MRI-based tumour control probability in skull-base chordomas treated with carbon-ion therapy

Purpose: To derive personalized tumour control probability (TCP) models, using diffusion-weighted (DW-) MRI for defining initial tumour cellular density in skull-base chordoma patients undergoing carbon-ion radiotherapy (CIRT). Materials and methods: 67 patients affected by skull-base chordoma were enrolled for a standardized CIRT treatment (70.4 Gy (RBE) prescription dose). Local control information was clinically assessed. For 20 of them, apparent diffusion coefficient (ADC) maps were computed from DW-MRI and then converted into cellular density. Radiosensitivity parameters (α, β) were estimated from the available data through an optimization procedure, taking advantage of a relationship observed between local control and the dose received by at least the 98% of the gross tumour volume. These parameters were fed into two poissonian TCP models, based on the LQ model, being the first (TCP LIT ) computed from literature parameters and the second (TCP ADC ) enriched by a personalized initial cellular density derived from ADC maps. Results: The inclusion of the cellular density derived from ADC into TCP ADC yielded slightly higher dose values at which TCP = 0.5 (D 50 = 38.91 Gy (RBE)) with respect to TCP LIT (D 50 34.16 Gy (RBE)). This suggested a more conservative approach, even if the prognostic power of TCP ADC and TCP LIT , tested with respect to local control, was equivalent in terms of sensitivity (0.867) and specificity (0.600). Conclusions: Both TCP ADC and TCP LIT exhibited good agreement with a clinically validated information of local control, the former providing more conservative predictions.