Plasmacytomas are localized neoplasms accounting for about 5% of all plasma cell dyscrasias [1]. Plasmacytoma is further classified into solitary plasmacytoma of bone (osseous plasmacytomas) and extramedullary plasmacytomas (extraosseous). Extraosseous lesions has a lower risk of progression to myeloma [2].The most common location is the axial skeleton and upper respiratory tract for osseous lesions and extramedullary lesions respectively [2]. SBP of clavicle is extremely rare, and only a few case reports are mentioned in the English literature [3].

A 18 year old female reported with complaints of swelling over the right shoulder region for 11months which was gradually increasing in size and swelling was associated with pain that was mild in intensity. She sustained a pathological fracture of the same following trivial injury. On local examination a spherical swelling of about 6x6 cm over the lateral aspect of clavicle, with normal temperature as compared to surrounding areas, non tender, smooth surface and well defined margins swelling was firm to hard in consistency. Swelling was fixed to underling structures but not to the skin. Skin was freely mobile over the swelling. All the movements of the shoulder joint were within normal range and were painless except for, overhead abduction that was associated with pain. MRI of the right clavicle showed expansile osteolytic lesion measuring 7.0 x 5.7 x 4.0 cm involving the lateral end of clavicle with erosion of cortex. Thin cortical Fractures were visualised in the margins of the lesion [Fig1]. Findings were suggestive of neoplastic lesion.

Fig1 showing expansile lesion in right clavicle with fracture of lateral end

The patient underwent tumour excision of clavicle & reconstruction using fibular graft with clavicle hook plate and the sample was sent for histopathology. Microscopic features of sections examined revealed a neoplastic lesion composed of sheets and cords of plasma cells [Fig.2].

Fig 2- Microscopic examination showing sheets of immature plasma cells.

These cells have eccentric, hyperchromatic nuclei with dispersed chromatin and occasional prominent nucleoli, and abundant cytoplasm. Histopathological examination (HPE) with immunohistochemistry revealed monoclonal plasmacytosis consistant with plasmacytoma (CD 138- fig 3 and kappa positivity-Fig 4)

Fig 3-Immunohistochemistry for CD 138 showing strong positivityin tumor cells

Fig -4 Immunohistochemistry for kappa showing strong positivity in tumor cells

Complete blood count, urea, creatinine, alkaline phosphatase, C-reactive protein, serum calcium and phosphorus levels showed normal values. Urinary Bence Jones Proteins were negative. Protein Electrophoresis were normal. Bone marrow aspiration showed a normocellular marrow with 2% plasma cells. A bone marrow biopsy showed scant marrow elements with trilineage haematopoiesis. Thereafter skeletal survey was negative for any other concomitant pathological lesion. Patient was taken up for External Beam Radiation therapy (EBRT) after appropriate immobilization using head and neck cast and CT based planning was done to deliver a dose of 5040 cGy in 28 fractions using 3DCRT (conformal) technique. The Patient tolerated radiation therapy well. A PET-CT done showed postoperative changes in right clavicle (SUV-3.92) with no evidence of residual lesion. She is currently under regular follow-up and she is radiologically, clinically, and immunologically disease free.

Solitary bone plasmacytoma (SBP) is characterized by proliferation of malignant monoclonal plasma cells. [1].The median age of presentation is 55 years with a female predominance (male to female Ratio is 1:2). Patients with SBP presents with a younger median age (ten years) than patients presenting with multiple myeloma.

SBP is commonly presented as an expansile lytic mass and are most commonly seen in the spine than any other bony sites.[4] SBP commonly involves axial skeleton that is vertebra, ribs, skull, pelvis, femur and scapula [2]. SBP of the clavicle is a very rare presentation. Pain is the most important presenting symptom. Typical features of myeloma like anemia, hypercalcemia, renal failure, or bone marrow involvement are not seen in SBP. [5]

Progression to myeloma in SBP patients is seen in 75% of patients with a median duration of two to three years and this proportion is steadily increased in due course of time. [1,6]. Hence, patients with SBP requires lifelong monitoring to detect progression to Multiple Myeloma with routine clinical and laboratory assessment. Factors predicting progression to myeloma includes tumor size >5 cm, age ≥60 years, high M protein levels (1 g/dL), persistence of M protein after treatment and spine lesions. [7]

The optimal treatment for most patients with SBP is moderate dose radiotherapy, approximately 40–50 Gy administered once daily at 1.8–2.0 Gy per fraction in a continuous course resulting in high local control rates of 83-96%.[8,9]

A study done by Mendenhall et al in 81 patients recommended a minimum dose of 40 Gy after analyzing the dose–response and review of Literature. The local failure rate in patients with SBP treated with doses of 40 Gy or above was 6% and in patients treated with doses below 40 Gy was found to be 31%.[10]

A study reported by Tsang et al. stated that 32 patients with SBP was treated with RT and they concluded that tumor bulk was the most important factor in local control. They also reported that that bulky SBP over 5 cm needs higher dose or combined multimodality approach for effective local control. [11]

We report this case of SBP due to its rare site and younger age of presentation. She is currently under follow up and asymptomatic. But she requires continuous further follow up for early detection and treatment of any systemic relapse of disease or progression into overt myeloma.