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.

A detailed description of the study is provided in the uploaded document 'Study Protocol with Statistical Analysis Plan'.

Inclusion Criteria:

• ECOG performance status (PS) 0-2
• Assessed to be able to complete the protocol scanning regime
• Referred for RT
• For head and neck cancer patients, a PET/CT scan must have been performed during the diagnostic evaluation for cancer
• Signed informed consent

Exclusion Criteria:

• Pregnancy
• Previous RT in the same anatomical area now referred for RT
• Previous surgery in the same anatomical area now referred for RT
• Participation in conflicting protocols
• If relevant for the protocol scanning regime: Allergic to contrast agent
• If relevant for the protocol scanning regime: Contraindication to iodine-based contrast agent for CT (as determined by the physician)
• If relevant for the protocol scanning regime: Contraindication to Gadolinium-based contrast agent for MRI (as determined by the physician)
• If relevant for the protocol scanning regime: Contraindication for MRI (pacemaker, metal, non-MRI, compatible implants etc.)
• Incapable of understanding the patient information

Cancer patients referred for radiotherapy at Aalborg University Hospital, Denmark. For the 'DART Bone (SBRT)' cohort, patients are also referred for radiotherapy at Odense University Hospital and Copenhagen University Hospital, Herlev and Gentofte, Denmark