Solitary Plasmacytoma of the Rib: A Report of a Rare Case

Introduction

Solitary plasmacytoma (SP) is a malignant neoplasm characterized by the uncontrolled proliferation of plasma cells. Based on its site of onset, SP can be classified into 2 distinct subtypes: SPB and extramedullary solitary plasmacytoma (EMP). Among these, SPB is particularly uncommon, constituting merely 6% of all malignant plasma cell tumors [1]. Typically, SPB demonstrates a predilection for the vertebrae and skull, with a notably lower incidence observed in other skeletal sites, such as the ribs. Owing to its rarity, SPB often eludes accurate diagnosis, frequently leading to missed or misdiagnosed cases during clinical evaluations [2]. This report details the comprehensive diagnostic and therapeutic journey of a patient with SPB originating from a rib. By integrating findings from previous case reports of rib-based SPB, a meticulous analysis of the imaging characteristics of this case was conducted, encompassing X-ray, CT, and SPECT. The overarching objective of this study is to augment the existing imaging database of SPB and to propose practical recommendations for enhancing the diagnostic accuracy of this elusive condition.

Case Report

A 66-year-old male patient presented with occasional cough, without sputum production, dyspnea, chills, or fever. Outpatient chest CT revealed: (1) scattered solid nodules of undetermined nature in both lungs; and (2) bone resorption and destruction of the right fifth posterior rib, accompanied by the formation of a soft-tissue-density mass, raising suspicion of a bone tumor or metastatic lesion. Subsequently, the patient was admitted to the Department of Thoracic and Cardiac Surgery with a preliminary diagnosis of “lung nodules, rib tumor?” Chest X-ray clearly demonstrated bone destruction in the posterior segment of the right fifth rib (Figure 1A, 1B). CT imaging further indicated localized bone resorption and destruction of the right fifth posterior rib, with the formation of an ill-defined soft-tissue mass measuring approximately 4.0×2.4×2.8 cm. Notably, the mass showed no significant enhancement on contrast-enhanced scans, strongly suggesting a bone tumor, a pleural-originated tumor, or a metastatic lesion (Figure 2A–2D). SPECT revealed a patchy area of increased radiotracer uptake in the right fifth posterior rib. When combined with fusion imaging, this finding was indicative of a neoplastic process. The remaining whole-body bone scan showed no evidence of bone tumors or bone metastases (Figure 3A–3E). Routine laboratory tests did not yield any remarkable or characteristic abnormalities. The patient underwent a minimally invasive laparoscopic surgical resection of the lesion. Gross pathological examination of the specimen showed a gray-red to gray-white, rib-like tissue, measuring 10.0×3.5×3.0 cm. A mass was observed on the rib surface, and was approximately 6.0×2.5×1.5 cm in size. The cut surface of the mass was gray-white, solid, brittle, and resembled fish flesh, firmly adherent to the bone surface. Histological analysis confirmed that the lesion originated from a malignant tumor of the lymphohematopoietic system in the rib. Immunohistochemical staining results were as follows: CD3 (−), CD20 (−), CD79a (+), Pax-5 (−), Ki-67 (approximately 50%), CD10 (+), Bcl-6 (−), Bcl-2 (weak +), SOX11 (−), Kappa (+), lambda (−), CD99 (−), CK (−), p53 (1+, wild-type), CD138 (+), CD38 (+), MUM-1 (+), myoglobin (−), SOX-10 (−), CD56 (+). Integrating these immunohistochemical findings with the morphological characteristics, the diagnosis of solitary plasmacytoma (SP) was established (Figure 4A, 4B). Considering the patient’s imaging results, postoperative pathological findings, and laboratory data, a definitive diagnosis of SPB originating from the rib was made.

Discussion

SP is a relatively rare hematopoietic disorder, accounting for only 10% of hematologic malignancies. It originates from monoclonal plasma cells in the bone marrow, which explains why it is commonly found in hematopoietically active sites such as the vertebrae, femur, pelvis, and ribs. Based on its anatomical origin, SP is classified into SPB and EMP, with SPB being the rarer subtype. Previous studies [1,4] have indicated that SPB is more common in men, with a male-to-female ratio of approximately 2: 1, and the median age of onset is 55 years. Clinically, SPB typically presents as a solitary osteolytic lesion with or without soft-tissue infiltration [5]. Despite extensive research efforts, the etiology of SPB has not been clearly defined yet. Current evidence suggests that genetics, radiation exposure, and chronic antigenic stimulation are recognized as risk factors [3–5].

