Linear energy transfer distributions in the brainstem depending on tumour location in intensity-modulated proton therapy of paediatric cancer.

BACKGROUND

For tumours near organs at risk, there is concern about unintended increase in biological dose from elevated linear energy transfer (LET) at the distal end of treatment fields. The objective of this study was therefore to investigate how different paediatric posterior fossa tumour locations impact LET and biological dose to the brainstem during intensity-modulated proton therapy (IMPT).

MATERIAL AND METHODS

Multiple IMPT plans were generated for four different simulated tumour locations relative to the brainstem for a five-year-old male patient. A prescribed dose of 59.4 Gy(RBE) was applied to the planning target volumes (PTVs). Plans with two lateral and one posterior non-coplanar fields were created, along with plans with modified field arrangements. The dose-averaged LET (LET) and the physical dose × RBE (D × RBE), where RBE=1+c × LET, were calculated using the FLUKA Monte Carlo code. A scaling parameter c was applied to make the RBE represent variations in the biological effect due to LET.

RESULTS

High LET values surrounded parts of the PTV and encompassed portions of the brainstem. Mean LET values in the brainstem were 3.2-6.6 keV/μm. The highest absolute brainstem LET values were seen with the tumour located most distant from the brainstem, whereas lower and more homogeneous LET values were seen when the tumour invaded the brainstem. In contrast, the highest mean D × RBE values were found in the latter case (54.0 Gy(RBE)), while the case with largest distance between tumour and brainstem had a mean D × RBE of 1.8 Gy(RBE).

CONCLUSIONS

Using IMPT to treat posterior fossa tumours may result in high LET values within the brainstem, particularly if the tumour volume is separated from the brainstem. However, the D × RBE was greater for tumours that approached or invaded the brainstem. Changing field angles showed a reduction of LET and D × RBE in the brainstem.