Wednesday, June 15, 2011

A Forearm Lump

Patients vary in their approach to lumps they find in their body. Some will seek medical attention immediately, while others will wait for the mass to enlarge or cause symptoms before going to see a physician.

A 50 year-old female presented to a hand surgeon with a slowly enlarging forearm mass:

The surgeon referred the patient for an MRI scan, to further characterize the mass. Coronal and sagittal T1-weighted images identify an encapsulated, fat-containing mass (red arrows) along the dorsal aspect of the forearm, with a few, thin internal septations (blue arrows):

An axial T1 image better depicts the relationship of the mass to the radius (R) and ulna (U):

An axial T2 fatsat image confirms that the lesion is composed of fat, and also better defines the deep extension of the lesion to the level of the interosseous membrane and anterior interosseous neurovascular bundle (green arrow):

An axial T1 fatsat image obtained after the administration of gadolinium shows that the lesion does not enhance:

Thus, the forearm mass is well-encapsulated, contains only a few-thin septations, has no nodular areas, and fails to enhance after the administration of gadolinium. The diagnosis of a simple lipoma was made, and important anatomic relationships were described in the radiology report, for preoperative planning. The lesion was excised, and was confirmed to be a simple lipoma.

MRI is often used to characterize soft tissue masses. The initial questions are usually:

- is the mass benign or malignant?

- what are the anatomic boundaries of the mass, and what is its relationship to critical structures such as major nerves and arteries?

- can a specific tissue diagnosis be provided?

A simple, but useful approach is to put the mass into one of three categories, which MRI can do with a high degree of reliability:

--> ganglion cyst

--> lipoma or other fat-containing mass

--> does not fit criteria for ganglion cyst or fat-containing mass, and needs further analysis

When a discrete, homogeneous fat-containing mass is encountered, a simple lipoma can be diagnosed with certainty. When an inhomogeneous fat-containing mass is found, further analysis is required. Gaskin and Helms examined 126 fat-containing masses, to better understand how to differentiate simple lipomas from well-differentiated liposarcomas (AJR 162:733-739, 2004) and concluded:

--> MRI is 100% specific for a simple lipoma, when the lesion has a characteristic appearance

--> infiltrating intramuscular lipomas also have a characteristic appearance, and can be diagnosed with confidence

They also noted that when a fat containing mass is heterogeneous (thickened or nodular septa, significant nonadipose tissue, prominent foci of high T2 signal, and prominent areas of enhancement), the lesion may be benign or malignant. In their series, 10/16 (63%) of the lesions suspicious for malignancy were benign lipoma variants, such as chondroid lipoma, osteolipoma, hibernoma, angiolipoma, lipoleiomyoma, and necrotic lipoma.

Thus, about two-thirds of soft tissue masses that contain fat and are suspicious for malignancy will actually turn out to be benign in nature. Nonetheless, it is important for the radiologist to alert the clinician and pathologist to the possibility of a liposarcoma, so that the patient can be managed appropriately.

A common question that is asked is, "Does the MRI need to be done with intravenous contrast?". When a simple lipoma or a typical ganglion cyst is found, there is usually no need to administer intravenous contrast for further characterization. Unfortunately, one does not know the nature of the mass before doing the MRI, so it is often best if the clinician orders the examination with intravenous contrast, "at the discretion of the radiologist".

Vic David MD