Target definition in radiotherapy of prostate cancer using magnetic resonance imaging only workflow

In magnetic resonance (MR) only radiotherapy, the target delineation needs to be performed without computed tomography (CT). We investigated in thirteen patients with prostate cancer, how the clinical target volume (CTV) was affected, when the target delineation procedure was changed from using both CT and MR images to using MR images only. The mean volume of the CTV_CT/MR was 61.0 cm³ as compared to 49.9 cm³ from MR-only based target delineation, corresponding to an average decrease of 18%. Our results show that CTV_MR-only was consistently smaller than CTV_CT/MR, which has to be taken into consideration before clinical commissioning of MR-only radiotherapy.