desmoid type fibromatosis is a benign fibroblastic tumor , which is previously known as extraabdominal desmoid , aggressive fibromatosis , and musculoaponeurotic fibromatosis ( 1 ) . although it is a benign tumor , it tends to be locally aggressive , and infiltrates into the surrounding structures , but does not metastasize ( 2 , 3 ) . it arises from the connective tissue of the muscle , and overlying fascia and aponeurosis . the most common extraabdominal sites include the shoulder girdle , followed by the chest wall and back ( 1 , 2 ) . spinal involvements with erosion of the vertebral bodies by paraspinal tumors have been reported ( 8 , 9 ) . however , to our knowledge , there have not been any reported cases of desmoid type fibromatosis originating from the facet joint of the spine . we report computed tomography ( ct ) and magnetic resonance ( mr ) findings of a desmoid type fibromatosis arising from the left facet joint of the lumbar spine in a 31-year - old man treated with partial facetectomy . a 31-year - old man was presented with back pain and mild numbness from the left buttock to the ankle , which developed after the previous motor vehicle accident , 1 year and 5 months ago . conventional radiographs of the lumbar spine demonstrated a bony erosive lesion with a sclerotic rim at the left l3 - 4 facet joint ( fig . ct revealed an isodense soft tissue mass to the muscle at the posterior portion of the left l3 - 4 facet joint , which eroded both l3 inferior and l4 superior articular processes , but the articular surfaces of the both eroded facets were well preserved ( fig . mr images demonstrated about 1.6  1.0  1.0 cm sized soft tissue tumor on the facet joint , which infiltrated beyond the posterior joint capsule . the tumor was isointense to the muscle on t1-weighted image ( repetition time [ tr]/echo time [ te ] 700/10.8 msec ) ( fig . 1d ) , and intermediate to low signal intensity of the tumor between the muscle and cerebrospinal fluid ( csf ) on t2-weighted image ( tr / te 4000/97 msec ) ( fig . the lesion was well enhanced by the gadolinium without evidence of the internal necrosis ( fig . the patient underwent bone biopsy , and pathologically , the tumor was diagnosed as a desmoid tumor . microscopically , the tumor was composed of cellular proliferation of the elongated , slender , spindle - shaped cells arranged in interlacing bundles of fibroblasts with abundant collagenous stroma ( fig . the cellularity varied from area to area with scanty mitotic activity , and the cells infiltrated into the adjacent bone . further , the tumor cells were found in the facet joint capsule with extracapsular infiltration . 1h ) , focal positive reactions to s100 protein , and negative reaction to cd34 , vimentin and desmin . after surgical treatment , the patient was relieved from pain in the lower back and buttock . desmoid type fibromatosis , which is also known as extraabdominal desmoid tumor , musculoaponeurotic fibromatosis and aggressive fibromatosis , is a benign fibroblastic tumor . it arises from the connective tissues , such as the muscles , fascia , aponeurosis or periosteum ( 1 , 5 ) , and may occur in various locations . the most common extra - abdominal sites include the shoulder or upper arm , followed by the chest wall and back , thigh , as well as the head and neck ( 1 - 3 , 10 ) . pericapsular location of the tumor was rarely reported in the knee joint ( 11 ) . in our case , the tumor developed in the facet joint of the lumbar spine . the incidence of desmoid type fibromatosis is 0.03% of all neoplasms and 3.5% of all fibrous tissue tumors ( 2 , 6 ) . it may occur at any age , but usually in young adults with peak prevalence between the ages of 25 and 35 years ( 2 , 5 , 10 ) . in adults , the tumor has a predilection in premenopausal women , but in children , the incidence was equal in both genders . when the tumor occurs in the paraspinal location , scoliosis can be developed with intraspinal extension ( 8 , 9 ) . macroscopically , desmoid type fibromatosis is a dense , coarse trabeculated , fibrous tumor with a whorled appearance , and usually infiltrates into fascicles of the skeletal muscles ( 2 , 5 , 6 ) . microscopically , the lesion consists of interweaving the bundles of spindle - shaped fibroblasts in various amounts of collagenous matrix , and the cells lack hyperchromatosia or atypia . immunohistochemically , the spindle cells stain with vimentin and smooth muscle actin ( 1 ) . our case revealed a positive stain for the smooth muscle actin , but negative for vimentin . the primary treatment of the