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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