Background:
Enchondromas are a common long bone benign tumor often discovered incidentally on imaging for adjacent pathology. These benign cartilaginous tumors can be difficult to differentiate from low-grade chondrosarcomas on imaging and histology. Multiple advanced imaging studies and clinic visits are required to confirm stability. Surveillance for these lesions can lead to significant patient costs without a clear oncologic or functional benefit. There is a lack of evidence-based consensus guidelines for the surveillance of enchondromas. The purposes of our study are: 1) to determine the number and proportion of low-grade cartilaginous tumors that demonstrate growth or require treatment and 2) to optimize the efficacy and cost-effectiveness of surveillance strategies for detecting biologically active lesions.

Methods:
A retrospective single-institution study was performed on 55 subjects, 18 years or older, with long bone enchondromas without concerning radiographic characteristics that were referred to our institution's orthopaedic oncology clinic from July 1, 2009 to November 30, 2016. All subjects had at least 12 months of radiographic follow-up. We performed a chart and imaging review to assess for growth of the lesion over time. The number of pre-referral imaging and the number of follow-up imaging studies were recorded. The costs of plain radiographs and advanced imaging were estimated using our institution's global charge list in 2016.

Results:
For stable enchondromas, 35 out of 52 lesions (67.3%) presented in the lower extremities compared to three out of three (100%) growing cartilaginous tumors. Three out of 55 (5.45%) long bone cartilaginous lesions exhibited growth at a median of 23 (range 21-25 months) follow-up. There was no apparent difference in median presenting age for stable versus growing lesions (58.5 versus 55.0 years old, p =0.5673) or median lesion size at presentation (4.1 cm versus 3.6 cm, p = 0.2923). None of these lesions presented with pain attributable to the lesion. One out of seven biopsied cartilaginous lesions (four stable and three growing) had a histology diagnosis of grade 1 chondrosarcoma. There was no significant difference in the median number of total clinical visits for stable (four) and growing (five) enchondromas (p = 0.0807). The median pre-referral costs per patient were: plain radiographs ($383.00), CT scans ($0.00), and MRI imaging ($3,969.00). The median post-referral costs for plain radiographs and MRI per patient were $1,326.00 and $4,668.00, respectively. The annual median costs for plain radiographs and MRI were $609.23 and $2,240.64, respectively.

Discussion:
In conclusion, enchondroma growth was a rare event and typically occurred at two years follow-up in our series. Given the low risk for malignant transformation, we propose surveillance with plain radiographic follow-up for stable enchondromas every 3-6 months for the first year and then annually for at least three years of total follow-up. The most significant costs savings can be made by limiting MRI imaging in the absence of clinical or radiographic concern. Additional studies are needed to determine the long-term risk of growth or declaration of chondrosarcoma.Level of Evidence: IV.





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