Background: There are limited epidemiologic studies characterizing skeletal metastasis, and fewer, if any, that comprehensively compare and contrast patients who are initially diagnosed with skeletal metastases of unknown primary (SMUP) to patients who have a known primary at the time that skeletal metastases are identified. Methods: This single-centered retrospective study provides a descriptive analysis of patient characteristics (demographics including but not limited to sex, age, and history of tobacco use), lesion characteristics (sites of primary lesion as well as number and location of metastatic lesions), and outcomes for patients identified as having SMUP compared to patients with a known primary cancer who have skeletal metastases. Results: In this study, 164 patients were identified as having bone metastases through record review of ICD-9 and ICD-10 codes. Of these patients, 65.9% had a known primary prior to identification of bone metastases while 34.1%were classified as having SMUP. Patients identified with SMUP were disproportionately male while patients with a known primary were disproportionately female. No other significant differences were identified for other patient characteristics. For patients with bone metastases with known primaries, the most common primary sites were lung, breast, and prostate carcinoma, in that order. However, the most common primary sites for patients with SMUP were, in order, lung, prostate, and breast carcinoma. The most common location for metastases, regardless of known or unknown primary was the spine, followed by the pelvis and the ribs. Lastly, with respect to long term prognosis, while there was a significant difference in patients’ survival time based on solely on SMUP status, there was no difference in survival once bone metastases were identified, regardless of whether the primary is known, as all bone metastases carried a poor prognosis. Conclusion: There are significant differences in the male to female ratio with respect to the initial presentation of bone metastases when comparing known primary to unknown primary malignancies. SMUP are more common in males than females, with 61% of males initially presenting with SMUP compared to only 39% of females presenting with SMUP. This finding warrants further evaluation of the etiology of this difference, which may potentially be clinically significant.
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