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Conclusion
This study highlights the need for chamber-depended
response maps when using LAICs for absolute and relative
dosimetry with proton pencil beams or small photon
beams.
OC-0150 Dual-energy CT-based proton treatment
planning to assess patient-specific range uncertainties
P. Wohlfahrt
1,2
, C. Möhler
3,4
, W. Enghardt
1,2,5,6
, S.
Greilich
3,4
, C. Richter
1,2,5,6
1
OncoRay - National Center for Radiation Research in
Oncology, Faculty of Medicine and University Hospital
Carl Gustav Carus- Technische Universität Dresden-
Helmholtz-Zentrum Dresden - Rossendorf, Dresden,
Germany
2
Helmholtz-Zentrum Dresden - Rossendorf, Institute of
Radiooncology, Dresden, Germany
3
German Cancer Research Center DKFZ, Division of
Medical Physics in Radiation Oncology, Heidelberg,
Germany
4
National Center for Radiation Research in Oncology
NCRO, Heidelberg Institute for Radiation Oncology HIRO,
Heidelberg, Germany
5
Department of Radiation Research in Oncology, Faculty
of Medicine and University Hospital Carl Gustav Carus-
Technische Universität Dresden, Dresden, Germany
6
German Cancer Consortium DKTK, Dresden, Germany
Purpose or Objective
To reduce range uncertainties in particle therapy arising
from a generic heuristic conversion (HLUT) of CT numbers
in stopping-power ratios (SPRs), an accurate patient-
specific SPR prediction is desirable. Treatment planning
based on dual-energy CT (DECT) can account for tissue
diversity and potentially contribute to shrink clinical
safety margins. Consequently, in this study dose
distributions derived from both a clinical HLUT and a
patient-specific DECT-based SPR prediction are compared
and range deviations are quantified for two different
treatment sites.
Material and Methods
Based on a database of more than 1000 clinical DECT scans
acquired with a single-source DECT scanner (Siemens
Somatom Definition AS), 10 prostate cancer and 54 head
tumor patients were selected to assess intra- and
interpatient tissue diversity and its impact on SPR
prediction. To evaluate age- and sex-dependent
variability, the head tumor cohort was divided in children,
women and men. DECT scans were converted in 79 keV
pseudo-monoenergetic CT scans (MonoCTs) and SPR
datasets derived by voxelwise calculations of electron
density and effective atomic number using syngo.via
(Siemens Healthineers). In XiO (Elekta) clinical proton
treatment plans were recalculated (a) on MonoCTs using
the clinical HLUT and (b) on SPR datasets to quantify range
and dose differences.
Results
The voxelwise correlation of SPR and CT number is similar
for men and women, but differs considerably between
adults and children in bony tissue, likely due to the
amount of calcium embedded in bones, which increases
with age. Based on voxelwise SPR comparisons, the clinical
HLUT predicts on average (2.2 ± 0.6) % larger SPRs for head
tumor patients and (1.7 ± 0.3) % larger SPRs for prostate
cases. The impact of both approaches on dose
distributions is shown in Fig. 1 and 2 for an exemplary head
tumor and prostate cancer patient. In the head case, the
HLUT predicts a 1.7 % shorter range (2.4 mm) resulting
from a 0.7 mm range underestimation in water-filled
ventricles (not precisely predicted by the HLUT) and
different SPR predictions for brain. A range deviation of
up to 3.0 % (7.1 mm) is obtained in the prostate case,
which is mainly caused by different SPR predictions for
bone marrow and muscle. These range differences in
single beams are not compensated in the overall
treatment plan.