¹⁸F-FDG PET/CT and ^99mTc-MDP Imaging of Non–Small Cell Lung Carcinoma Osseous Metastases

QUESTION 1

Based on the ¹⁸F-FDG PET/CT images, which of the following statements best describes the findings in the skeleton:

• A. Multiple traumatic fractures.

• B. Multiple metabolically active osseous metastases.

• C. Lesions that cannot be characterized and are most likely degenerative disease.

• D. Bone marrow stimulation.

One week later, a bone scan was performed to evaluate if ⁸⁹Sr could be used to manage the patient’s intractable lower back pain. A whole-body bone scan was obtained 2 h after intravenous injection of 925 MBq (25 mCi) of ^99mTc-methyldiphosphonate (^99mTc-MDP):

QUESTION 2

Based on the anterior and posterior whole-body bone scan images, and considering the prior PET/CT scans, which of the following statements is true (Note: there is a history of trauma to the right ankle):

• A. The bone scan appearance is expected because ¹⁸F-FDG PET is more sensitive than bone scanning for detection of osteoblastic metastases.

• B. The discrepancy between the bone scan and ¹⁸F-FDG findings is most consistent with a benign process.

• C. ¹⁸F-FDG PET allows earlier detection of metabolically active metastases in marrow that may not yet have generated an osseous response.

• D. Bone scintigraphy is generally an insensitive modality for staging lung cancer skeletal metastases.

CASE DISCUSSION

In this patient with non–small cell lung cancer, ¹⁸F-FDG PET and ^99mTc-MDP bone scintigraphy were strikingly discrepant for visualization of skeletal metastases. Although the literature shows varying results, the consensus is that ¹⁸F-FDG PET and ^99mTc-MDP bone scintigraphy have equivalent sensitivity (∼90%), with ¹⁸F-FDG PET having the higher specificity (98% vs. 61% in 1 study).

The 2 agents show different sensitivities in osteoblastic versus osteolytic skeletal metastases, based on the mechanisms of tracer localization. ¹⁸F-FDG uptake represents increased glucose metabolism in malignant cells, whereas diphosphonate uptake reflects the remodeling response surrounding the metastatic deposits, which may take longer to develop. ¹⁸F-FDG PET thus potentially has an advantage for detection of early bone disease. Several authors have shown that ¹⁸F-FDG PET is more sensitive in detecting osteolytic metastases than bone scintigraphy, whereas bone scintigraphy is more sensitive in detecting osteoblastic metastases. This suggests a complementary role for ¹⁸F-FDG PET and bone scintigraphy depending on the predominate type of bone metastases expected.