Focal bone activity on F18-FDG PET/CT images in oncology patients - How relevant is it clinically ?

Objectives Focally increased bony FDG activity on FDG PET/CT (PET/CT) in oncology patients is often considered malignant, especially in the presence of corresponding CT abnormality. However, when such foci are noted without CT changes, the clinical relevance is unknown. Very few existing reports address this subject matter. We therefore undertook this IRB approved analysis of the incidence and significance of newly detected, focally increased bone activity in patients presenting for PET/CT imaging for treatment strategy.

Methods Reports of 938 PET/CT scans were reviewed for any focally increased FDG activity in bones. Criteria for including patients was presence of focal osseous FDG uptake, who had a definite follow up. Patients with previously known bone metastases and with significant diffuse bone marrow uptake that precluded evaluation for focally abnormal activity, were excluded. Final diagnoses were based on a reference standard which included biopsy, additional imaging like a diagnostic CT or MRI and/or follow up imaging. 57 patients were eventually included in our study. The number of lesions in each patient were studied individually if there were three or less lesions. The presence or absence of corresponding CT abnormality in the area of focally increased activity was also noted.

Results Out of the 57 patients in our study, 20 patients who had multiple lesions were considered as a single entity. Of these,12 had corresponding CT changes and 8 were without CT changes. The remainder of the 37 patients had 52 lesions (each lesion was considered as an individual entity) - 36 with CT changes and 16 without CT changes. Therefore, in total we studied 72 (52 + 20) entities. Our results showed that 85% of the entities with no CT change and 87% entities with some CT change were truly malignant. 15% of entities with no CT abnormality and 13% with CT abnormality were proven to be benign. The p value for these results was 1, suggesting no statistically significant difference between the 2 groups. The sensitivity and positive predictive value of having a CT correlate with FDG uptake was expectantly high, 63% and 87% respectively. However, the negative predictive value of having a CT correlate was only 15%.

Conclusions Foci of FDG uptake overlying bone structures are highly predictive of metastatic disease when associated with bone changes on low dose CT scan. However, the absence of a CT correlate on FDG PET study does not rule out the malignant process. FDG PET can detect metabolic changes earlier than morphological changes are apparent on anatomical imaging. A considerable number of such lesions represents malignant disease and further imaging or pathological confirmation is warranted.

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