Prognostic value of MIB-1 proliferation index in solitary fibrous tumors of the pleura implemented in a new score - a multicenter study.

Background: Although the majority of solitary fibrous tumors of the pleura (SFTP) follow a benign course, 10-25% of patients suffer from recurrence or metastatic disease. Several scoring models have been proposed to predict the outcome. However, none of these included immunohistochemical (IHC) markers as possible prognosticators.

Methods: In this multicenter study, we collected clinical data and formalin-fixed and paraffin-embedded (FFPE) tissue blocks of patients with histologically proven SFTP which had been surgically resected between 2000 und 2015. After systematic and extensive IHC staining on tissue microarrays, the results were analyzed and compared to histomorphological and clinical data for their possible prognostic value.

Results: In total, 78 patients (mean age 61 ± 11 years) were included. Of these, 9 patients (11%) had an adverse outcome including SFTP recurrence (n = 6) or SFTP-related death (n = 3). Mean overall survival was 172 ± 13 months. 1 and 10-year event-free survival rates were 99% and 93%. In the multivariable analysis only MIB-1 proliferation index (Ki-67) ≥10% (HR 12.3, CI 1.1-139.5, p = 0.043), ≥4 mitoses per 10 high power fields (HR 36.5, CI 1.2-1103.7, p = 0.039) and tumor size larger than 10 cm (HR 81.8, CI 1.7-4016.8, p = 0.027) were independently associated with adverse outcome.

Conclusion: A high proliferation rate by MIB-1 IHC was associated with impaired outcome. Upon this, we established a new score using mitosis, necrosis, size of the tumor and MIB-1, which performed better than the traditional scores in our data set. This prognostic score could help to better evaluate outcome of SFTP, but requires external validation.