Orthotopic glioblastoma models for evaluation of the CTV concept

Background and purpose: In times of high-precision radiotherapy, the accurate and precise
definition of the primary tumour localisation and its microscopic spread is of enormous
importance. In glioblastoma, the microscopic tumour extension is uncertain and therefore
population-based margins for clinical target volume (CTV) definition are clinically used, which
could either be too small leading to increased risk of loco-regional recurrences or too large thus
enhancing the probability of normal tissue toxicity. Therefore, the aim of this project is to
investigate an individualized definition of the CTV in preclinical glioblastoma models, based
on specific biological tumour characteristics.
Material and methods: The microscopic tumour extensions of two different orthotopic brain
tumour models (U87MG_mCherry; G7_mCherry) were evaluated before and during
fractionated radiotherapy and correlated with corresponding histological data. Representative
tumour slices were analysed using Matrix-assisted Laser Desorption/Ionization (MALDI) and
stained for putative cancer stem cell markers as well as invasion markers (Nestin, MMP14,
Musashi 1, CD44).
Results: The edges of the tumour are clearly shown by the MALDI segmentation via
unsupervised clustering of mass spectra and are consistent with the histologically defined
border in H&E staining in both models. MALDI component analysis supposed specific peaks
as potential markers for normal brain tissue (e.g. 1339 m/z), whereas other peaks demarcated
the tumours very well (e.g. 1562 m/z for U87MG_mCherry) irrespective of treatment. MMP14
staining revealed only a few positive cells mainly in the tumour border, which could reflect the
invasive front in both models.
Conclusion: The results of this study indicate that a step towards an individualized CTV
definition based on biological tumour characteristics, especially using MALDI information, in
glioblastoma models seems possible. Visualization of tumour volume and protein heterogeneity
can be potentially used to define radiotherapy-sensitive and resistant areas. In-depth image
analyses will be performed in order to further improve the model for CTV definition.