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The reproducibility of film measurements was on average
of 2%. Measurements made on the Cheese phantom
surface without mask showed an overestimation of the TPS
of 28.6% with fine grid, which is commonly used in clinic.
In presence of the mask there was an improvement of the
agreement between EBT3 measurements and TPS
estimated doses, achieving 0.7%. A considerable number
of measurements was performed on 8 patients. The mean
absolute value of the difference between measured and
TPS-calculated dose and its standard deviation was 11.6%
± 2.8% for all treatments. The average differences were
9.1% for brain and H&N (in these case measurements were
performed with mask), and -9.2% for the sarcoma. Hence,
there was an overestimation of the TPS without the
thermoplastic mask.
Conclusion
In vivo surface dose measurements with EBT3 are a useful
tool for quality assurance in tomotherapy, since the TPS
does not give accurate dose values in the first millimeters
of skin. Measurements performed both on phantom and in
vivo have shown a bolus effect due to the thermoplastic
mask, that compensates for the overestimation of the skin
dose calculated by the TPS.
EP-1490 A 3-class density method to monitor doses to
the parotid glands and spinal cord in oropharynx IMRT
N. Perichon
1
, S. Couespel
1
, C. Hervé
1
, O. Henry
1
, C.
Lafond
1,2,3
, J. Castelli
2,3,4
, A. Largent
2,3
, O. Acosta
2,3
, E.
Chajon
4
, R. De Crevoisier
2,3,4
1
Centre Eugène Marquis, Unité de Physique Médicale,
Rennes CEDEX, France
2
INSERM, U1099, RENNES, France
3
Université Rennes 1, LTSI, RENNES, France
4
Centre Eugène Marquis, Radiation Oncology
Department, Rennes CEDEX, France
Purpose or Objective
Within a perspective of dose guided/dose monitoring
adaptive radiotherapy, a crucial issue is the possibility to
calculate the dose distribution on Cone Beam CT scans
(CBCTs). The parotid glands (PGs) and the spinal cord (SC)
are among the main organs at risk (OAR) exposed to an
overdose during the course of IMRT for oropharynx
carcinoma. Dose calculation is particularly complex on
non-CT images. One clinically applicable option would be
to apply three density classes (soft tissue, air, bone) in the
CBCTs, corresponding to the density values of the planning
CT. The aim of this study was therefore to estimate the
accuracy of the dose distribution calculation within PGs
and SC by affectation of three density classes.
Material and Methods
Fifteen patients receiving IMRT for oropharyngeal cancer
had a weekly CT scan along their treatment. OAR and
target volumes were manually delineated in each CT. A 3-
class tissue (soft tissue, air and bone) segmentation was
performed in each CT scan using a manual threshold
method: over 110 UH for the bone and under -150 UH for
the air contained into the external patient contours. Soft
tissue was deduced by Boolean operation from air, bone
and external patient contour. Mean density values were
affected to the 3 classes in the weekly CTs, corresponding
to those read on the planning CT for each patient. A plan
was first generated on each planning CT scan using a 3
dose levels simultaneously integrated boost protocol
(70Gy/63Gy/56Gy in 35 fractions). The beam parameters
defined on the planning CT scan were transferred to each
weekly CT. Two dose distributions were then calculated in
these CT using an adaptive convolution algorithm: either
based on the “standard reference” CTscan, or based on
the 3 density class CT scan. The doses to the PGs (DVH and
mean dose) and the SC (D2%) calculated according to the
two CT modalities were compared (Wilcoxon test). Finally,
3D gamma index were also calculated to compare the 3D
dose distributions. We report the results for the first 5
patients.
Results
The PGs DVH and mean doses were not significantly
different according to the two CT modality based
calculation. On average, the difference for the mean dose
was 0.1 % (SD=0.7 %). The SC D2% doses were slightly
significantly higher when calculation is based on the
standard CT with a mean value of 42.94 Gy (SD=3.03 Gy)
compared to 42.52 Gy (SD=2.76 Gy) when calculated on
the 3 density classes. Figure 1 represents dose distribution
in sagittal plane calculated on 3 density class CT. Figure 2
shows the 3D gamma index map on the sagittal plane
(criteria DTA/DD 1mm/1% local dose, dose threshold 10
%): 91.7 % accepted point; gamma mean value 0.6. Most
differences between the two dose distributions seems to
appear on bone volumes.
Conclusion
This 3-class density method can be used to monitor the
fraction dose in the PGs during oropharynx cancer IMRT.
Small significant differences are observed for the highest
dose received in the spinal cord, likely due to the bone
heterogeneity.
EP-1491 Verification of FFF VMAT plans with PDIP and
GLAaS algorithms by using the new imager of
TrueBeamSTx
T. Ercan
1
, A. Levent
2
, T. Cagin
3
, S.M. Igdem
1
1
Gayrettepe Florence Nightingale Hospital, Radiation
Oncology, Gayrettepe - Istanbul, Turkey
2
Medideal Medical Projects and Solutions Inc., Medical
Physics, Istanbul, Turkey