283 / 1082
S270
ESTRO 36 2017
_______________________________________________________________________________________________
LET
d
was also scaled by a parameter c=0.04 µm/keV
(IJROBP 2016), such that the resulting product quantifies
the additional biological dose stemming from the LET
effect.
Results
High LET
d
values surrounded part of the PTV and
encompassed portions of the brainstem (Fig 1). The mean
LET
d
values to the brainstem were 4.0, 5.6, 5.9 and 8.6
keV/µm for cases A, B, C and D, while the mean LET
d
values for the PTVs were between 3.0-3.5 keV/µm for
cases A-D. In cases where the target volume overlapped
the brainstem, the c x LET
d
x dose resulted in 'hot-spots”
within the brainstem. The highest absolute LET
d
values
were seen in the cases with the PTV located most distant
from the brainstem, whereas lower and more
homogeneous LET
d
values were seen for the full overlap
case (Fig 2). When examining dose-weighted LET
d
,
however, the opposite trend was seen: An increase in
dose-weighted LET
d
was observed in tumours closer to the
brainstem.
Conclusion
When treating children with posterior fossa tumours with
IMPT, our modelling study demonstrated that elevated
LET
d
values are located in the brainstem, in particular if
the tumour volume was distant from the brainstem.
However, the impact of LET
d
on overall biological dose was
greater for tumours that approached or invaded the
brainstem.
OC-0517 Investigation of the RBE variation of protons
in the rat spinal cord within a spread-out Bragg peak
M. Saager
1
, P. Peschke
2
, S. Brons
3
, M. Scholz
4
, P.E.
Huber
2
, J. Debus
5
, C.P. Karger
1
1
DKFZ, Medical Physics in Radiation Oncology,
Heidelberg, Germany