Metastatic
Bone Lesions
Metastatic bone lesions are the predominant malignancy to effect bone,
with 15 times the occurrence rate of the next most common bone malignancy. There are 5 tumors notorious for their
capacity to spread to bone that include Breast,
Lung, Thyroid, Renal Cell and Prostate (a popular memory aid is BLT Kosher
Pickle.) Primarily they spread to
spine, but lung cancer is known to metastasize to the peripheral extremities.
Only 9% of the time is a single metastasis detected.
Symptoms are variable depending on the
site, type and size of the lesion. Blastic
metastases are usually non-painful and likely relates to the decreased risk
of pathologic fracture when compared to lytic metastases. Hypercalcemia
is not uncommon, especially with breast and lung metastases. For the most part breast lesions are blastic but the other lesions are more variable
(Figure 17).
|
Figure
# 17
Blastic metastases
to the pelvis and proximal femur |
From the orthopedic standpoint,
pathologic fractures forebode an average
survival of 19 months depending on the type of lesion with the worst
average survival with lung metastases being an average of 4 months. Before any intervention is to be undertaken
a minimum life expectancy of 6 weeks
should be expected. The most common
lesion about the hip occurs at the intertrochanteric region and should be
treated with a long-stem, calcar-replacing arthroplasty. The next most common lesion of the hip
involves the femoral neck and should be treated with a cemented bipolar. Femoral shaft lesions should be treated with
a cephalomedullary device to protect the
femoral neck and intertrochanteric region if the metastasis spreads. As previously stated, isolated metastases
are uncommon and radiographs of the
entire bone to be treated must be taken to confirm the absence of other
lesions that could compromise surgical interventions. For example, an intertrochanteric lesion to be treated with a
long stem arthroplasty would suffer catastrophic failure if a second missed
lesion were to be present at the distal extent of the arthroplasty stem causing
a large stress riser and eventual periprosthetic fracture. Lesions of the spine have different
characteristics depending on location.
Cervical spine metastases are rarely problematic due to the wide canal
with relation to spinal cord diameter.
Thoracic spine lesions, on the other hand, are well known to cause paralysis due to the narrow canal with
relation to a flare in the spinal cord diameter. These lesions should be treated with a thoractomy, vertebrectomy,
and placement of a femoral allograft with cement. This is one of the few indications for allograft when dealing
with metastatic lesions, as most lesions will require post-operative XRT rendering bone graft non-viable. In this case, the femoral allograft acts as
a structural graft to prevent collapse without the hope of bony
integration. Low thoracic and lumbar
vertebra can also progress to paralysis, though the canal to cord diameter
ratio is more favorable. These lesions
should also be approached anteriorly to perform vertebrectomy with placement of
femoral allograft and cement. A
posterior approach with laminectomy to decompress the cord is a tempting option
but should be resisted for fear of
further destabilizing the spine. In
many cases, the intact posterior elements provide the only resistance to
complete mechanical failure of the bone.
Acetabular lesions are some of the most challenging metastases to
treat. In general they are treated with
hip arthroplasty after the lesion has been removed with a curette and replaced
with cement that contains structural metal rebar reinforcement.
Special mention of pre-operative planning
in the case of renal cell carcinoma metastasis needs to be made. As one would expect from the tissue of
origin, these lesions are highly
vascular. Exposure during attempts
to remove or biopsy these lesions can cause exsanguination in a matter of moments. For this reason, pre-operative angioablation should be performed.
As previously mentioned, XRT is often
used in conjunction with the above measures.
Prostate metastases and lymphoreticular lesions are the most radiosensitive; while breast is
intermediately sensitive. Tamoxifen is
only 30% effective against breast cancer metastases unless they are known
estrogen receptor positive in which case the effectiveness increases to 70%. Finally, renal and GI metastases are the
least radiosensitive.
