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ESTRO 36 2017
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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
The patients were CT scanned with a 2
nd
generation dual
source CT scanner, SOMATOM Definition Flash (Siemens
Healthcare, Forchheim, Germany). DECT images were
acquired at 100/Sn140 kVp, and SECT images were
obtained as a weighted summation of the low and high
DECT images. The DECT scans were acquired at the same
day as the control CT scan midway through the treatment
course and using the same dose settings as used for the
control scan. The CT scans covered the whole anatomical
region of the head down to the top of lungs – the SPR
comparison was thereby performed over very
heterogeneous tissue regions. SPR images were calculated
from both the DECT and SECT scans for the four first
patients included in the study. For DECT, SPR images were
calculated using a noise-robust method previously
developed in our group. For SECT, the stoichiometric
method was used. Based on SPR images, difference maps
were calculated. Seven regions of interest (ROIs) were
placed, each covering a single tissue type. Relative SPR
differences between the DECT and SECT calculations were
extracted from the ROIs.
Results
For bone, SECT systematically underestimated the SPR
compared with DECT, while the reverse was the case for
the soft tissues (Fig. 1). The relative SPR differences
ranged from -2.2% to 0.9%, with a mean difference of -
0.6% (Fig. 2). Large variations of up to 1.5 percentage
points were seen for the SPR difference across the
patients. However, the differences for the individual
patients were systematically either positive or negative
for each ROI (Fig. 2).