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Case Report: Ossifying Fibroma of the Zygomatic Arch in a Corgi

A 20-month-old, male, neutered Corgi was referred for assessment of left facial swelling over the region of the zygomatic arch. 

fig. 1 — dorsal view of the head showing focal swelling over left zygomatic arch

The mass had appeared suddenly, six weeks prior to referral.  There was no history of trauma.  On initial examination, the referring veterinarian found no evidence of ocular or dental disease.  Routine blood-testing was unremarkable.  Re-examination two weeks later, revealed the swelling to have increased in size, prompting further investigation; radiographs had revealed osteolysis with concurrent osteogenesis of the central part of the zygomatic arch; a radiologist’s report had offered calcinosis circumscripta or chondrosarcoma as the two most likely differentials.  Wedge-biopsy of the mass was consistent with a benign reactive process, likely secondary to trauma.  After biopsy, the owners noted that the mass had continued to grow.

fig. 2 - ventrodorsal view of the skull showing mixed lysis and new bone production of the mid zygomatic arch

On presentation at the author’s clinic, the dog appeared bright and comfortable.  General physical examination was unremarkable apart from a firm, fixed, non-painful swelling over the left zygomatic arch (fig.1).  In addition, there was focal, soft-tissue swelling ventral to the mass.  Abdominal ultrasound and thoracic radiographs were unremarkable. 

fig. 3—Dorsal, STIR MR-slice showing focal, low-intensity mass within the cortices of the left zygomatic bone.

Repeat radiology of the skull revealed the mass to have grown since the initial radiographs (fig. 2). MR-scans of the head revealed a well-circumscribed mass bounded by the cortices of the left zygomatic arch, centred over the region of the frontal process/ suture line of the zygomatic bone, measuring 2.5 x 1.5 cm.  The mass itself was characterised by a heterogeneous, but generally low intensity signal on T1W, T2W and STIR sequences (fig. 3).  In addition, there was an area of increased T2W/STIR signal in the soft tissues ventrolateral to the mass, consistent with the clinical presentation.  The regional lymph nodes were found to be of normal size.  Nevertheless, fine-needle-aspirates were taken of the left submandibular and prescapular glands which revealed no evidence of metastatic disease.  A repeat wedge-biopsy was taken from the zygomatic mass, when a small seroma was found in association with the previous biopsy tract.  Tissue samples were submitted for further histopathology; there was proliferating cartilage maturing into trabecular bone, most consistent with the earlier findings of a benign reactive process.  Microbiology of tissue samples yielded no bacterial or fungal growth.

fig. 4 — Intraoperative photograph:osteotomy of the zygomatic bone, cranial to the mass

Two weeks after the second biopsy, the dog was re-admitted for definitive excision of the mass via a ventrolateral approach, the seroma was excised and the palpebral nerve was reflected dorsally. The soft tissue attachments to the zygomatic arch were freed and the arch was severed rostral and caudal to the mass using an osteotome (fig. 4). A closed-suction drain was sutured into the dead-space before the wound was closed in layers and a light dressing applied (fig. 5).

fig. 5 — postoperative photograph: Closed suction drain in place to minimise risk of seroma formation.

Final histopathology identified an expansile, progressive and lytic fibro-osseous mass developing within the bone of the zygomatic arch. There was no evidence of malignancy and excision appeared to be complete.  The appearance of the mass was unusual and could not easily be categorized, but the most likely diagnosis offered was a variant of ossifying fibroma, giving a good long term prognosis; at the time of writing (two months post surgery) the dog has made an uneventful recovery from the surgery, with no recurrence of the mass.

Discussion:

Ossifying fibromas are rarely reported in the veterinary literature.  They have been described as benign, bone neoplasms that have been reported in the mandible, maxilla, skull and in one case, the os penis of a dog1, but most commonly in the rostral mandible of young horses where they are classified as juvenile, mandibular, ossifying fibromas2.  In humans, the condition is classified as a benign fibro-osseous lesion of the mandible or maxilla and considered to be a variant of fibrous dysplasia.  These are described as well circumscribed osteolytic lesions radiographically, with varying degrees of calcification and cortical thinning.  They are further considered as a developmental disorder in which normal bone is replaced with abnormal fibrous tissue that contains small, abnormally arranged bone trabeculae3.  This description would fit with the clinical presentation seen in the above case, as well as the radiographic appearance of the mass.  However, it is noted that such lesions often produce increased signal on T2W/ fat suppression MR-sequences in man, in contrast to the low intensity signal observed in this case.  This suggests the disease process in humans may not be directly analogous to the case described above, given the apparently higher bone metabolism in man.

Surgical excision generally carries a favorable prognosis.  However, the mass on the os penis recurred with a return of clinical signs1.  From a surgical planning perspective, resection with narrow margins should be curative, provided clear margins are achievable.  Without surgery, the mass outlined above might reasonably be expected to continue growing and eventually cause functional impairment to neighbouring structures (e.g. the globe).  

Removal of the ventral support to the globe may require consideration of reconstructive techniques involving the implantation of prosthetic material4.  However, in this case, it was felt that repair of the soft tissues surrounding the resected portion of the zygomatic arch was sufficient.

Summary:

There remains the slight possibility that histopathology was not representative of the lesion, although a malignant process was deemed unlikely given consistent results from three separate biopsy procedures.  This was a very favourable outcome, given the initial aggressive radiographic appearance of the lesion.

References:

1.   Mirkovic T.K., Schmon C.L., Allen A.L. (2004) Urinary Obstruction secondary to an ossifying fibroma of the os penis in a dog. Journal of the American Animal Hospital Association 40 152-156. 

2.   Miller M.A., Towle H.A.M., Heng H.G., Greenberg C.B., Pool R.R. (2008) Mandibular Ossifying Fibroma in a dog. Vet Pathology 45 (2): 203-6

3.   Alam A. Chander B.N. (2003) Craniofacial Fibrous Dysplasia presenting with Visual Impairment. Medical Journal of the Armed Forces India 59 342-343

4.   Tutt CLC (personal communication).