Acute generalized exanthematous pustulosis induced by sorafenib.

Discussion | Periosteal ganglia are uncommon single or multiloculatedsubcutaneouscysticnodules.These lesionsare rarely encounteredbydermatologists and areusually seen in the orthopedic setting.Althoughdescribedmainly inmen, these lesions also have been reported1,2 in children. Periosteal ganglia typically involvethetibia,but reports2-4havealsodescribed involvementof themedialmalleolus, femur, ilium, radius, and ulna. Duration before presentation varies from several weeks to years.1 Lesions can be asymptomatic or tender, and a history of trauma is variable.3 Mucoid degeneration of the periosteum is the most frequently proposed pathogenesis for the formation of periosteal ganglia.1-6Fibroblastsare thought to formintercellularmucin, which coalesces to form cystic lesions. Accumulation of mucoidmaterial compresses the surrounding tissue, thereby inducing further fibroblast proliferation, collagen production, and ultimately an encapsulating fibrous wall.4 The central cystic contents are composed of an acellularmucinous or gelatinous fluid.4 Although communication with the underlying joint space has not been reported, cases have shown3,5 varying degrees of underlying cortical erosion with scalloping and spiculated bone reactions. Choi and colleagues4 described a case with an underlying interosseous component. However, as in our patient, these cysts frequently havenounderlying connection to the cortical bone. Several imagingmodalities to evaluate periosteal ganglia have been described. Plain radiographs, although helpful in detectingunderlyingbonychanges, arenonspecific anddonot differentiate pretibial ganglion cysts from other surface tumors.3 Computed tomography is helpful in further discerning characteristics of the soft-tissuemass, butmagnetic resonance imaging is the modality of choice.3 Magnetic resonance imagingdemonstrates ahomogeneous signal intensity, whichappears isointense tomuscleonT1-weighted imagesand has a high signal intensity when compared with fat on T2weighted images.3,5 Definitive treatmentof periosteal ganglia is by surgical excision. Someauthors1-3 recommendexcising an adjacentmargin of normal periosteum toprevent recurrence. Although recurrenceafter surgical excisionhasbeendescribed,1,3 thismay represent continuedmucoiddegeneration rather than incomplete excision. The clinical differential diagnosis for pretibial subcutaneousmasses or nodules is broad and includes erythema nodosum, nodular pretibial myxedema, subcutaneous sarcoidosis, periosteal chondroma, parosteal lipoma, subperiosteal hematoma, subperiosteal abscess,periosteal aneurysmalbone cyst, chondromyxoid fibroma, orperiosteal osteosarcoma.1-3,5 Although uncommon and rarely encountered by dermatologists, periosteal ganglion cysts remain an important condition to consider in the differential diagnosis of subcutaneous pretibial lesions. This case highlights the need for dermatologists to recognize this uncommon diagnosis to facilitate appropriate workup and referral.