desmoid type fibromatosis is a wide resection with negative resection margin ( 2 , 6 , 10 ) . when wide resection can not be achieved due to infiltrative feature , adjuvant radiotherapy is necessary . the radiograph of desmoid type fibromatosis is usually normal , or may show a nonspecific soft - tissue mass . osseous involvement may be seen in 6 - 37% of the cases , which manifests as pressure erosion or periosteal frond - like irregularity , and sometimes as cortical destruction and cancellous involvement ( 4 , 5 , 10 ) . ct of desmoid type fibromatosis reveals a soft - tissue mass of the variable attenuation density . the lesions usually show isoattenuation to that of the skeletal muscle , but may be higher or lower with variable amount of collagen matrix or myxoid component . in addition , ct after intravenous contrast enhancement also demonstrates the enhancement of variable degrees . however , there is no relationship between the vascularity of tumor and its histologic features ( 12 ) . the mr signal intensities of the desmoid type fibromatosis are variable along with the extent of collagen and the degree of cellularity . usually , t1-weighted images demonstrate homogeneously similar to slightly hyperintense signal intensity in relation to that of the muscle . on non - fat - suppressed t2-weighted image , most lesions show heterogeneous intermediate signal intensity , lower than that of the muscle and higher than that of fat , but less frequently , predominantly low or high signal intensity of the lesions are also found ( 7 , 10 , 13 - 16 ) . the low signal areas on t2-weighted image are related to relatively decreased cellularity and abundant collagen ; whereas , the hyperintense areas are correlated with an increased cellularity in the abundant collagen matrix . prominent low - signal - intensity bands without contrast enhancement are often seen with all pulse sequences and are thought to be the denser areas of the collagen . after injection of gadolinium contrast , the lesions show moderate to strong enhancement , particularly , more in the cellular regions , but less in the collagenous portion . the desmoid type fibromatosis has variable signal intensities on t2-weighted images according to the histologic evolution ( 10 , 16 ) . the initial lesions are more cellular with large extracellular spaces and less areas of hyalinized collagen , which demonstrate increased signal intensity . as the amount of collagen deposition is increased in the central and peripheral areas , the signal intensities of the lesions become variable . finally , as the fibrous compositions are predominant with a decrease in cellularity and extracellular spaces , the tumors overall appear as low signal intensity . our case revealed intermediate to slightly high signal intensity on t2wi , which was correlated with typical cellularity with collagenous stroma . differential diagnoses of our case , arising from the facet joint include collagenous fibroma , nodular synovitis , and pigmented villonodular synovitis ( pvns ) . collagenous fibroma ( desmoplastic fibroma ) is a benign fibrous soft tissue tumor , which is composed of spindle shaped fibroblasts embedded in a dense fibrous stroma . vertebral involvement with bony erosion ( 17 ) and intraarticular location in the hip joint ( 18 ) have been reported . the nodular fasciitis is a benign myofibroblastic proliferating tumor - like lesion with a predilection for the subcutaneous tissue , fascia and within the muscle . intraarticular location of the nodular fasciitis has been also reported ( 19 , 20 ) . the ct and mr imaging findings of the nodular fasciitis are nonspecific , according to the amount of cellularity and fibrous component . pvns is a benign , local aggressive lesion , which is characterized by villous or nodular proliferation of the synovium in the joints , bursae or tendon sheaths . involvement of the facet joint by pvns with bony erosion of the adjacent lamina is rarely reported ( 21 , 22 ) . desmoid type fibromatosis can involve the facet joint , and should be considered in the differential diagnosis of the fibrous soft tissue tumor in the facet joint of the spine .
desmoid type fibromatosis is a benign fibroblastic tumor arising from the fascia or musculoaponeurosis . 
 it may occur in various locations , but most commonly in the shoulder girdle and neck ; to our knowledge , there has been no reported case originating from a facet joint of the spine . 
 we report ct and mr imaging findings of a desmoid type fibromatosis , involving the facet joint of the l3 - 4 spine with bone involvement .