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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.
Conclusion
In contrast to a generic HLUT, a DECT-based SPR
prediction can individually consider age- and sex-
dependent tissue variability in proton treatment planning.
This diversity information can also provide suggestions for
subgroup-specific improvements of the heuristic CT
calibration. The assessment of relative SPR and dose