use a threshold regarding
couch shifts. If the CBCT demands shifts over the
threshold, the patient must be moved, while shifts below
the threshold remain as residual errors. The aims of this