br Criteria for diagnosing of skeletal metastases
Criteria for diagnosing of skeletal metastases by FDG PET/CT are increased standardized uptake value (SUV) on PET image, and osteo-blastic lesions, osteolytic lesions, mixed osteoblastic/osteolytic lesions, or no demonstrable anatomical change on CT image. Presence of Journal of Bone Oncology 15 (2019) 100219
Characteristics Number Percentage
fracture lines or callus formation was interpreted as a fracture.
The clinical data of the 290 patients including age, gender, patho-logical diagnosis reports and the results of PET/CT and bone scanning were retrospectively analyzed. The patients were divided into four groups according to primary tumor regions: head and neck, thorax, abdomen and pelvis. These classifications are shown in Table 1. The skeleton was classified into six regions: skull (including bones of the cerebral Pimozide and facial cranium); cervical spine; thoracic bones (including ribs, sternum, collarbone, bladebone and thoracic spine); lumbar spine; pelvis (including sacrococcyx, ilium, ischium, and pubis); and extremities (including humerus, femur, radioulnar, and tibio-fibular) (Fig. 1). According to the results of PET/CT and bone scanning of the 298 patients, distribution and pattern of skeletal metastases were analyzed.
2.4. Statistical analysis
The chi-square test was performed to compare diﬀerences in pro-portions of skeletal metastases between diﬀerent groups using SPSS statistical software (version 20.0). And P values less than 0.05 was considered statistically significant.
Of the 102 patients with only one bone containing a metastasis, 8
Fig. 1. Six regions of human skeleton: skull, cervical spine, thoracic bones, lumbar spine, pelvis and extremities.
Fig. 2. PET/CT images of a patient with breast cancer who had only one bone metastasis, in her thoracic spine (T3).
Overall, the thoracic skeleton was more frequently metastasized in patients with thorax tumors than the other patients (80.9% vs. 67.0%, P = 0.007). The cervical spine or thoracic bones were more frequently metastasized in patients with primary tumors above the diaphragm than those below the diaphragm (82.0% vs. 65.5%, P = 0.002). Patients with pelvic tumors had a higher incidence of pelvis metastasis than other patients, but not significantly so (82.9% vs. 68.2%, P = 0.077) (Table 2).
Reportedly, about two-thirds of patients with cancer will develop bone metastasis , and bone metastasis complications are sometimes the first manifestation of an occult primary tumor [20,21]. Thus, ex-ploring the distribution features of bone metastasis is important. Al-though studies on bone metastasis have been published for diﬀerent tumor types, such as breast cancer , prostate cancer  and others [24,25], this study addresses primary tumors in all bodily parts.
Our cohort excluded patients with visceral metastasis to focus on bone metastasis that were likely spread by the vertebral venous plexus, and we found that the metastases are usually found in the bones close to the primary tumor, which is reminiscent of the first-pass organ model for metastatic dissemination. The most frequently involved area was the spine, followed by the pelvis and thoracic bones, which is consistent with previous reports of bone metastases spread by the circulatory system [26,27].
Especially among patients with only one bone containing a metas-tasis but no visceral metastases, we observed the characteristics of bone metastatic distribution that neighboring bones to the primary were more likely to be first metastasized. This also correspond to previous findings that the diﬀerences in the distribution of bone metastases be-tween pulmonary, breast and prostate cancers [7,11].
Hematogenous spreading is the most common pathway for bone metastasis, in addition, vertebral venous plexuses play very important roles. Due to the low pressure, large volume, slow blood flow and abundant vessel branches of the vertebral venous plexus, tumor cells can easier transfer to nearby bones. Whether the bone metastasis pat-terns of cancer patients with bone metastasis but no visceral metastasis
Fig. 3. Metastases of neighboring bones by tumors in diﬀerent parts of the body among the 102 patients who had only one bone containing a metastasis. **P<0.05.
are clues to primary tumor locations in patients with CUP warrants further investigation.
Although we found no evidence that indicates the skull and lumbar spine were more frequently metastasized from certain tumors, among patients with only one bone containing a metastasis, the cervical spine was more frequently metastasized in patients with head and neck tu-mors than in other patients, the thoracic skeleton was more frequently metastasized in patients with thorax tumors than in other patients, and the pelvis was more frequently metastasized in patients with pelvic tumors than in other patients. These distributions may provide valuable clues to primary tumor locations in patients with CUPs, especially those with single or few bone metastases; the CUP is likely to be in a neighboring organ.