The main standards currently used in diagnosis of SPB are as follows [5,6]: (1) histopathology: single-bone clonal plasma cell proliferation; (2) normal bone marrow image, with the proportion of plasma cells <10%; (3) imaging suggests that there are no distant bone and multiple foci of disease, which may be accompanied by invasion of neighboring bone; (4) no end-organ damage (eg, hypercalcemia, renal insufficiency, anemia, bone damage); (5) no or low levels of serum and urinary M protein, and normal immunoglobulin levels [3,6]; and (6) no or low level of serum and urine M protein and normal immunoglobulin level [3,6]. As emphasized in the guidelines [5], imaging modalities are indispensable for evaluating the extent and severity of plasmacytoma. In our case, CT and SPECT each offered unique diagnostic insights. CT provided detailed visualization of the localized bone resorption and destruction, as well as the morphology, margins, and size of the soft-tissue mass in the right fifth posterior rib, thereby serving as a crucial basis for the initial assessment of the lesion’s nature. Given that SPB is predominantly characterized by osseous invasion with minimal soft-tissue involvement, whole-body bone scanning proved pivotal. By detecting the metabolic activity of the entire skeletal system, it confirmed the absence of additional bone tumors or metastatic lesions beyond the right fifth posterior rib. This finding is fundamental in differentiating solitary lesions of SPB from multifocal bone conditions such as MM and other diseases causing multiple bone lesions.

Clinically, SP typically presents with chest wall pain. However, in this particular case, the onset was insidious, the course of the disease was relatively short, and there were no characteristic clinical symptoms. Since this disease is rather rare, and similar to the research findings of Yao [2], Pi [7], and Ahyouni [8], our hospital lacked sufficient understanding of this condition, which led to failure in making a definite diagnosis before the operation. Fortunately, the patient consented to surgical treatment, which laid a solid treatment foundation and offered a better opportunity for subsequent aggressive treatment. Therefore, when adult patients show localized or solitary bone resorption, destruction, or invasion of adjacent soft tissues on imaging examinations such as X-ray, CT, and SPECT, even in the absence of characteristic clinical manifestations, clinicians should remain vigilant and consider the possibility of SPB.

During the clinical diagnosis process, great attention should be paid to the differential diagnosis of SPB from the following 3 diseases: (1) MM is characterized by multifocal skeletal involvement and is often complicated with anemia (hemoglobin <10 g/dL), hypercalcemia (>2.75 mmol/L, >11 mg/dL), and renal impairment (creatinine clearance <40 mL/min or serum creatinine > 177 μmol/L, >2 mg/dL). Bone marrow examination reveals that the proportion of plasma cells is ≥10%, and the serum/urine M protein levels are significantly elevated. In contrast, SPB presents as a single lesion without systemic bone marrow infiltration; the proportion of bone marrow plasma cells is <10%, and the M protein level is low or negative [9]. (2) Bone lymphoma can show osteolytic or osteogenic destruction on imaging and is often accompanied by lymphadenopathy. Immunohistochemical examination shows positive expression of lymphocyte markers (such as CD20+, CD3+). However, SPB is mainly characterized by single osteolytic destruction without lymph node involvement, and immunohistochemistry shows positive expression of plasma cell markers (such as CD138+, CD38+) [3]. (3) Bone metastatic carcinoma is another disease that needs to be differentiated from SPB. Most patients with bone metastatic carcinoma have a clear history of primary tumors. Imaging examination can reveal rib destruction accompanied by soft-tissue masses, and osteogenic reactions can be observed in some cases. Immunohistochemical examination shows positive expression of epithelial markers (such as cytokeratin CK and epithelial membrane antigen EMA). In contrast, SPB expresses plasma cell markers (CD138+, CD38+) but is negative for epithelial markers [10].

In terms of treatment strategies, solitary plasmacytoma (SPB), as a malignant tumor highly sensitive to radiotherapy, has been regarded as a malignancy for which radiotherapy is the primary treatment modality in numerous studies [5,6]. In terms of therapeutic effect, a large-scale clinical study [11] has clearly demonstrated that compared with patients who did not receive radiotherapy (RT), the survival rate of patients who received RT treatment increased significantly. The median overall survival times were 50 months and 118 months, respectively, showing a remarkable difference. SPB has a high recurrence risk, and there is a 50% chance of progressing to MM [12,13]. Therefore, long-term and systematic follow-up monitoring is essential to promptly detect disease changes and implement precise interventions. Some studies [13] also indicate that although chemotherapy can reduce the risk of patients progressing to MM, this treatment is mainly applicable to SPB patients with a poor prognosis. Other research [14] has confirmed that regular monitoring of laboratory indicators, including M protein levels, immunoglobulin levels, free light chain ratios, and cytogenetic indicators, is of great importance for early detection of SPB recurrence and progression. By comprehensively monitoring clinical symptoms, laboratory test results, and imaging findings, clinicians can dynamically adjust treatment plans, thereby effectively improving long-term survival rates and quality of life. Thus, standardized regular follow-up plays an irreplaceable and crucial role in the early detection of SPB recurrence or progression.

Conclusions

This case report enriches the diagnostic data for asymptomatic SPB of the ribs. In clinical practice, when imaging examinations of adult patients reveal localized and solitary bone resorption, destruction, and invasion of adjacent soft tissues, even if the patients lack characteristic clinical manifestations, the possibility of SPB should be considered. Clinicians should conduct comprehensive relevant laboratory tests and implement regular monitoring, thereby reducing the risks of misdiagnosis and missed diagnosis. This approach can secure a better opportunity for subsequent treatment and improve patient prognosis.