CORR Insights®: Is Skeletal Imaging Essential in the Staging Workup for Conventional Chondrosarcoma?

Where Are We Now?

Gulia et al. [3] retrospectively studied 427 patients with conventional chondrosarcomas to help answer an important question: Is skeletal imaging needed in patients with conventional chondrosarcoma? They found that only three patients had bone-only metastases detected by bone scan or fluorodeoxyglucose positron-emission tomography (FDG PET)/CT, a low proportion that supports earlier work by Douis et al. [2], who found that skeletal scintigraphy detected no chondrosarcoma metastases in a smaller number of patients (n = 188). Gulia et al. [3] bring to light two topics of interest: the ideal modality for detecting skeletal metastases in patients with chondrosarcoma and the appropriate use of limited imaging resources.

Skeletal staging in patients with bone sarcomas is typically performed using a bone scan or FDG PET/CT. Bone scan, although sensitive to mineralized components of chondroid lesions, may be limited by complex planar anatomy and overlapping structures, such as that encountered in the chest wall and pelvis. FDG PET/CT allows for cross-sectional imaging of complex anatomy and is augmented by concurrent data from CT images, but is limited by low uptake of chondrosarcomas [1]. There are no studies of which I am aware that compare bone scan and FDG PET/CT, and the decision to use one over the other is left to the discretion of the treating physician and is often affected by availability and institutional habit. Additionally, there are no data on the most-sensitive and specific modality for detecting skeletal metastases in patients with chondrosarcoma or on factors that predict the risk of metastases at baseline.

Where Do We Need To Go?

As noted by Gulia et al. [3] it is currently unknown whether screening for skeletal metastases in a patient with chondrosarcoma is an appropriate use of imaging resources. Even with more-sensitive imaging modalities than bone scans, the frequency with which skeletal metastases occur in patients with conventional chondrosarcomas is expected to be low, and risk-stratification schemes will be needed to select patients for skeletal screening in order to improve outcomes. Is screening for skeletal metastases in these patients is an appropriate use of resources, since those metastases are so rare? What would we need to do in order to combine what Gulia et al. [3] and others [5] have discovered, so that we can have a robust risk-stratification approach?

In addition, evidence-based data are needed to determine the best modality for skeletal screening and to guide recommendations for initial workup for patients with chondrosarcoma. As with all diagnostic studies, determining what is best will need to balance sensitivity and specificity and account for the costs of missed diagnoses because of false negatives and unnecessary additional studies because of false positives. The cost of additional chest imaging for detecting lung metastases, as would be needed with whole-body MRI (but not necessarily with PET/CT) would need to be accounted for.

How Do We Get There?

There are existing data for risk stratification, including those in the present study. Gulia et al. [3] suggest reserving skeletal imaging for patients with a higher risk of having skeletal metastases, which, in their study, were those with lung metastases. However, the study did not consider additional data that can be used to create more-robust risk stratification schemes. Thorkildsen et al. [5] have identified an axial skeletal location and a soft-tissue component ≥ 1 cm as adverse prognostic indicators. However, the endpoint of interest in that report was the eventual development of metastases and not the presence of metastases at baseline. Future studies can assess the utility of these and other prognostic indicators for selecting the patients who are most likely to benefit from skeletal imaging.

There are imaging modalities that theoretically would be more-sensitive to detect chondrosarcoma metastases than either a bone scan or FDG PET/CT. Sodium fluoride PET/CT is more sensitive than bone scans are for detecting mineralized and blastic lesions and includes added data from concurrently acquired CT images [4]. Another promising modality for detecting skeletal metastases in patients with chondrosarcoma is whole-body MRI. Chondrosarcomas have a high T2 signal on MRI and are easily detectable on fluid-sensitive MR sequences. A rapid MRI protocol consisting of fluid-sensitive sequences could be developed and tested for this purpose.

The current study [3] can be used to appropriately power a prospective study comparing the standard of care bone scan with sodium fluoride PET/CT and whole-body MRI and to develop selection criteria to improve the use of resources. Owing to the relative rarity of chondrosarcomas and skeletal metastases, a multi-institutional effort would be needed to accrue a sufficient number of patients. Such an effort would require standardized data collection, imaging techniques, and histopathologic and imaging interpretation.