Mandibular Solitary Plasmacytoma in an Elderly Patient: a Rare Case Highlighting the Role of Exclusive Radiotherapy

1. Introduction

Solitary plasmacytoma (SP) is histologically characterized by a localized accumulation of monoclonal neoplastic plasma cells, without signs of systemic plasma cell proliferation, absence of bone marrow dissemination, and a unique lesion. It is a rare form of plasma cell neoplasm, accounting for 5 to 10% of all plasma cell neoplasms according to the literature (1-2). It can be classified into two groups based on its location: solitary bone plasmacytoma (SBP) or intramedullary plasmacytoma, and extramedullary plasmacytoma (EMP) (1-3). The bone forms are the most common and primarily affect the vertebrae, as well as the ribs, clavicles, sternum, and long bones (4). Mandibular involvement has been rarely described in the literature.
The frequent progression of solitary bone plasmacytoma to multiple myeloma, reported in 55% of cases in the literature, continues to raise questions about its nosological authenticity (5).
Based on a case of solitary bone plasmacytoma of the mandible, complemented by a literature review, we aim to revisit the diagnostic, therapeutic, and evolutionary aspects of this condition.

The aim of this case report is to examine the role of exclusive radiotherapy in the curative treatment of an unresected mandibular solitary plasmacytoma in an elderly patient.

2. Case presentation

Our patient is an 80-year-old man with a history of hypertension and diabetes, both of which are well controlled with medical treatment. The disease started approximately three months prior to admission in the Radiation Oncology department, with the onset of localized swelling and dental mobility, prompting him to consult his dentist. A panoramic dental X-ray and a CT scan of the facial bones were performed, revealing a semi-recent left parasymphyseal mandibular fracture with significant mandibular osteocondensation and an associated anterior periosteal reaction. These findings were highly suggestive of chronic osteomyelitis complicating the aforementioned fracture. There was also evidence of a phlegmon involving the soft tissues surrounding the left mandibular body in the fracture area, without a clearly organized collection.

Figure 4: PET-CT scan slice showing hypermetabolism corresponding to the left mandibular fracture involvement, with a SUVmax of 8.6 compared to a hepatic reference of 3.6.

Upon admission to our department, the clinical examination found a patient in good general condition (Performance Status 1), without local pain (Visual Analog Scale, VAS score 0), weighing 85 kg, and standing 1.80 m tall.
The locoregional examination revealed hypoesthesia in the territory of the left mandibular branch (V3), along with a slight leftward deviation of the mouth.
There was also a swelling of the lower left mandibular region, in the context of prior dental avulsions (teeth 27 and 34), with ongoing healing progressing well.

A planning CT scan was then performed using a five-point thermoplastic mask and knee supports to ensure immobilization. Alignment was performed using wall lasers set to the level of the tragus. The images were transferred from the treatment planning system (TPS) to the contouring console.

Helical tomotherapy was selected for treatment delivery, utilizing 6 MV photon beams generated by a linear accelerator equipped with a multileaf collimator (MLC) to conform the dose precisely to the target volumes. Target volume delineation included the GTV-T (Gross Tumor Volume – Tumor), corresponding to the visible tumor mass; the CTV-T (Clinical Target Volume – Tumor), defined by adding a 5 mm margin around the GTV-T to account for potential microscopic spread; and the PTV-T (Planning Target Volume – Tumor), obtained by expanding the GTV-T with a 10 mm margin to compensate for setup uncertainties and patient motion. A total dose of 46 Gy was prescribed to the PTV, normalized to the 50% isodose line, in accordance with ICRU Report 83. The maximum dose (Dmax) remained below 105% of the prescribed dose. Target volume coverage was excellent, with more than 98% of the PTV receiving the prescribed dose. Organs at risk (OARs) were delineated using rigid image fusion with MRI to enhance anatomical accuracy. These included the brain, eyes, crystalline lenses, optic nerves, brainstem, cochleae, oral cavity, limbs (if within the field), larynx, parotid glands, submandibular glands, pharyngeal constrictor muscles, skin, pituitary gland, and spinal cord. All dose constraints to the OARs were respected. Daily patient positioning was verified using on-board cone-beam computed tomography (CBCT) fused with the planning CT to ensure precise alignment and treatment accuracy. The overall treatment duration was 32 days, completed without interruption. During the course of radiotherapy, the patient developed grade 2 radiomucositis (according to CTCAE), which was managed with mouthwashes containing sodium bicarbonate, xylocaine, and corticosteroids. Grade 2 radiodermatitis also developed and was treated with emollients and adhesive dressings; both conditions improved with appropriate symptomatic management. At the six-month follow-up visit, clinical evaluation confirmed significant tumor regression, as observed at the three-month post-treatment assessment. However, hypoesthesia in the V3 (mandibular) territory persisted. A follow-up MRI revealed a 62% reduction in tumor volume compared to the initial imaging. To date, no late toxicities have been observed, and the radiodermatitis that developed during treatment has completely resolved.

