Intramuscular lesion with spinal canal extension : An imaging diagnostic dilemma

INTRODUCTION

Extramedullary plasmacytoma (EP) is less common than intramedullary plasmacytoma, comprising only 2% of all overall plasma cell disorder, but demonstrates a better prognosis. The occurrence of EP is reported rarely at the skeletal system, even rare at the paraspinal region. EP is commonly associated with systemic multiple myeloma and some even reported extramedullary plasmacytoma in a relapse or post treatment for multiple myeloma.

Radiography is done as a primary survey and usually showed muscle enlargement with or without bone involvement. MRI is performed for further characterization and delineation of the lesion.

We report a rare case of solitary intramuscular plasmacytoma in the absence of systemic involvement or prior history of multiple myeloma.

CASE DESCRIPTION

Mr NY, a 61-year-old male, initially presented in 2019 complaining

of lower back pain without neurological deficit. No prior history of trauma and no fever or

constitutional symptoms. He was treated as degenerative spine disease and non-contrasted

MRI of the lumbar spine at that time showed paravertebral lesion within the right psoas with

intraspinal canal extension. He refused any intervention then.  In 2021, he complains of progressive lower limb weakness and a repeated MRI showed enlargement of the lesion with more extensive intraspinal canal extension from L2 to L4 causing compression unto the spinal canal and nerve roots [Figure 2(a) and 2(b)]. The lesion demonstrated isointense on T1W, hyperintense on T2W and homogenously enhanced post contrast [Figure 1(a), 1(b) and 1(c)].

Imaging differential diagnoses of metastases, neurofibroma and malignant peripheral nerve sheath tumour was given.  Histopathological examination is compatible with plasma cell neoplasm. Bone marrow aspiration showed reactive marrow with no evidence of increased plasma cells.

LEARNING POINT

A paraspinal EP is an extremely rare site for its manifestation. In the presence of nerve compression, the symptoms might include lower limb weakness and numbness. The most common MRI findings of EP on T1 weighted images is isointense to surrounding muscle, hyperintense on T2 weighted and marked enhancement post contrast. These are also commonly seen in other intramuscular lesions, thus providing imaging diagnostic dilemma. However, due to the rarity of the disease, site of manifestation and common MRI findings in other intramuscular mass; a diagnosis of EP was not listed. However, in view of the patient’s age group and typical imaging findings, despite the low incidence rate, a diagnosis of EP should be considered.