The purpose of this study is to compare whole body magnetic resonance (MR) imaging, whole body positron emission tomography (PET)/MR imaging, and (if available) PET/Computed Tomography (CT) imaging for the diagnosis of tumors in children and young adults. Sensitivities, specificities and diagnostic accuracies of the different imaging modalities will be compared for significant differences.

Inclusion Criteria:

• Patients with proven solid tumors or newly diagnosed mass strongly suspected to represent a solid tumor.
• We might also include some patients with leukemia if there is concern for a focal lesion in any of their organs.
• Participant's parents (if participant is under 18) or the participant (if participant is 18 years or older) must willingly give written informed consent prior to any image transfer to the image registry.

Exclusion Criteria:

• Patient has contraindications for MRI or PET/MR. This includes patients with cardiac pacemakers or intracranial vascular clips as well as patients with a blood glucose level > 200 mg/dl.
• Lack of parental permission (if participant is younger than 18) or lack of informed consent (if participant is at least 18 years of age).
• Patient has a CNS primary tumor.
• Pregnant women and fetuses

Pediatric and young adult (<40 years) male or female subjects with proven solid tumors or newly diagnosed mass strongly suspected to represent a solid tumor