3. Discussion

Plasma cell neoplasms account for 1 to 2% of all human malignancies. Several clinical forms exist, among which solitary plasmacytoma represents less than 10% of these proliferations. It was first described by Schridde in 1905 (6-7). Solitary plasmacytoma is classified into two groups. The most common is solitary bone plasmacytoma (SBP), which frequently evolves into a multiple form. It commonly affects the axial skeleton and rarely involves the jaws. Only about 4.4% of SBPs occur in the mandible, most often in areas rich in bone marrow such as the body, angle, and ramus of the mandible (8). The second group is solitary extramedullary plasmacytoma (SEP) (9).

The etiology of solitary plasmacytoma remains uncertain, but several hypotheses have been proposed, including radiation exposure, chemical agents, viral infections, and genetic factors. Cytogenetic studies have identified losses of chromosomes 13, 1p, and 14q, and gains of chromosomes 19p, 9q, and 1q. Interleukin-6 is considered a key growth factor in the pathogenesis of this condition (10).

SBP most often occurs in patients aged 50 to 80 years, with a mean age of onset around 60. It is rare before the age of 40 (11). The condition is more frequent in men, with a reported male-to-female ratio of 2:1 (12).

The clinical presentation of solitary plasmacytoma of the mandible (SPM) is not specific; facial swelling, dental mobility, and sensory disturbances are the most frequent clinical signs. In rare cases, a solitary plasmacytoma of the mandible may be discovered during a pathological fracture, post-extraction bleeding, or a dental joint disorder in the case of condylar localization (13).

Malignant plasma cells produce cytokines and release an osteoclast-activating factor that stimulates osteoclasts to resorb bone; consequently, on radiographic examination, they appear as well-defined osteolytic lesions with either unilocular or multilocular radiolucency (10-14). According to Lae et al., three radiographic patterns have been described in SBP: multilocular ‘soap bubble’ lesions, unilocular radiolucency (as seen in our patient) with a cystic appearance, and poorly defined destructive bone resorption (15). CT helps to refine the radiological image and assess the extension toward cortical and soft tissues. MRI remains a valuable imaging modality for detecting bone-involved plasmacytomas, assessing soft tissue involvement and bone marrow infiltration, and evaluating disease throughout the body. Solitary plasmacytomas show low signal intensity on T1-weighted images and high signal intensity on T2-weighted images, with homogeneous enhancement after gadolinium contrast injection (16).

In our case, there were signs of periosteal reaction with cortical destruction, potentially extending widely into the surrounding tissues.

Under the microscope, plasma cells exhibit varying degrees of differentiation within a sparsely cellular stroma. Sukpanichnant et al. classified plasmacytomas into mild, moderate, and severe dysplasia based on the degree of plasma cell differentiation (17). Nuclei may be binucleated. The spherical nuclei are eccentrically located and display regular or irregular chromatin margination, often in a cartwheel pattern (18). Occasionally, the chromatin appears coarsely clumped, showing a clock-face configuration. The cells present a pale, globular perinuclear cytoplasmic area known as a hof. Pseudoangiomatous areas, multinucleated giant cells, amyloid deposition, and myxoid changes may also be observed in some cases. Plasma cells may contain intracytoplasmic inclusions (Russell bodies) and intranuclear inclusions known as Dutcher bodies. Immature plasma cells have larger or more irregular nuclei, less condensed chromatin, and may display prominent nucleoli. These immature cells are larger and more pleomorphic, with abundant, mildly basophilic cytoplasm (10).

The neoplastic process is secretory in approximately 99% of cases, producing monoclonal immunoglobulins of either light or heavy chains, which can be detected in the serum or urine (15). In accordance with the literature, the present case revealed elevated serum lambda light chains, exceeding the normal range. To diagnose a case of solitary bone plasmacytoma (SBP) and to exclude multiple myeloma (MM), thorough clinical investigations are required, including skeletal radiological survey, bone marrow aspiration, complete blood count, serum calcium levels, and renal function tests (15). Specific diagnostic criteria for SBP were proposed by Bataille and Sany (Table 1)(19).

The present case meets all of the aforementioned criteria. In addition to laboratory investigations, determining monoclonal restriction to either kappa or lambda light chains is a crucial approach in the evaluation of suspected plasmacytoma.

