periosteal chondroma , also known as juxtacortical chondroma , is a rare benign cartilaginous neoplasm of bone surface that occurs predominantly in the metaphases of long tubular bones , particularly , the proximal humerus and distal femur . it presents as a swelling or palpable mass that may be painful in children and young adults , with a slight male predominance . we herein present the imaging features of periosteal chondroma involving the proximal phalanx of the left index finger in an adolescent boy and provide a brief review of the relevant literature . a 12-year - old boy presented with a 1-year history of a slow - growing , painless mass in the radial aspect of the proximal phalanx of the left index finger . physical examination revealed a 2  cm , firm , immobile , nontender , subcutaneous mass ( figure 1 ) . plain radiographs revealed a discernible soft tissue lesion with saucerization and sclerosis of the underlying cortex ( figure 2 ) . computed tomography ( ct ) scans confirmed the presence of a periosteal - based lesion without calcification ( figure 3 ) . magnetic resonance imaging ( mri ) exhibited a well - circumscribed juxtacortical mass measuring 1.8  1.2  0.8  cm . the mass showed intermediate signal intensity on t1-weighted images ( figure 4(a ) ) and high signal intensity on t2-weighted images ( figure 4(b ) ) . contrast - enhanced fat - suppressed t1-weighted images demonstrated peripheral and septal enhancement ( figure 4(c ) ) . the mass was adherent to the periosteum of the proximal phalanx and extended into the bone cortex . it was completely excised together with the overlying peritoneum , and the underlying bone cortex was curetted . histologically , the tumor was well demarcated and surrounded by a periosteum - like fibrous capsule . the tumor consisted of hyaline cartilage with clusters of bland chondrocytes ( figure 5 ) . the postoperative course was uneventful , and the patient is doing well without evidence of local recurrence eight months after surgery . surface - based bone tumors may present as palpable subcutaneous masses in the hands or feet , as in our case . in fact , periosteal chondromas involving finger phalanges were confused with primary subcutaneous or skin tumors at initial presentation [ 3 , 4 ] . clinicians should therefore be aware of the possibility of periosteal chondroma when evaluating subcutaneous lesions in the fingers . periosteal chondroma is much less common than enchondroma , accounting for less than 2% of all chondromas . it is usually small , measuring less than 3  cm in diameter , and can erode the underlying cortex without penetrating into the medullar cavity . patients with periosteal chondromas of the fingers are usually asymptomatic and they complain only focal swelling or palpable masses [ 3 , 4 ] . the pathogenesis of periosteal chondroma is poorly understood , but isocitrate dehydrogenase 1 ( idh1 ) mutations have been identified in 71% of cases . histologically , the lesions occasionally show hypercellularity , nuclear enlargement , binucleation , and myxoid change of the matrix , which can lead to a mistaken diagnosis of low - grade chondrosarcoma . it is therefore important to be familiar with the imaging features of periosteal chondroma for its accurate diagnosis and appropriate treatment . plain radiographs often show a discernible soft tissue mass with underlying cortical saucerization or scalloping , subjacent cortical sclerosis , and overhanging margins . ct may be helpful in identifying the presence of scattered calcification and the lack of intramedullary extension . on mri , periosteal chondroma typically appears as a well - circumscribed , juxtacortical mass with intermediate signal intensity on t1-weighted images and high signal intensity on t2-weighted images . soft tissue chondroma occurs most commonly in the fingers and typically affects adults between 30 and 60 years of age . radiographically , the lesion is well demarcated and does not involve the underlying bone , although some tumors cause compression deformity or bone erosion . histologically , soft tissue chondroma and periosteal chondroma are similar , and the distinction is best made on the basis of radiological features . periosteal chondrosarcoma occurs predominantly in the metaphases of long tubular bones and has a peak incidence in the second to fourth decades of life . the lesion is generally larger with a more prominent soft tissue component . according to robinson et al . however , permeation into the soft tissue is an important characteristic of periosteal chondrosarcoma that can be used to distinguish it from periosteal chondroma . resection of the underlying bone cortex after excision of this tumor has been suggested to reduce the possibility of recurrence . in summary , we have described a unique case of periosteal chondroma of the proximal phalanx in the left index finger manifesting as a subcutaneous mass in an adolescent boy . knowledge of the characteristic imaging features can lead to an accurate diagnosis of periosteal chondroma , thereby avoiding unnecessary radical surgery .
periosteal chondroma is a rare benign hyaline cartilage neoplasm that occurs most commonly in the metaphases of long tubular bones . 
 we present a unique case of periosteal chondroma arising in the proximal phalanx of the left index finger in a 12-year - old boy . 
 physical examination revealed a slightly protuberant , subcutaneous mass . 
 plain radiographs and computed tomography scans showed a periosteal lesion producing saucerization of the cortex and subjacent cortical sclerosis , without internal matrix calcification . on magnetic resonance imaging , 
 the lesion exhibited intermediate signal intensity on t1-weighted images and high signal intensity on t2-weighted images . 
 contrast - enhanced fat - suppressed t1-weighted images demonstrated peripheral and septal enhancement . 
 the patient underwent a marginal excision with curettage of the underlying bone cortex . 
 histological examination confirmed the diagnosis of periosteal chondroma . there has been no evidence of local recurrence eight months after surgery . 
 periosteal chondroma can protrude into the subcutaneous soft tissue causing a palpable mass . 
 recognition of the typical radiological features can lead to an accurate diagnosis of this rare condition .