Osseous Metastasis

While PET/CT is of limited diagnostic value in the evaluation of primary bone tumors (addressed in detail, here), it is a powerful tool for detecting most metastatic lesions – even lesions not identifiable on CT bone window images. In fact, up to 50% of osseous metastases noted on the PET portion of the exam are radiographically occult on the co-registered CT images.

• PET/CT imaging is clearly superior to convention bone scanning for lytic metastases and for mixed lytic/sclerotic lesions.  
• For blastic/sclerotic metastases (e.g. breast & prostate cancers), PET imaging considered less sensitive than a traditional bone scan.
• Recent literature, however, now suggests that while a bone scan may be more sensitive than a PET scan alone for blastic lesions, it is probably less sensitive when compared to a PET scan plus a CT scan (e.g. a PET/CT scan).

Our general approach to assessing a potential osseous metastasis relies on both the PET and CT appearance of the lesion, analyzing it according to the following algorithm:

1.“PET Positive” & “Suspicious on CT”: 

This appearance is highly suspicious for malignancy (some state “consistent with”). We generally report: 

“An intensely avid lytic bone lesion is present within the L3 vertebral body (SUV 5.5), most consistent with an osseous metastasis.” 

2. “PET Positive” & “Normal on CT”: 

Despite the lack of a CT abnormality, such PET abnormalities are still very suspicious (an estimated 50% of metastatic bone lesions present on the PET portion of the exam are radiographically occult on CT). We generally report:

“A focus of intense uptake is noted within the L3 vertebral body. Although an associated osseous abnormality is not present on the co-registered CT images, the appearance is highly suspicious for an osseous metastasis.”

Obviously, if there are multiple sites of FDG uptake in the skeleton, the likelihood of metastatic disease dramatically increases.

3. Non-Avid CT “Abnormality” (Non-Sclerotic):  

Such findings generally represent either benign lesions (e.g. hemangioma) or treated metastases. In these cases, we generally report:

“An ill-defined lucency is noted in the L3 vertebral body, non-avid. Although of unclear etiology, its lack of metabolic activity suggests either a benign finding or treated lesion.”

4. Non-Avid Sclerotic CT Abnormality: 

Nearly always, non-avid sclerotic bone lesions do not represent active malignancy, usually representing either treated metastases or benign lesions such as bone islands.

As PET can have decreased sensitivity for some blastic metastases (especially in breast cancer or prostate cancer), the possibility of a false negative (e.g. non-avid active metastasis) is a possibility. In light of this concern, we often report: 

“A 1.3 cm non-avid sclerotic focus is noted in the L3 vertebral body. While its lack of metabolic activity suggests a benign lesion or treated metastasis, PET imaging can have decreased sensitivity for some blastic lesions. 

If multiple non-avid sclerotic densities are present in a breast cancer or prostate cancer patient without a history of prior treatment, these would be suspicious for “active” metastatic lesions despite their lack of FDG uptake.

“Flare Phenomenon”:

Osteoblastic activity associated with healing of bone lesions can lead to an increase in the metabolic activity of a bone lesion on a follow-up scan (may also see an increase in the number of lesions, as an occult lesion on the prior exam now becomes apparent).

This phenomenon typically occurs 2 weeks to 3 months after therapy (can be as late as 6 months).

We consider this possibility when the osseous metastatic disease appears worse, yet the non-osseous findings have improved.  In such cases, recommend follow-up. 

Associated most frequently with breast, prostate and NSCLC.