Table 1. Specific Diagnostic Criteria for Solitary Bone Plasmacytoma (SBP) (19)

* Radiologic examination, bone scanning, and computed tomography will be useful for such an accurate assessment. ** In sternum and pelvis

From a therapeutic perspective, surgical treatment may be considered for diagnostic purposes, in the presence of neurological complications such as spinal cord compression, or to treat or prevent a pathological fracture in bone weakened by tumor-induced osteolysis. In the specific case of mandibular involvement, surgical intervention is feasible in the vast majority of situations.
Complete resection may be indicated for peripheral, easily accessible lesions; however, mutilating surgery should be avoided, especially since radiotherapy offers comparable efficacy while allowing better preservation of function (5-6-20-21)

Moreover, the radiosensitivity and radiocurability of solitary bone plasmacytoma have been documented for over half a century (19,22). Radiotherapy is considered the standard treatment for solitary bone plasmacytoma. When administered at optimal doses of 40 to 50 Gy, it provides a local control rate exceeding 90%, with excellent tolerance and rapid, durable pain relief.
A dose–response effect was reported by Mendenhall et al. (23), who observed a 94% local control rate at doses above 40 Gy, compared to 69% at lower doses. Similarly, Frassica et al. (24) reported a 15.6% local failure rate in a series of 46 patients when the radiotherapy dose was below 45 Gy, whereas the local control rate was 100% with higher doses.
The addition of chemotherapy is not recommended, as it has not been shown to improve outcomes in terms of recurrence or progression to multiple myeloma (4-5).

Predictive factors for local recurrence appear to include, in addition to insufficient radiotherapy doses, the anatomical location of the solitary bone plasmacytoma — with spinal or pelvic lesions being more difficult to manage than peripheral ones — the persistent presence of monoclonal protein after treatment, as well as tumor size (5).

The median survival rate for solitary plasmacytoma is longer than that for multiple myeloma, due to the localized nature of the disease. However, clinical remission remains rare (25). SBPs carry a high risk of progression to multiple myeloma (65% to 84% at 10 years and nearly 100% at 15 years), whereas SEPs have a lower risk of progression (10% to 30% at 10 years) but a higher risk of local recurrence(26). Scientific data suggest that patients with an SBP larger than 4 to 5 cm are at increased risk of developing multiple myeloma (27).

Due to the progressive nature of SBP and the extended period (6 to 8 months) required to achieve maximal radiotherapy response, periodic monitoring is essential. A complete blood count, serum biochemistry (calcium and creatinine), and systematic evaluation of potential biomarkers such as persistent M-protein, clonal expansion of free light chains (FLC), and the presence of clonal plasma cells are standard practices(10). PET/CT, when available, can be particularly useful for follow-up in skeletal studies

4. Conclusion

Solitary plasmacytoma of the mandible is a rare entity, often difficult to diagnose and manage due to its potential for local aggressiveness and progression to multiple myeloma. Through this case, we highlight the efficacy of exclusive radiotherapy in achieving local control of the disease, even in anatomically complex locations such as the mandible. A thorough diagnostic evaluation, precise treatment planning, and close post-treatment monitoring remain essential. Further studies and larger case series are required to refine therapeutic strategies, particularly for aggressive or atypical forms. This work contributes to the literature supporting the use of radiotherapy as an effective conservative treatment in selected cases.

Abbreviations

CBCT – Cone Beam Computed Tomography

CD138 / CD20 – Cluster of Differentiation (Immunohistochemical markers)

CRAB – Calcium elevation, Renal insufficiency, Anemia, and Bone lesions (Multiple Myeloma diagnostic criteria)

CT – Computed Tomography

CTCAE – Common Terminology Criteria for Adverse Events

CTV-T – Clinical Target Volume – Tumor

Dmax – Maximum Dose

EMP – Extramedullary Plasmacytoma

ENT – Ear, Nose, and Throat

FDG-PET – Fluorodeoxyglucose Positron Emission Tomography

GTV-T – Gross Tumor Volume – Tumor

ICRU – International Commission on Radiation Units and Measurements

IgM – Immunoglobulin M

Ki-67 – Cellular proliferation biomarker

MLC – Multileaf Collimator

MM – Multiple Myeloma

MRI – Magnetic Resonance Imaging

OAR – Organs At Risk

PTV-T – Planning Target Volume – Tumor

SBP – Solitary Bone Plasmacytoma

SEP – Solitary Extramedullary Plasmacytoma

SP – Solitary Plasmacytoma

SPM – Solitary Plasmacytoma of the Mandible

SUVmax – Maximum Standardized Uptake Value

TPS – Treatment Planning System

V3 – Mandibular branch of the Trigeminal Nerve

VAS – Visual Analog Scale (for pain assessment)

VIDE – Vincristine, Ifosfamide, Doxorubicin, and Etoposide (Chemotherapy protocol)

Statements

Authors’ contributions: OAS: Writing – original draft, Writing – review & editing. FEZA Writing – review & editing. NB Conceptualization, supervision, and guidance.

Consent for publication: As the corresponding author, I confirm that the manuscript has been read and approved for submission by all named authors

Funding: This case report has received no financial support.

Conflict of interest: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Informed consent: The informed consent was obtained from the patient for publication of this case report and any accompanying images.

Statement of Ethics: The case management was approved by the tumor board of the Emile Muller Hospital Of The South Alsace Regional Hospital Group, Mulhouse, France.

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