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

• Skull-base chordoma response to carbon-ion therapy is not fully characterized. • Diffusion-weighted MRI provides ADC maps that can be related to cellular density. • MRI can provide patient-specific information for tumour control probability models. 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). 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. 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). Both TCP ADC and TCP LIT exhibited good agreement with a clinically validated information of local control, the former providing more conservative predictions. [ABSTRACT FROM AUTHOR]