eosinophilic granuloma ( eg ) , a benign local bone disease , is classified as the mildest form of langerhans ' cell histiocytosis ( lch ) . eg predominantly affects children , adolescents , and young adults , and the skull is the most common site of involvement.5 ) the pathologic diagnosis of eg can be confirmed by positive stains of cd1a antigen and protein s-100 or when intracytoplasmic organelles ( birbeck granules ) are detected using electron microscopy.11 ) the most common symptom of eg is a tender , growing scalp mass , but it can be asymptomatic and incidentally found on skull x - rays . the clinical course is benign with a tendency to spontaneous regress ; however , eg may also recur . it is common to prove the antecedent of a mild head trauma preceding clinical symptoms . a-6-year - old boy was referred to our hospital with a soft mass on the midline of his forehead . he had undergone surgery for eg in the vertex 19 months ago ( figure 1 ) . one month prior to minor head trauma , a follow - up computed tomography ( ct ) ( figure 2 ) was performed to check for recurrence , and none was found . one week before his arrival at the hospital , he had hit his head , and the resulting mass grew rapidly after the trauma . the findings of the neurological examination were unremarkable , except for a non - tender , soft , and immobile mass . the three - dimensional ( 3d ) ct showed a lytic bony defect on the frontal bone without hemorrhage ( figure 3a and b ) and the enhanced magnetic resonance imaging ( mri ) revealed a 1.4 cm - sized growing mass on the midline of the forehead ( figure 3c and d ) . the level of erythrocyte sedimentation rate ( esr ) and c - reactive protein ( crp ) was within normal limits . the protruding mass extended from the scalp through the defect in the frontal bone , and was visualized when the scalp flap was reflected ( figure 4a ) . after removal of the bone flap and tumor , it was noted that there was no hematoma and the dura mater was intact without invasion ( figure 4b ) . the tumor was removed completely and the bony defect was replaced with bone cement ( figure 4c ) . the histopathological examination revealed numerous oval langerhans ' cells and eosinophils ( figure 5a and b ) . the langerhans ' cells were stained for cd1a and s-100 ( figure 5c and d ) . the patient was followed - up six months after the operation and had a normal neurologic examination at that point . lch is a unifocal or multifocal disorder of the bone or soft tissue , most often appearing as a lytic lesion of the skull . eg is the mildest form of lch.5 ) the exact incidence of eg is not well known but vandenberg and coley12 ) found that it accounted for only 2.4% of benign skull tumors . in the series of cases studied by jelsma and ross,4 ) the most common symptom of eg is a tender , enlarged scalp mass on the parietal and frontal bones . eg may be asymptomatic and only discovered on a skull x - ray by chance . the x - ray radiologic findings consist of a punched - out , skull lesion with sharply defined margins . as the clinical and radiologic findings are not specific enough to determine the diagnosis , cytology is very helpful in diagnosing eg . the key feature is the identification of characteristic langerhans cells in a background of eosinophils and red blood cells.5 ) there is no definite disease etiology for eg . however , autoimmune diseases , inflammatory processes , and uncontrolled langerhans cell replication can be considered to be etiological factors.3 ) also , minor trauma is considered an etiologic factors . there have been six reported cases of eg associated with epidural hematoma ( table 1).1678910 ) the possible mechanisms of epidural hematomas in the six cases consisted of a rupture of the tumor cyst , communicating with the epidural vein and the sinus pericranii , or a head trauma - related intratumoral hemorrhage , which subsequently penetrated into the epidural space . all six cases were associated with epidural hemorrhages , while our case was not . in our case , no hematoma or hemorrhage was found in the ct / mri image , or in intraoperative and pathologic findings . the patient in our case regularly visited the clinic for follow - up appointments due to his history of eg . the ct scan taken one month prior to the trauma showed no definite recurrence ; however , a newly developed eg lesion was found one week after the trauma . this indicates that the development of eg may be associated with trauma . in this case , it is thought that eg developed through a local inflammatory response due to a minor trauma . trauma may be a trigger factor of the inflammatory response instead of the etiology of the disease . the elevated levels of esr and crp are found in some , but not all , cases of eg . this is because eg lesions are limited to local level and the time between diagnosis and trauma varies . in the present case , the author finds the period between the trauma and diagnosis to be relatively long . factors related to hematoma formation may include the trauma 's intensity and the time interval after the trauma . for example , in one case , a patient with an epidural hematoma had a two - week time interval , with a fluid level confirmed four weeks later . in the management of solitary eg , surgical excision is recommended and radiotherapy can be considered as an adjuvant therapy if necessary . some authors advocate surgical excision followed by postoperative treatment that includes low - dose radiotherapy , intralesional application of corticosteroids , or a combination of chemotherapy and radiotherapy.11 ) other studies found that solitary calvarial eg in skeletally immature patients 14 years of age or younger resolved fully without any recurrence.2 ) eg is responsive to steroid treatment and this indicates that the etiology of eg is associated with local inflammatory processes . to avoid surgical procedures , the prognosis depends on the age of the patient at the time of diagnosis and the number of lesions . the local recurrence rate is 6% , and new lesions appear in about 22% of patients , therefore , long - term follow - up is recommended.311 ) our case suggests that minor trauma may be an aggravating factor for the progression of eg , based on the radiological findings before and after the trauma . thus , careful observation with regular follow - up is needed for patients with eg after trauma .