Serial volumetric analysis of meningioma after primary external beam radiation therapy and the factors predict the therapeutic response: a retrospective study

Background: External beam radiation therapy (EBRT) is one of the treatment modalities for meningioma, and there are a few clinical reports about its short-term efficacy. However, studies reporting the long-term clinical outcomes with serial volumetric analysis are rare. This study reports the long-term clinical outcome of primary EBRT for meningioma with a comprehensive analysis of the volumetric changes and the annual reduction rate. Methods: A retrospective analysis was performed of patients who received conventional EBRT as a first-line treatment for meningioma between 1998 and 2020. The clinical outcomes and volumetric changes after primary EBRT were analyzed. Results: A total of 43 cases with a mean follow-up of 86.4 months were included. Most of the meningiomas were located in the skull base (40 cases, 93%). During the follow-up period, the tumor volume decreased in 38 (88.4%) patients and increased in 5 (11.6%) patients. The tumor volume generally decreased up to 6 months after EBRT, and transient volume expansion was observed between 6 and 12 months. When the tumor volume decreased 10% or more during the first 6 months after EBRT, a favorable therapeutic response was achieved. The mean overall reduction rate of tumor volume was 37.1%, and the mean annual reduction rate was 11.3%. Conclusions: Primary EBRT could be a feasible treatment option for meningioma from the perspective of volume control with favorable clinical outcomes.