Predicting Behavior of Solitary Fibrous Tumor: Are We Getting Closer to More Accurate Risk Assessment?

Predicting clinical behavior of solitary fibrous tumor (SFT) has long been a notoriously difficult task for both oncologist and surgical pathologist. It is well established that morphologically benign tumors may occasionally metastasize and conversely that many tumors, *45 %, classified as malignant based on histologic findings have an indolent clinical course. In this regard, SFT represents a biologic continuum with regard to both histologic appearances and clinical behavior. Although many studies have examined clinical and pathological parameters that may predict risk of relapse in SFT, the majority of these studies have examined small case numbers with limited follow-up. In this issue of Annals of Surgical Oncology, two separate groups have attempted to define clinical and pathologic factors that may predict local recurrence and distant metastasis, by retrospective analysis of 83 and 81 patients with SFT, at a wide variety of anatomic sites, respectively. In both studies, the presence or absence of certain histologic features was gleaned from review of pathology reports. Wilky and colleagues found that anatomic site, specifically extrathoracic location, and the presence of histologic features said to be diagnostic of malignancy as described by England in 1989 (hypercellularity, [4 mitoses per 10 high-power fields [HPF], pleomorphism), as well as ‘‘atypical’’ histologic features that were considered ‘‘borderline’’ by Wilky et al. (0–4 mitoses per HPF, foci of hypercellularity, and atypia) were significantly associated with either local recurrence or distant metastasis. However, the relative impact of each of these histologic features was not evaluated by multivariate analysis. They reported overall rates of 12 and 5 % for local recurrence and metastasis, respectively, with a median follow-up time of 37 months. As might be expected (because of inevitably narrow or positive excision margins), the highest rate of local recurrence occurred in SFT of the CNS, which (in this study) represented 29 % of the study population, but the association between extrathoracic location and increased recurrence rate persisted when CNS tumors were excluded. Although, in this study extrathoracic SFT was more likely to have atypical or malignant histologic features, the difference in recurrence rate (local or distant) seen in extrathoracic locations persisted after correction for size, histologic features, and margin status. Tumor size was not significantly associated with local recurrence or metastasis, but size [10 cm was associated with malignant histology. Van Houdt et al. found that tumor size [10 cm and mitoses [4 per 10 high-power fields predicted metastasis in a cohort of 81 patients. The rates of local recurrence (29 %) and metastasis (34 %), with a median follow-up of 34 months, were substantially higher than those in the cohort studied by Wilky et al., perhaps reflecting referral bias to the tertiary centers involved in the Dutch study, as the follow-up periods were similar in duration in both studies. Local recurrence was significantly associated with positive resection margins at the time of initial surgery, reflecting the unusually high proportion of head and neck tumors in this series, accounting for 27 % of cases. In contrast to Wilky and colleagues, these authors did not find a significant correlation between anatomic location and the development of recurrent disease. With regard to anatomic site, most previous studies have not shown a significant correlation between anatomic site and the development of metastasis, although studies have been limited by small sample size and short follow-up. Approximately 10 % of extrathoracic SFTs showed local recurrence or metastasis in a study by Vallat-Decouvelaere Society of Surgical Oncology 2013