The primary aim of this clinical study, 'Dual Energy CT - a tool for delineation of tumor and organs at risk in radiotherapy' (DART) is to evaluate whether dual-energy CT (DECT) is at least as effective as magnetic resonance imaging (MRI) in delineating both tumors and organs at risk (OARs) in patients referred for radiotherapy (RT). This primary aim will be explored in patient groups where the performance of DECT for RT has been described in the literature (but mostly based on signal-to-noise ratio and/or contrast-to-noise ratio), such as brain metastases (sub-cohort 'DART Brain') and head and neck cancer (sub-cohort 'DART H\&N'). Additionally, DART will explore diagnoses not yet studied in the literature, such as bone metastases (sub-cohort 'DART Bone'). If DECT could be demonstrated to achieve delineations for RT in both tumors and OARs that are equally accurate as those based on MRI, it could offer significant advantages by being faster and more cost-effective, making DECT a valuable alternative to MRI in clinical practice. As a secondary aim, the study will evaluate whether DART offers benefits when added to standard RT imaging for tumor and OAR delineation: (1) For lung cancer patients (sub-cohort 'DART Lung'), where MRI scans are typically not valuable due to tumor motion caused by breathing, 'DART Lung' will assess whether DECT performed in breath-hold provides added value compared to the single-energy CT (SECT) scan conducted during free breathing, as used in current clinical practice. (2) For head and neck cancer patients, 'DART H\&N' will evaluate whether DECT offers added value compared to the positron emission tomography (PET)/CT scan currently used in clinical practice.