11-year-old male Springer Spaniel

• There is a very large well marginated mass occupying much of the right side of the head and causing pressure atrophy of the lateral portion of the temporalis muscle on that side.
• It is approx 13 cm long x 5 cm wide x 8cm in height at its greatest. It is closely associated with the horizontal ear canal, parotid and zygomatic salivary glands, temperomandibular joint and extends into the lateral portion of the right orbit where it causes medial displacement of the extraocular muscles and rostral displacement of the globe. It nonetheless remains well marginated at these regions and a definitive connection to or origin from any of these is not seen.
• The mass is in close association with the right tympanic bulla and temporomandibular joint.
• It is relatively homogenous on all sequences. It is hyperintense on both T1 and T2 and is markedly hypointense on STIR. It is similar in intensity to subcutaneous fat on all sequences. There is no significant contrast enhancement.
• There was also debris consistent with wax/fluid/pus/polyp in the left middle ear. There was no suggestion of active inflammation or of bone destruction so this was considered clinically silent and of no current clinical significance.
• The signal characteristics here strongly support this being a fatty mass such an an infiltrative lipoma. STIR is a special technique called an inversion recovery sequence which acts to null out the high signal produced by fat. As a result, fat will appear dark on STIR sequences.

The dog was taken to surgery and a large fatty mass was removed. Histopathology confirmed the diagnosis of a lipoma.

Infiltrative lipomas are locally aggressive where they invade muscle, fascia, fibrous tissue, joint capsule and even bone for example. They have been reported as occurring on the extremities, body wall, perineum and head. Diagnosis requires demonstration of fatty tissue invading surrounding normal tissue. This may be difficult or impossible with FNAs, core or surgical biopsies. Their appearance has been described on radiographs but demonstration of invasion of surrounding tissue is often not possible. Cross-sectional imaging provides more detailed information in this regard and the appearance of invasive lipomas has been described using CT. This case shows an example of an infiltrative lipoma as imaged with MRI. Treatment is usually surgical but recurrence rates vary from 36-50%. Radiation therapy has also been reported in addition to a single dog treated with doxorubicin.