Towards tumour hypoxia imaging: Incorporating relative oxygen extraction fraction mapping of prostate with multi‐parametric quantitative MRI on a 1.5T MR‐linac

Summary Hypoxia plays a central role in tumour radioresistance. Reliable tumour hypoxia imaging would allow the monitoring of tumour response and a more personalized adaptation of radiotherapy planning. Here, we showed a proof of concept of the feasibility and repeatability of relative oxygen extraction fraction (rOEF) mapping of prostate using multi‐parametric quantitative MRI (qMRI) achieved for the first time on a 1.5T MR‐linac. T2, T2* relaxation times maps, and intra‐voxel incoherent motion (IVIM) parametric maps mapping were computed on a 29 years old healthy volunteer. R2′ and rOEF maps were calculated based on a multi‐parametric model. Long‐term repeatability and repeatability coefficient (RC) were determined for each parameter according to QIBA recommendations. Mean values for the entire healthy prostate were 0.99 ± 0.14 × 10 −3 mm/s 2 , 81 ± 2.1 × 10 −3 mm/s 2 , 21.6 ± 3.6%, 92.7 ± 19.7 ms and 62.4 ± 17.3 ms for D slow , D fast , f , T2 and T2*, respectively. R2′ and rOEF in the prostate were 6.1 ± 3.4 s −1 and 18.2 ± 10.1% respectively. The RC of rOEF was 4.43%. Long‐term repeatability of quantitative parameters based on a test–retest ranged from 2 to 18%. qMRI parameters are measurable and repeatable on 1.5T MR LINAC. From T2, T2* and IVIM parameters maps, we were able to obtain a rOEF mapping of the prostate. These results are the first step to a non‐invasive imaging of tumour hypoxia during radiotherapy leading to a biological image‐guided adaptive radiotherapy.