Marrow Fat in Bone Lesions Can Be Reliably Detected and Is Rare in Malignant Lesions

A recent study was conducted to determine if the presence of focal normal bone marrow fat signal within a tumor on MRI excludes malignancy and to assess inter-observer accuracy.

The results have shown that fat in a bone lesion found on MRI strongly favors a benign etiology, and management of incidentally found lesions should be adjusted accordingly.

Design
This was a retrospective review at 2 separate institutions of 111 malignant and 73 benign de novo tumors. Fat signal within the tumor was determined by non-fat suppressed T1-weighted imaging and fat suppressed T2-weighted and/or STIR images.

Results
There was good interobserver reliability of 95.3% and 96.7% at sites 1 and 2, respectively. At site 1, normal marrow fat signal was identified in 1 of 50 (2.0%) malignant and 3 of 14 (21.4%) benign tumors (P =0.030). The positive predictive value (PPV) and negative predictive value (NPV) at site 1 was 81.7% and 75.0%, respectively.

At site 2, normal marrow fat signal was found in 3 of 61 (4.9%) malignant and 14 of 59 (23.7%) benign tumors (P =0.004). The PPV and NPV at site 2 was 56.3% and 82.4%, respectively. For the pooled consensus, the frequency of intratumoral fat in benign lesions (23.3%) was significantly greater than the frequency in malignant lesions (3.6%) (P <0.001).

Conclusions
Marrow fat in bone lesions can be reliably detected and is rare in malignant lesions.

Fat in a bone lesion found on MRI strongly favors a benign etiology, and management of incidentally found lesions should be adjusted accordingly.

Reviewer’s Comments
There was only 1 lesion containing intralesional fat that had no soft tissue component, which proved to be a lymphoma. This lesion was followed clinically and showed no change over 4 years.

The other malignant lesions containing fat were clearly aggressive, not resulting in wrong lesion assessment. It is comforting to know that fat-containing lesions, if they clearly lack aggressive features, are generally benign, even in the older patient population.

Author: Cornelia Wenokor, MD

Reference:
Simpfendorfer CS, Ilaslan H, et al. Does the Presence of Focal Normal Marrow Fat Signal Within a Tumor on MRI Exclude Malignancy? An Analysis of 184 Histologically Proven Tumors of the Pelvic and Appendicular Skeleton. Skeletal Radiol; 2008; 37 (September): 797-804