Solitary bone plasmacytoma is a plasma cell disorder characterized by the formation of a single tumor in the bone.

The tumor, also called a plasmacytoma, occurs when abnormal plasma cells originating in the bone marrow accumulate on the interior surface of the bone. However, in patients with solitary bone plasmacytoma, these malignant plasma cells are typically not present throughout the bone marrow itself or in the soft tissues surrounding the bone.

The following article provides a detailed introduction to solitary bone plasmacytoma, including what it is, how it is diagnosed, how it is treated, and how it relates to multiple myeloma.

What Is Solitary Bone Plasmactyoma?

In patients with solitary bone plasmacytoma, abnormal plasma cells in the bone marrow aggregate to form a single tumor in any bone in the body.

Most commonly, the tumor develops in a bone along the spinal column. In order of decreasing frequency, the tumor may also develop in the pelvis, ribs, upper extremities, face, skull, femur, and sternum.

Unlike multiple myeloma, solitary bone plasmacytoma does not include the presence of abnormal plasma cells throughout the bone marrow or in the soft tissues surrounding the bone, a condition known as extramedullary myeloma (see related Beacon news). In some cases, however, patients with solitary bone plasmacytoma may also have a small amount of abnormal plasma cells in the bone marrow (see “Diagnosis”).

According to Dr. Vincent Rajkumar of the Mayo Clinic in Rochester, Minnesota, experts do not fully understand how abnormal plasma cells (which originate in the bone marrow) are able to travel from the bone marrow to the surface of the bone and form a tumor in patients with solitary bone plasmacytoma.

It is likely, however, that the location of the bone tumor is the same location at which the abnormal plasma cells initially originated in the bone marrow.

“That is why in roughly 50 percent of cases, solitary bone plasmacytoma is curable by radiating that one site alone,” said Dr. Rajkumar.

Patients with solitary bone plasmacytoma do not have the typical features of myeloma, which include low red blood cell counts, elevated calcium levels in the blood, or kidney deficiencies, although patients with solitary bone plasmacytoma may eventually progress to multiple myeloma (see "Prognosis").

Diagnosis

A patient is diagnosed with solitary bone plasmacytoma when a biopsy reveals a single tumor inside the bone comprised of abnormal plasma cells.

However, bone surveys and positron electron tomography (PET) or magnetic resonance imaging (MRI) must show no other lesions in the bone or in the soft tissues. Physicians recommend PET or MRI scans of the entire spine and pelvis in order to confirm that there are no abnormal plasma cell tumors anywhere other than the single site. Past studies have shown that one-third of patients suspected to have solitary bone plasmacytoma actually have evidence of other plasma cell tumors in other parts of the skeleton, as detected by PET and MRI scans.

In addition, blood tests must also reveal none of the typical myeloma-related features, such as low red blood cell counts, elevated calcium levels in the blood, or kidney deficiencies, in patients with solitary bone plasmacytoma.

The presence of monoclonal proteins in the blood or urine may be present in 30 percent to 75 percent of patients with solitary bone plasmacytoma. Monoclonal proteins are produced by abnormal plasma cells and are often measured in blood tests to track the progress of plasma cell disorders such as multiple myeloma.

Patients with solitary bone plasmacytoma who also have monoclonal proteins in the blood or urine are diagnosed differently but treated the same way as patients who do not have monoclonal proteins.

If the patient has less than 10 percent of abnormal plasma cells in the bone marrow, he or she is diagnosed with solitary bone plasmacytoma plus monoclonal gammopathy of undetermined significance, a common myeloma precursor disease.

However, if the patient has more than 10 percent of abnormal plasma cells in the bone marrow, he or she is diagnosed with stage 1 multiple myeloma.

Prevalence

Approximately 5 percent of all plasma cell disorders are solitary bone plasmacytomas. In the United States, roughly 450 new cases of solitary bone plasmacytoma are diagnosed each year. Men are diagnosed twice as often as women. Solitary bone plasmacytoma occurs most commonly in African-Americans and least commonly in Asians and Pacific Islanders.

The median age at diagnosis is 55 to 65 years, compared to a median age of 71 years for patients diagnosed with multiple myeloma.

Symptoms

The most common symptom of solitary bone plasmacytoma is pain at the tumor site as a result of bone destruction from the infiltrating plasma cells.

Some patients may also experience a fracture at the tumor site. Patients with compression fractures in the spine often experience severe spasms and back pain.

According to Dr. Rajkumar, pain or fractures at the tumor site are characteristic symptoms of solitary bone plasmacytoma, but more specific symptoms depend on which bone is involved.

“For example, a solitary bone plasmacytoma in the back may grow out of the vertebra to cause spinal cord compression and paralysis, while a solitary bone plasmacytoma in the rib may cause pain while breathing,” said Dr. Rajkumar.

Treatment

Radiation Therapy

The primary mode of treatment for patients with solitary bone plasmacytoma is radiation therapy localized to the tumor site.

A previous retrospective study involving 206 patients with solitary bone plasmacytoma indicated that patients who received localized radiation therapy had a lower rate of relapse (12 percent) than patients who did not receive radiation (60 percent).

However, according to the literature, the ideal radiation dose is controversial. Published studies have reported ideal doses ranging from 30 gray (Gy) to 60 Gy, although most radiation oncologists recommend doses between 40 Gy and 50 Gy. Gray is the radiation dose expressed in terms of absorbed energy per unit mass of tissue.

