INTRODUCTION: Metastatic bone tumors frequently occur in the spinal region. Patients with spinal metastases of unknown primary origin who present only with back or radicular pain at the first visit are often initially treated by orthopedic surgeons, and identification of the primary can be difficult. 18F‐fluoro‐2‐deoxyglucose positron emission tomography (18F-FDG PET-CT) is a non-invasive and highly sensitive whole-body imaging, which is generally performed prior to biopsy for patients with suspected bone metastases of unknown primary at our institution. The purpose of the present study is to review and report the efficacy of 18F-FDG PET-CT ordered by orthopedic surgeons for the identification of the primary lesions in patients with back or radicular pain and spinal metastases.
METHODS: Out of 136 patients with suspected bone metastasis of unknown primary and 18F-FDG PET-CT ordered by orthopedic surgeons between 2009 and 2023, of whom each medical chart was retrospectively reviewed, 23 patients with suspected spinal metastases who present only with back or radicular pain at the first visit were included. Diagnosis of all patients was confirmed by histopathological examination. Age, sex, metastatic lesion, histopathological diagnosis, and oncological outcome were collected. Efficacy of 18F-FDG PET-CT was classified into 5 groups, based on how 18F-FDG PET-CT contributed to the identification of the primary, as follows: Essential: identification of primary was difficult without 18F-FDG PET-CT, Beneficial: although identification of primary may have been possible without 18F-FDG PET-CT, the findings of 18F-FDG PET-CT were effective, Unnecessary: identification of primary was not affected by F18F-FDG PET-CT, Misleading: identification of primary was interfered with by 18F-FDG PET-CT, No information: no accumulation was detected.
RESULTS: The median age was 61 years (range, 35-80 years). Patients consisted of 15 male and 8 female. Metastatic lesions were various from the seventh thoracic vertebra to the sacral vertebra. The histopathological diagnosis of the primary tumor was as follows: 5 cases of lung cancer, 3 cases of malignant lymphoma, 2 cases of prostate cancer, 1 case of each breast cancer, uterine cancer, thyroid cancer, liver cancer, gastric cancer, rectal cancer, multiple myeloma, osteosarcoma in the tibia. In one case, diagnosis could not be confirmed despite histopathological examination. In four cases, spinal tumors were diagnosed as primary, not metastatic. Oncological outcomes at the last follow-up were as follows: alive with disease (AWD) in 12 patients and died of disease (DOD) in 11 patients. Efficacy of 18F-FDG PET-CT was analyzed in 19 patients with spinal metastases as follows: 2 cases of essential, 12 cases of beneficial, 1 case of unnecessary, 1 case of misleading, 3 cases of no information.
DISCUSSION: Although 18F-FDG PET-CT is expensive and time-consuming, it is noninvasive and provides visually clear results, which can contribute to identification of primary lesions. In the present study, 18F-FDG PET-CT provided useful clues to the identification of the primary in 14 out of 19 patients with spinal metastases of unknown primary presenting only with back or radicular pain. 18F-FDG PET-CT may be a useful first-line imaging modality for patients with spinal metastasis of unknown primary.