Most bone metastases of unknown origin come from the lungs or kidneys

In patients over 40 years old, the incidence of bone metastasis is higher than that of primary malignant bone tumors such as chondrosarcoma. Bone metastasis often occurs in the proximal bones of the limbs or the spine. The most common primary sites include the lungs, kidneys, prostate, breasts, and thyroid glands, but it is not ruled out that the source of bone metastasis in some patients is unknown.

In patients with a history of primary malignant tumors, breast pain and prostate cancer are the most common sources of bone metastasis, while most bone metastases of unknown origin come from the lungs or kidneys. About 40% of the primary tumors are located in the lungs, 8% in the kidneys, 5.6% in the prostate, 4.8% in the liver, and 4% in the breast. This may be because lung or kidney tumors are usually difficult to detect and can grow to a larger size without symptoms, or it may be because these tumors are more likely to metastasize early than breast or prostate cancer. Therefore, most primary tumors may be found through examination of the chest and abdominal organs. For primary sites such as the breast that are less common in patients with bone metastases of unknown origin, there is no need for immediate special examinations unless the medical history and physical examination prove abnormalities.

Because most primary tumors are usually deep and small in size, it is sometimes difficult to obtain positive results through physical examination. Clinical scholars have reported that only 8% of patients can find primary tumors through physical examination. The focus of the physical examination should be on the prostate, breast, thyroid and abdomen, which may provide more clues.