Imaging growth as a predictor of grade of malignancy and aggressiveness of IDH-mutant and 1p/19q-codeleted oligodendrogliomas in adults.

BACKGROUND We quantified the spontaneous imaging growth rate of oligodendrogliomas. We assessed whether: 1) it discriminates between grade II and grade III oligodendrogliomas; 2) grade III oligodendrogliomas with neoangiogenesis are associated with more fast growth rates (≥8 mm/year). METHODS Retrospective bicentric cohort study (2010-2016) of adult patients harboring a newly diagnosed supratentorial oligodendroglioma, IDH-mutant and 1p/19q-codeleted (World Health Organization 2016 classification), with a minimum of two available MRI's before any treatment (minimum six-week interval) to measure the spontaneous tumor growth rate. RESULTS We included 108 patients (44.7±14.1 years, 60 males). The tumor growth rate was higher in grade III oligodendrogliomas with neoangiogenesis (n=37, median 10.4 mm/year, mean 10.0±6.9) than in grade III oligodendrogliomas with increased mitosis count only (cutoff ≥6 mitoses, n=18, median 3.9 mm/year, mean 4.5±3.2; p=0.004), and higher than in grade II oligodendrogliomas (n=53, median 2.3 mm/year, mean 2.8±2.2; p<0.001). There was increased prevalence of fast tumor growth rates in grade III oligodendrogliomas with neoangiogenesis (54.1%) compared to grade III oligodendrogliomas with increased mitosis count only (11.1%; p<0.001), and to grade II oligodendrogliomas (0.0%; p<0.001). The tumor growth rate trends did not differ between centres (p=0.121). Neoangiogenesis (p<0.001) and mitosis count at 9 and > (p=0.013) were independently associated with tumor growth rates ≥8 mm/year. A tumor growth rates ≥8 mm/year was the only predictor independently associated with shorter progression-free survival (p=0.041). CONCLUSIONS The spontaneous tumor growth rate recapitulates oligodendroglioma aggressiveness, permits identification of grade III oligodendrogliomas preoperatively when ≥8 mm/year, and questions the grading by mitosis count.