Mammary-Type Myofibroblastoma of the Elbow : A Case of a Rare Upper Extremity Tumor

Mammary-typemyofibroblastoma (MMFB) is a rare tumor often found in the breast tissue of older males as was originally described in 1987 by Wargotz [1]. MMFB belongs to the family of benign stromal cell tumors, which includes solitary fibrous tumors, nodular fasciitis, pseudoangiomatous stromal hyperplasia and primary mammary fibromatosis, all of which are characterized by the presence of myofibroblasticcells [2,3]. Classic type MMFB is a bland spindle cell tumor with features of fibroblastic and myofibroblastic differentiation [4].


Introduction
Mammary-typemyofibroblastoma (MMFB) is a rare tumor often found in the breast tissue of older males as was originally described in 1987 by Wargotz [1].MMFB belongs to the family of benign stromal cell tumors, which includes solitary fibrous tumors, nodular fasciitis, pseudoangiomatous stromal hyperplasia and primary mammary fibromatosis, all of which are characterized by the presence of myofibroblasticcells [2,3].Classic type MMFB is a bland spindle cell tumor with features of fibroblastic and myofibroblastic differentiation [4].
MMFB most commonly presents as a slow growing, painless mass that develops over the course of months to years [5,6].Though often found in the breast, the tumor also occurs in extramammary sites commonly arising from subcutaneous tissue of the trunk, from the mid-axilla to the groin, which has led to a suspected origin of the embryonic mammary lines.While other locations have been described, MMFB in the limbs is rare [2].We report a case of histologically confirmed left upper extremity mammary type myofibroblastoma in a 58 year old male with an unusual clinical presentation.

Case Report
A 58 year old male with a past medical history of hypertension, gout and type II diabetes presented at our institution with a mass in his left elbow that had become apparent over the course of three months.He possibly recognized it six months before the initial office visit, but over the three months prior to presentation, it doubled in size, and became tense and uncomfortable.There was no reported history of trauma and family history was noncontributory.
On physical exam, there was no overlying erythema or ecchymosis.The mass was hard and immobile, measuring approximately 8x6cm in greatest diameter.The patient had full active and passive range of motion of the affected extremity and no neurological deficit.malignancy.The preoperative MRI revealed a large soft tissue mass measuring 5.3x5.7x8.0cmwithin the brachioradialis muscle that demonstrated lesional enhancement with the addition of contrast.These findings were consistent with a liposarcoma, which prompted a metastatic workup including a chest CT and incisional biopsy.The chest CT found no evidence of metastatic disease (Figure 1).
A thick yellow mass was identified during incisional biopsy.The specimen was sent for pathology.Histologically, the lesion was composed of bland spindle cells with short, stubby nuclei in a predominantly myxoidmatrix containing hyalinized collagen bundles, scattered mast cells, and a prominent adipocytic component.No atypia was seen.Surgical excision of the mass was scheduled as definitive treatment.A 12cm incision was madeutilizing an anterior approach overlying the left elbow joint.A plane between the brachioradialis and brachialis muscles was developed through which a large mass was apparent.It was densely adherent to the brachioradialis muscle.Through meticulous dissection, the mass was traced down to the joint, where there was no apparent capsular involvement.The mass was removed as a whole and sent to pathology with a confirmed diagnosis of MMFB.
The patient continues to do well and has had no recurrence or further complications at six month follow-up.

Discussion
In most cases benign masses of the upper extremity can be distinguished from malignant lesions based on their clinical presentation.Benign masses have a tendency to be soft, mobile, and slow growing, while malignant masses are more likely to be fast growing and immobile.Rare upper extremity tumors can fall outside of these general characteristics.
MMFB is a benign spindle cell tumor with neoplastic cells demonstrating variable fibromyofibroblastic differentiation [4,5].MMFB fibroblasts characteristically demonstrate CD34 positivity, which is thoughtto indicategrowth from an uncommitted mesenchymal cell origin, capable of a wide array of differentiation.This may explain the many variants of MMFB, and the presence of fibroblasts, myofibroblasts, smooth muscle cells, adipocytes, chondrocytes and osseous cells within the same lesion [3].Additionally, MMFB is characterized by positivity for vimentin, desmin and smooth muscle actin as well as loss of expression of Rb protein due to deletions of chromosome 13q [7].
Originally described as occurring in older males, MMFB has been found to have an equal distribution in postmenopausal women.Patients often report a history of slow growth over months to years [4].Physical exam findings consist of a unilateral, solitary, firm and painless mass, frequently 1-4cm in diameter [8].MMFB has a predilection for areas associated with accessory breast tissue and other breast pathology such as the breast, axilla,abdominal wall and perineal region [2].
While MMFB is generally considered to present sporadically, there is documented association with gynecomastia and androgen deprivation therapy for prostate cancer [8,9].Additionally, lesions characteristically stain for androgen, estrogen and progesterone receptors, which has resulted in speculation regarding thepossible pathogenic role of sex steroids [2,4].
On MRI, MMFB presents as a well-circumscribed, nodular mass with homogeneous enhancement and internal septations that lack microcalcifications [2,7].Successful diagnosis can often be made with the use of incisional biopsy.Gross examinationmay reveal a well circumscribed, smooth to lobulated rubbery mass with a solid pale white to grayish cut surface with a whorling pattern and a variable fatty component [2].
The differential diagnosis includes spindle cell lipoma and cellular angiofibroma, both of which show overlapping morphologic and immunophenotypic characteristics and demonstrate loss of chromosome 13q.Also in the differential is a variety of other bland looking spindle cell lesions such as nodular fasciitis, inflammatory

Figure 1 :
Figure 1: A. Coronal T1 demonstrating a large soft tissue mass, B. Coronal T2 Fat-suppressed post-contrast image demonstrating lesional enhancement, C. Sagittal T1 showing location over anterior radiocapitellar joint, D. T2 fat-suppressed image showing post contrast enhancement.
The lesional cells were diffusely positive for CD34 and demonstrated focal desmin staining.Immuno histochemical stains for MDM2 and CDK4 were negative.An immunohistochemical stain for Rbwas negative, indicating a loss of expression of the Rb protein as a consequence of chromosome 13q deletion.The differential diagnosis on initial evaluation included spindle cell lipoma, but the desmin positivity was more indicative of MMFB.Due to an unusual clinical presentation and MRI findings, the tissue was sent for a second opinion to the Department of Pathology at Brigham and Women's Hospital, Boston, MA, where the diagnosis of MMFB was confirmed (Figure2).

Figure 2 :
Figure 2: A. Hematoxylin and Eosin stain demonstrating short spindle cells, course collagen bundles and admixed fat in a myxoid stroma.B. Loss of expression of Rb protein (negative staining) with retained expression in non-neoplastic endothelial cells (brown staining, inset).C. Positive immunohistochemical staining for CD34 (brown staining).D. Focal positivity for desmin (brown staining).