From the Guidelines
A PET scan from skull to mid-thigh is the recommended imaging procedure for cancer staging, detecting metastases, evaluating treatment response, and identifying infection or inflammation sources, as it provides comprehensive coverage of the most common sites for disease while limiting radiation exposure and scan time. This scan typically involves injecting a radioactive glucose solution (usually F-18 FDG) intravenously about an hour before imaging, and the procedure covers from the base of the skull down to mid-thigh, capturing all major organs and potential disease sites in this range 1. The skull-to-mid-thigh protocol is particularly valuable because it shows functional activity rather than just anatomical structure, helping to distinguish between benign and malignant processes.
Some key considerations for patients undergoing a PET scan from skull to mid-thigh include:
- Fasting for 4-6 hours before the scan
- Avoiding strenuous exercise for 24 hours prior
- Informing their doctor about diabetes, pregnancy, or claustrophobia
- The actual scanning time is typically 20-30 minutes, though the entire appointment may take 2-3 hours including preparation and tracer uptake time. The use of PET/CT over CT for the evaluation of metastatic disease is supported by NCCN guidelines, and FDG-PET/CT has been shown to be useful in detection of local and distant disease, as well as in excluding metastatic disease in patients with pelvic recurrence who are amenable to radical surgery 1.
In terms of specific clinical scenarios, a PET scan from skull to mid-thigh is usually appropriate for the initial staging of patients with newly diagnosed esophageal cancer, as well as for the evaluation of patients with esophageal cancer undergoing treatment, and for patients who had esophageal cancer with no suspected or known recurrence after treatment 1.
From the Research
PET Scan Skull to Mid Thigh
- The study 2 assessed the utility of whole-body 18F-fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG PET/CT) imaging relative to the standard field of view (skull base to mid-thigh) in patients with primary melanoma site that is not located in the lower extremities.
- The results of the study 2 suggest that 18F-FDG PET/CT imaging of the lower extremity may not be justified if the primary neoplasm is not located in the lower extremities, as it may not change patient management.
- Another study 3 found that including the entire brain on body imaging with FDG PET/MRI can provide added clinical value to the management of oncology patients, as it can detect important brain abnormalities that may be missed when the head is not included.
Limitations of PET/CT Imaging
- The study 4 highlights one of the main limitations of PET-CT imaging, which is its inability to distinguish between cancerous lesions and post-treatment inflammatory conditions.
- The study 5 also notes that false-positive FDG uptake can occur in several infectious diseases and aseptic inflammatory processes, which can lead to false-positive diagnosis.
- The study 6 discusses the potential of hybrid PET/MR for imaging infection and inflammation, which can provide more accurate diagnosis and management of these conditions.
Clinical Implications
- The study 2 suggests that elimination of lower extremity imaging can reduce scanning time and additional radiation exposure, which can be beneficial for patients.
- The study 3 found that routine body imaging with FDG PET/MRI of the area from the skull base to the mid thigh may miss important brain abnormalities when the head is not included, which can have significant clinical implications for patient management.