A normal cisterna chyli is a structure that may simulate retrocrural adenopathy on unenhanced CT. Our case report is significant because it highlights two cases in which patients with prior primary malignancies (medullary thyroid carcinoma, esophageal carcinoma) underwent PET-CT for re-staging, and in each case, a prominent right retrocrural structure is seen representing either a cisterna chyli or retrocrural metastasis. The physiologic activity of the structure on PET-CT is considered and cross-sectional abdominal images are examined in full as well. In both cases, the structure has a low attenuation on CT and a low SUVmax on PET-CT, consistent with a benign process. These cases demonstrate that with the combined use of CT attenuation and PET SUVmax as guiding parameters, a cisterna chyli should be more readily distinguishable from retrocrural adenopathy, and thus can help to avoid unnecessary invasive management of the patient.