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S908
ESTRO 36
_______________________________________________________________________________________________
2UDepartment of Mathematical Physics and of Fluids,
Madrid, Spain
Purpose or Objective
Assessment of the differences between CT and CBCT based
dose calculation for a volumetric modulated arc therapy
(VMAT) in head and neck radiotherapy treatment.
Material and Methods
CBCT images of an Alderson RANDO phantom with tissue-
equivalent material were acquired in a Varian`s On-Board
Imager OBI (v1.5) installed on a Varian DHX accelerator,
using its standard-dose head protocol (100 kV and 2.5 slice
thickness). On the other hand, planning CT images were
acquired in a Toshiba Aquilion LB using the same phantom
and with our own clinical head and neck protocol (120 kV
and 3 mm slice thickness). Different OAR (Body, spinal
cord, parotids, mandible, oropharynx, dermis, an inner
ring with 1 cm thickness and shoulder) and a PTV were
delimitated. Additionally, reference points were inserted
over all these structures. All defined structures and points
were registered with the CBCT images by means of the
Varian rigid registration software. Both the delimitation of
volumes and the design of the treatment plan have taken
into account the limited field of view of CBCT (length 16
cm, diameter 25 cm). A head and neck VMAT plan has been
calculated in Eclipse (v10) using both sets of images. For
CT images, we only used a standard calibration curve and
3 different calibration sets of curves for CBCT images, i.e.,
standard, measured with a CATPHAN 504 phantom and
measured with a CIRS 062M head phantom placed between
head and neck RANDO slices. Dose and HU were calculated
in all reference points as well as dose-volume-histograms
for the anatomical locations for both CT and CBCT. A
gamma analysis was used for HVD comparison.
Results
The mean HU differences are less than 50 UH and the
relative dose differences are less than 3% for all the
calibration curves (Table 1) on every reference point over
all the structures.
The gamma (2%, 2 mm) DHV analysis shows an excellent
agreement for almost all the structures (>95%) (Image1).
Ring and dermis have gamma >85% .The non-pass regions
correspond to very low dose regions. The worst gamma
(>50%) corresponds to the left parotid because it is a very
small structure (10 cc) into a high gradient dose zone.
Furthermore, there is a difference of 1.8% on its volume
as measured on the CBCT and the CT images, probably due
to interpolation errors. These results are similar for all the
calibration
curves analysed.
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