“Our radiation therapy specialists advise 40 Gy, similar to most practices elsewhere,” said Dr. Raymond Alexanian of the MD Anderson Cancer Center in Houston.

Evidence-based guidelines published by the United Kingdom Myeloma Forum recommend at least 40 Gy over the course of 20 radiation sessions. They also recommend that radiation should encompass the entire tumor plus a margin of at least 2 cm beyond the tumor region detected by MRI. For tumors larger than 5 cm, a higher dose (up to 50 Gy over 25 sessions) may be used.

Surgery

Surgery is rarely necessary but may be required for patients who have structural instabilities or deformities in the bone resulting from the tumor.

In these cases, radiation therapy may be delayed but is still administered after the surgery.

Chemotherapy

According to the literature, the use of chemotherapy as an additional or preventative measure, also called adjuvant chemotherapy, is controversial for patients with solitary bone plasmacytoma. While some studies have shown no benefit, others have suggested that adjuvant chemotherapy prevents or delays the time to progression to myeloma.

A small, retrospective study of 32 patients with solitary bone plasmacytoma revealed that adjuvant chemotherapy delayed the median time to progression to myeloma from 29 months to 59 months. However, chemotherapy did not decrease the frequency of progression to myeloma.

Another small study involving 53 patients with solitary bone plasmacytoma showed that patients who received radiation therapy plus chemotherapy had a significantly higher rate of disease-free survival after nine years than patients who received radiation therapy alone.

However, because these studies are small and therefore inconclusive, some physicians believe that there is currently no role for chemotherapy in the treatment of solitary bone plasmacytoma. Instead, they recommend observing patients after the initial radiation therapy.

According to Dr. Rajkumar, there are currently no data from randomized trials that support the use of adjuvant chemotherapy in the treatment of solitary bone plasmacytoma.

“We are planning clinical trials of adjuvant chemotherapy [in patients with solitary bone plasmacytoma]. But as with everything else in medicine, our treatments can either harm or help; that is why we need Phase 3 data with appropriate endpoints before embarking on new treatments in routine practice,” said Dr. Rajkumar.

Dr. Alexanian agreed that adjuvant chemotherapy should not yet be administered alongside radiation therapy as frontline treatment for solitary bone plasmacytoma.

“Since radiation therapy is curative for approximately one-third of properly staged patients [with solitary bone plasmacytoma], I do not favor adjuvant chemotherapy. [It] should be reserved for recurrent disease if or when it develops,” commented Dr. Alexanian.

Follow-Up

The Mayo Clinic recommends that patients with solitary bone plasmacytoma should have their blood and urine protein levels as well as their blood counts, creatinine and calcium levels tested every four to six months for one year after completion of radiation therapy. Thereafter, patients should continue to receive blood and urine tests on an annual basis.

A bone survey, MRI scan of the spine, or whole-body PET/CT scans should be administered if blood or urine tests reveal the presence of monoclonal protein in the blood, or if existing monoclonal protein levels increase.

Prognosis

The median overall survival time of patients with solitary bone plasmacytoma is 10 years. Approximately 75 percent of patients are alive after five years, and 45 percent of patients are alive after 10 years. Disease-free survival rates at five years and 10 years are 45 percent and 25 percent, respectively.

Approximately 50 percent to 60 percent of patients who are diagnosed with solitary bone plasmacytoma progress to multiple myeloma even after receiving radiation therapy.

According to Dr. Rajkumar, patients who are cured after radiation therapy are likely patients with true solitary bone plasmacytoma. On the other hand, patients who progress even after treatment with radiation therapy may have had myeloma prior to therapy.

“[The solitary bone tumor] is likely the first bone lesion of myeloma [in patients who relapse after radiation therapy]. However, this is not obvious given the limits of the radiologic tests that can be done,” said Dr. Rajkumar.

Previous retrospective studies have also found that two-thirds of patients with solitary bone plasmacytoma develop multiple myeloma within the first four years, although progression can occur as late as 13 years following diagnosis.

Progression to myeloma is thought to arise either from undetectable, abnormal plasma cells that were outside the field of radiation or from abnormal plasma cells that survive after radiation therapy.

In addition, the presence of monoclonal protein in the blood is a prognostic factor associated with progression to myeloma.

“Persistence of monoclonal proteins after radiation therapy obviously signifies residual disease outside of the radiation field,” said Dr. Alexanian.

One study showed that patients with solitary bone plasmacytoma whose monoclonal protein disappeared after radiation therapy had a 10-year progression-free survival rate of 91 percent, compared to 29 percent for patients who still had monoclonal protein following radiation therapy.

Another study involving 116 patients with solitary bone plasmacytoma showed that two risk factors predicted disease progression after five years. These included a persistent blood monoclonal protein level of above 0.5 g/dl one to two years after solitary bone plasmacytoma diagnosis, as well as an abnormal free light chain ratio at the time of diagnosis.

Patients who had neither of these risk factors had a 13 percent chance of progressing to myeloma after five years. Patients who had one risk factor had a 26 percent chance, and patients who had both risk factors had a 62 percent chance.

Other factors that increase the likelihood of progression to myeloma include a tumor size of at least 5 cm, old age, low bone density (osteopenia), and fast-growing blood vessels in the tumor sample (high-grade angiogenesis).

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