Saturday, May 12, 2012

Lucent Bone Lesions: Differential Diagnosis #12

Today's Daily Diff concerns lucent bone lesions, which are commonly encountered incidentally when evaluating plain x-rays, especially of long bones. Sometimes, the term lytic is also used to describe these lesions. The word lucent implies a non-aggressive or benign process, while the term lytic implies a more aggressive appearance. However, some radiologists will either use one of these terms exclusively, or use them interchangeably. The table below lists a broad differential for any lucent lesion, but in practice, only two or three entities should be considered for any particular finding when the clinical context is taken into account. The phrase FOG MACHINES (which are apparently a real thing!) can be used to recall these diagnoses. Alternatively, the phrase FEGNOMASHIC can be used, but that's even worse than fog machines, no?

FFibrous dysplasia
GGiant cell tumorAge > 20 after physeal closure

MMetastasesRenal, thyroid, lung
AAneursymal bone cystCan have a fluid-fluid level
Chondromyxoid fibroma

HHyperparathryoidismLesions also called 'brown tumors'
NNonossifying fibromaaka 'fibrous cortical defect' if < 2 cm
EEosinophilic granuloma
EG is part of the Langerhans Cell Histiocytosis spectrum
SSolitary bone cystsaka 'unicameral bone cysts'
Can have the 'fallen fragment sign'

Two major criteria for narrowing the differential down are age, multiple lesions, and location. The entities with multiple lesions are fibrous dysplasia, EG/enchondromas, hyperparathyroidism, metastastes, and infection (FEMHI). The common lesion locations are shown in the diagram below:
Common lucent bone lesion locations
Source: U. of Washington Radiology

* Lucent Lesions of Bone | University of Washington Radiology