Examination Methods That are Effective in Diagnosing The Primary Origins Of Metastatic Bone Tumors of Unknown Primary Origin During The Initial Visit: A Retrospective Case Series

Background: The occurrence of skeletal metastases of unknown primary origin (SMUP) is uncommon. To provide appropriate treatments for the primary and metastatic lesions, early diagnosis of the primary origin of SMUP is important. Therefore, this study aimed to assess the types of examination method effective in diagnosing the primary origin of SMUP and their order in terms of efficacy. Methods: We retrospectively examined 63 patients with SMUP. The primary origin was examined via physical examination, blood test including tumor markers, chest radiography, thoracoabdominal computed tomography (CT) scan, positron emission tomography (PET)-CT scan, metastatic lesion biopsy, and other assessments. Examination methods considered effective for the diagnosis of the primary origin in a specific type of cancer were investigated. Results: The lung was the most common primary origin, followed by the lymph nodes, prostate, and breast. Meanwhile, biopsy (87.0%) was the most effective examination, followed by PET-CT scan (60.0%) and thoracoabdominal CT scan (38.6%). The first four steps; physical examination, blood tests, chest radiography, thoracoabdominal CT were effective in diagnosing the primary origin in 41 (65%) of 63 patients. Finally, four patients were diagnosed with SMUP. Conclusions: The examination steps used in this study are effective in diagnosing the primary origin of SMUP during the initial visit. Blood tests are useful for hematological malignancy and prostate cancer. CT scan can be used to identify cancers in the lung, breast, and kidney, which are the common primary origins. Moreover, it is more useful than PET-CT scan from a comprehensive perspective.