Diagnostic Performance of PET/CT Scans in Evaluation of Patients with Skeletal Metastases of Unknown Primary

While it may be an ineffective tool for use in the identification of primary malignancies in patients with a skeletal metastases of unknown primary (SMUP), PET/CT could have utility as a screening tool for treating orthopaedic oncologists, a new study suggests. The study, presented today at the 2019 Annual Meeting of the American Association of Orthopaedic Surgeons (AAOS) in Las Vegas, evaluated the diagnostic yield of PET/CT imaging in identifying a primary tumor in patients with SMUP. Secondarily, the study assessed the ability of PET/CT scans to identify primary and metastatic sites in all patients with skeletal metastases.

Identifying primary cancer impacts patient outcomes

“Despite utilizing a well-known standard diagnostic evaluation, the diagnosis of a primary malignancy remains elusive for some patients” states Lukas Nystrom, MD, a Staff physician in Cleveland Clinic’s Department of Orthopaedic Surgery. “Identifying the primary cancer source, and any metastases, is an essential component in staging and treatment.”

In order to determine the usefulness of PET/CT imaging in this process, this retrospective study included 35 patients at a tertiary cancer center who had both a standard diagnostic evaluation to assess the location of their primary cancer and subsequent PET/CT imaging. The standard diagnostic evaluation included history and physical, pertinent laboratory blood tests, CT of chest/abdomen/pelvis, whole body bone scan and a bone biopsy of the skeletal lesion. A primary cancer was identified in 63 percent of patients via the standard testing regimen, leaving 37 percent with SMUP.

PET/CT scans add little diagnostic insight

PET/CT scans were unable to identify a primary cancer in 92 percent of patients with SMUP. However, in all patients with a known primary cancer, PET/CT confirmed the site of the known primary cancer.

In terms of metastatic sites, PET/CT identified 176 sites in the 35 patients, 55 percent of which were in bone. There were 61 areas of uptake that were thought to be metastases, the majority of them also in bone. PET/CT scans had a false positive rate of 7.7 percent. PET/CT failed to identify only three of the 115 known metastatic sites in the patient cohort, for a false negative rate of 2.6 percent.

Skeletal metastases are the most common type of malignant bone tumor. Studies show that primary malignancy remains elusive in 15 percent of patients with a new lytic bone lesion. Biopsy alone is unable to detect cancer primary in 60 percent of cases; thus, the diagnostic odyssey often contains multiple, time consuming steps. A delayed diagnoses, or failure to achieve a diagnosis, has negative impacts for patients and may contribute to a poorer overall prognosis. The median survival for SMUP patients whose primary cancer site remained unknown at final diagnosis is 11 months.

“We hope that this study sheds light on the utility of PET/CT in this patient group. Although PET/CT will rarely aid in the identification of a primary cancer when our standard diagnostic examination fails to do so, there may be a role for their use. Given that these scans have been able to confirm the site of a primary cancer and demonstrate accuracy in confirmation and identification of additional metastases, it is reasonable to consider the PET/CT replacing the standard imaging evaluation. Further research, with multicenter collaboration, is warranted,” Dr. Nystrom notes.