Tumor biology and incidence
Chondromyxoid fibroma is a rare benign tumor characterized by lobulated areas of spindle shaped and myxoid (stellate) cells with abundant chondroid or myxoid matrix and interspersed giant cells.  It represents less than 5% of all benign cartilage tumors and < 1% of all bone tumors. 
Age
This disease usually presents in the second or third decades of life.
Gender
Male:Female = 2:1
Presentation
Chondromyxoid fibromas typically present (85%) as chronic pain near a joint without a history of trauma.  Patients can also present with swelling 65% of the time.  Patients present with pathologic fracture less than 5% of the time.  Patients may also present with a soft tissue mass and restriction of motion.  Usually constitutional symptoms are absent.
Physical findings
Tenderness to palpation of the affected area as well as decreased range of motion is common. 
Plain films
Chondromyxoid fibromas are eccentric, radiolucent lesions with well-defined margins.  It usually has a sclerotic border and "soap bubble" appearance.  Most commonly it is purely a lytic lesion, however, can have mineralization and calcification 12% of the time.  There is no periosteal reaction.  
Site
Chondromyxoid fibromas are found in the metaphysis of long, tubular bones.  They are more often found in the lower extremity with the proximal tibia being the most common site. 
Size
The size is variable but typically ranges from 3-10 cm.
Tumor effect on bone
Chondromyxoid fibromas may cause expansion or thinning of the cortex. 
Bone response to tumor
A sclerotic rim of bone may be present.
Matrix
Matrix is a zonal and lobular pattern with loose myxoid areas and dense chondroid areas. 
Cortex
Expansion and thinning of the cortex is common.
Soft tissue mass
Patients often have swelling and a soft tissue mass.
Bone scan
There is increased uptake of chondromyxoid fibromas on bone scan.  However, this information usually contributes little additional information in making the diagnosis.  If the bone scan reveals multiple lesions this is highly uncharacteristic of chondromyxoid fibroma.  
CT Scan
CT scans helps to define architecture and cortical integrity.  It can also show mineralization of the tumor (12% of cases). 
MRI
Chondromyxoid fibromas are typical cartilaginous lesions which demonstrate low signal intensity on T1-weighted images and high signal intensity on T2-weighted images. 
Differential Diagnosis
Giant cell tumor (generally older, do NOT have calcifications)
Chondrosarcoma
Fibrous dysplasia and non-ossifying fibroma
Chondroblastoma
Unicameral Bone Cyst
Aneurysmal Bone Cyst
Natural history
No cases of metastases of chondromyxoid fibromas have been reported.  This disease can be locally aggressive.  It can cause bone destruction and has a propensity to recur once treated.  If untreated this lesions tends to grow and infiltrate the surrounding tissues.
Pathology
Histologically, these lesions are similar to chondrosarcoma.  Chondromyxoid fibromas are composed of spindle cells, myxoid (stellate) cells and multi-nucleated giant cells.  However, the zonal architecture and lobular pattern of loose myxoid and dense chondroid differentiate it from other pathologies.    This lesion has microscopic calcifications 35% of the time.
Diagnosis and treatment
After imaging (usually plain films, CT scan and MRI) and confirmatory biopsy this lesion is usually treated surgery as there is little evidence to support spontaneous resolution.   Treatment is intralesional excision, curettage and autograft or allograft bone grafting.  Polymethalmethacrylate cement may also be used for lesions that require structural support.  Some authors recommend en bloc resection and reconstruction due to recurrence risk. 
Complications
Local recurrence rates have been reported between 15-25%.  This depends on the age of the patient and type of resection (higher with intralesional excision).   Pathologic fracture through the lesion occurs 5% of the time.  Complications related to surgical treatment include infection, failure of allograft, and fracture. Malignant transformation is rare but has been reported in association with radiation treatment.  Due to this XRT is a contra-indicated treatment option.
Recommended Reading

Nielsen GP, Keel SB, Dickersin GR, Selig MK, Bhan AK, Rosenberg AE.Chondromyxoid fibroma: a tumor showing myofibroblastic, myochondroblastic, and chondrocytic differentiation. Mod Pathol. 1999 May;12(5):514-7.
Wu CT, Inwards CY, O'Laughlin S, Rock MG, Beabout JW, Unni KK. Chondromyxoid fibroma of bone: a clinicopathologic review of 278 cases. Hum Pathol. 1998 May;29(5):438-46.
 Yamaguchi T, Dorfman HD.Radiographic and histologic patterns of calcification in chondromyxoid fibroma. Skeletal Radiol. 1998 Oct;27(10):559-64.