Evaluation of mild focal bone marrow uptake on 18F-FDG PET/CT

Objectives A diffuse or focal bone marrow (BM) FDG uptake usually reflects hyperplastic BM or pathologic process such as involvement of malignancy or hematopoietic disease. We have sometimes encountered mild focal marrow FDG uptake and it is difficult to differentiate from early sign of metastasis. This study evaluated mild focal marrow uptake on PET/CT to find out their characteristics such as the benignancy.

Methods From January 2006 to October 2011, PET/CT for clinical evaluation of known or suspected malignancy were retrospectively evaluated. Patients with hematologic malignancy and bone tumor were excluded. Nineteen patients (15 men; mean age, 56.4 years) were enrolled who had foci of slightly higher FDG uptake than surrounding BM on visual inspection. They had follow up (FU) studies (FU PET/CT in 15 patients) including the other imaging investigations such as MRI, bone scintigraphy (10 and 5 patients, respectively) and biopsy was done in 2 patients.

Results Sixteen of 19 patients had known primary cancer (GIT 8, lung 3, breast 2, tongue 1, thyroid 2). Eleven patients had single marrow uptake and 8 patients presented with focal marrow uptake more than two site. The mean SUVmax of the focal BM uptake is 2.9 ± 0.7. The proximal femur is the most common uptake site (8 patients). All patients showed no interval change or decreased uptake on FU PET/CT. MRI revealed abnormal signal in the corresponding sites, and interpreted as metastases, focal island of red marrow and fibrous dysplasia. Two patients presented with metastatic osteolytic lesions on FU PET/CT but previously identified marrow uptake had no interval change. Two patients with multifocal marrow uptake underwent bone biopsy and the results were negative. However 1 patient had multiple bone metastases on FU bone scintigraphy finally. The mean FU period was 18.1 ± 14.1 months.

Conclusions Our study showed that most of the mild focal BM uptake has a tendency of benign condition such as functional hematopoietic marrow rather than metastases. Further studies including pathologic examination and more patients are needed to conclude