Bone Metastasis

Bone
is the third most common site of metastatic disease. Cancers most likely to metastasize to bone include breast,
lung, prostate, thyroid and kidney. Carcinomas are much more likely to metastasize to bone than sarcomas. The axial
skeleton is seeded more than the appendicular skeleton, partly due to the persistence of red bone marrow in the
former. The ribs, pelvis and spine are normally the first bones involved and distal bones are rarely affected.
Metastases are established when a single tumor cell or a clump of cells gain access to the blood stream, reach the
bone marrow through blood vessels in Haversian canals, extravasate, multiply and neovascularize. Batson’s vertebral
venous plexus allows cells to enter the vertebral circulation without first passing through the lungs. The sluggish
blood flow in this plexus is more conducive to tumor survival, accounting for the high rate of prostate cancer
metastasis to the spine.

Pain,
pathological fractures and hypercalcemia are the major sources of morbidity with bone metastasis. Pain is the most
common symptom found in 70% of patients with bone metastases. l Pain is caused by stretching of the periosteum by the
tumor as well as nerve stimulation in the endosteum. Pathological fractures are most common in breast cancer due to
the lytic nature of the lesions. They are uncommon in lung cancer due to short life span and rare in prostate cancer
which tend to be osteoblastic lesions. Hypercalcemia only occurs in 10% of patients.2

Lytic
bone metastases must be greater than 1 cm and have destroyed 30-50% of the bone density 3 in order to be seen by
x-ray. It is also difficult to distinguish between metastases and benign lesions such as Paget’s disease or
osteoporosis on plain film. On bone scan, radiolabeled bisphosphonates are taken up by in areas of bone formation but
not by the tumor cells. CT is more specific than bone scan and can distinguish between osteolytic and osteoblastic
lesions. MRI is the most sensitive method of detection bone metastases because cells can spotted before local bone
reaction has occurred. Metastatic bone lesions can be described as osteolytic, osteoblastic and mixed. The osteolytic
lesions are most common where the destructive processes outstrip the laying down of new bone. New treatments with
medicines that may block bone lysis by tumor cells are currently in clinical trials. Osteoblastic lesions result from
new bone growth that is stimulated by the tumor. Microscopically, most lesions are mixed.

Treatment
for bone metastasis is normally palliative. An assessment of the risk of pathological fracture must be made by an
experienced orthopaedic surgeon. Lesions that do not represent a risk for fracture may be treated with radiation or
by appropriate chemotherapy directed at the tumor. Lesions that are regarded as a risk for pathologic fracture should
be surgically stabilized on an elective basis before a fracture occurs. The goals of surgery are to preserve
stability and function of the musculoskeletal system as well as alleviate pain. Emergency surgery is done for spinal
metastasis in the hope of preserving neurological function.

References
^lVinholes, J. et al., Effects of Bone Metastases on Bone Metabolism: Implications for Diagnosis, Imaging and Assessment
of Response to Cancer Treatment, Cancer Treatment Reviews 22:289-331, 1996. 2 Stoll, B. and Parbhoo, S.,
Bone Metastasis, Raven Press Books, Ltd.:New York, NY, 1983, p. 14. 3 Vinholes, et al. 1/14/98 8:46 AM
1

Adapted, with permission from bonetumor.com By Henry DeGroot III, M.D.