ESTRO 36 Abstract Book

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