Untreated plasmacytoma of bone containing macroscopic intralesional fat and mimicking intraosseous lipoma: A case report and review of the literature

Highlights

• Non-treated microscopic or macroscopic fat-containing bone lesions are overwhelmingly benign

• We report the first case of a non-treated solitary plasmacytoma with macroscopic intralesional fat

• Image-guided percutaneous biopsy should be considered for symptomatic fat-containing bone

Abstract

Solitary plasmacytoma is a rare form of plasma cell neoplasm defined by local neoplastic accumulation of monoclonal plasma cells in the absence of systemic proliferative plasma cell disease. In this case report, a 65-year-old female with remote past medical history of papillary thyroid cancer presented with shoulder pain and radiographs showing an aggressive osteolytic lesion presumed to represent an osseous metastasis. The subsequent MRI and CT examinations demonstrated diffuse intralesional macroscopic fat without a nonlipogenic soft tissue component or focal, nodular mass-like enhancement. The presence of macroscopic fat in an untreated osseous lesion suggested a benign lesion with the favored diagnosis an intraosseous lipoma with non-displaced pathological fracture. Therefore, the decision was made to forego image-guided percutaneous biopsy and instead proceed directly to open surgical biopsy and partial distal claviculectomy. Pathology of the resected specimen showed focally dense infiltration of plasma cells within the marrow space and scant hematopoiesis compatible with a plasma cell neoplasm. To the best of our knowledge, this is first case report of solitary plasmacytoma of bone, or any untreated plasma cell neoplasm, containing macroscopic fat upon imaging. The decision to forego image-guided percutaneous biopsy had significant treatment implications as the primary therapy for patients with SPB is not surgical, but localized radiation therapy. Based on this case, solitary plasmacytoma of bone may be included as one of the rare fat containing malignant bone lesions and imaging guided percutaneous biopsy should be considered in symptomatic fat-containing bone lesions.

Introduction

Solitary plasmacytoma is a rare form of plasma cell neoplasm defined by local neoplastic accumulation of monoclonal plasma cells in the absence of systemic proliferative plasma cell disease. This unusual form of plasma cell neoplasm accounts for 5–10% of all plasma cell neoplasms and is subdivided into solitary plasmacytoma of bone (SPB) and extramedullary plasmacytoma, each type representing approximately 5% of all plasma cell neoplasms. SPB preferentially occurs within the axial skeleton including the skull and vertebrae while extramedullary plasmacytoma most often arises in the head and neck at the nasopharynx [[1], [2], [3], [4]]. The magnetic resonance imaging (MRI) appearance of SPB and extramedullary plasmacytoma have been well-described in the literature and characterized as generally isointense to hypointense to skeletal muscle on T1-weighted imaging, hyperintense, but heterogeneous on T2-weighted imaging with heterogeneous to robust enhancement after the administration of gadolinium [[5], [6], [7], [8], [9]].

To the best of our knowledge, we present the first case of untreated solitary plasmacytoma containing macroscopic fat on imaging. This is also the first reported case of any type of untreated plasma cell neoplasm containing macroscopic fat. Unlike soft tissue masses, the vast majority of intraosseous lesions containing macroscopic fat are benign and the presence of either macroscopic and/or microscopic intralesional fat on cross-sectional imaging has proven to be extremely useful in excluding malignant etiologies [[10], [11], [12]]. Our working diagnosis based on the cross-sectional imaging was a distal clavicular intraosseous lipoma (IOL) with multifocal, non-displaced pathologic fractures. This initial misdiagnosis based on the available imaging had significant treatment implications for the patient which are discussed in further detail below.