926 / 1096
S910
ESTRO 36
_______________________________________________________________________________________________
Conclusion
A compromise between the patient uncertainty
positioning and the associated workload is needed. The
optimization of the threshold used for couch shifts is
subjective and depends on the importance given to both
factors. We showed that using a threshold <2 mm doesn’t
effectively reduce the total uncertainty. We believe that
a threshold of 3 or 4 mm is adequate, keeping the
positioning uncertainty below 1 mm and a reasonable
clinical workload.
EP-1671 Calculation of the skin dose-of- the-day during
Tomotherapy for head and neck cancer patients
M. Branchini
1,2
, C. Fiorino
1
, M. Mori
1
, I. Dell'Oca
3
, M.G.
Cattaneo
1
, L. Perna
1
, N.G. Di Muzio
3
, R. Calandrino
1
, S.
Broggi
1
1
San Raffaele Scientific Institute, Medical Physics, Milan,
Italy
2
IRCCS Istituto Oncologico Veneto, Medical Physics,
Padova, Italy
3
San Raffaele Scientific Institute, Radiotherapy, Milan,
Italy
Purpose or Objective
Late fibrosis is known to depend on the severity of acute
skin toxicity; an increase of skin dose during RT due to
anatomy deformation may translate into an increased risk
of acute toxicity, suggesting a potential benefit from
planning adaptation to counteract this effect. Within this
scenario, current study started to explore a previously
suggested method for dose-of-the-day calculation in
quantifying changes of the skin dose during Tomotherapy
(HT) for head and neck (HN) cancer.
Material and Methods
Planning CTs of 9 HN patients treated with HT (SIB:
54/66/69 Gy/30fr or sequential boost: 54/66.6-70.2Gy in
37-39 fr) were deformable registered to MVCT images
acquired at the 15
th
fraction (processed with anisotropic
diffusion filter) using a constrained intensity-based
algorithm (MIM software). At the same day, a diagnostic
kVCT was acquired with patient in treatment position
(CT15) and taken as reference. The original HT plans were
recalculated on both the resulting deformed images
(CTdef) and CT15 using the DQA (dose quality assurance)
HT module. In order to validate the method in computing
the dose-of-the-day of the skin, the superficial layers
(SL) of the body with thickness of 2, 3 and 5 mm (as a
surrogate of the skin dose distribution: SL2,SL3,SL5) were
considered in the body cranial-caudal extension
corresponding to the high-dose PTV. The SL V95%, V97%,
V98%, V100%, V102%, V105% and V107% of the prescribed
PTV dose (i.e: likely to correlate with skin toxicity) were
extracted for CT15 and CTdef and compared. In addition,
trendlines’ R
2
of the graphs with Vd% of CT15 vs CTdef
were computed to assess correlation between the twos.
Then, as a first example of clinical application, skin dose
differences between fraction 15 and planning (V95%-
V107% of SL) were retrospectively analyzed for 8 patients
treated with SIB.
Results
The differences between SL2/SL3/SL5 V95%-V107% in CT15
and CTdef were very small (<1%/1cc Figure 1). The
correlation between SL DVHs parameters estimated on
CT15 and CTdef was high (mean R
2
=0.91), with higher
correlation for lower doses (i.e.: V95%, R
2
: 0.97, 0.98 and
0.99 for SL2, SL3 and SL5, respectively). When looking to
the changes during HT, small average differences
between planned vs dose-of-the-day values of SL V95%-
V107% were found (< 2 cc), excepting one patient (out of
8) who showed a much more relevant difference between
the planned skin dose and the delivered dose at fr 15
(V102%=7cc for SL5, Figure 2).
Conclusion
The calculation of the skin dose-of-the-day using planning
CT-to-MVCT DIR is sufficiently reliable. The method was
proven to be able of pointing out early superficial
overdosing, to inform adaptive strategies. Preliminary
results suggest that clinically relevant changes at half
treatment should occur in a minority of patients,
reinforcing the utility of our approach to select patients
who may really benefit from adaptive replanning.
Electronic Poster: Physics track: CT Imaging for
treatment preparation
EP-1672 Dual energy CT for improved proton stopping
power estimation in head and neck cancer patients
V. Taasti
1
, L. Muren
1
, K. Jensen
2
, J. Petersen
1
, J.
Thygesen
3
, A. Tietze
4
, C. Grau
2
, D. Hansen
1
1
Aarhus University Hospital, Dept. of Medical Physics,
Aarhus, Denmark
2
Aarhus University Hospital, Dept. of Oncology, Aarhus,
Denmark
3
Aarhus University Hospital, Dept. of Clinical
Engineering, Aarhus, Denmark
4
Aarhus University Hospital, Dept. of Neuroradiology,
Aarhus, Denmark
Purpose or Objective
Pre-clinical and phantom studies have established that
dual energy CT (DECT) improves estimation of the proton
stopping power ratio (SPR) compared to single energy CT
(SECT), leading to increased accuracy in treatment
planning dose calculations. However, proton SPR
estimation using DECT vs. SECT has only been compared in
a single study of tumours in the cranial region with limited
anatomical variations, with inconclusive results. We have
therefore initiated a clinical imaging study of proton SPR
estimation in the head and neck region comparing DECT
and SECT. The aim of this study was to investigate if SPR
differences between the two CT modalities were found
when evaluating heterogeneous tissues of the head and
neck region.
Material and Methods