In Reply: Association Between Pseudoprogression of Vestibular Schwannoma After Radiosurgery and Radiological Features of Solid and Cystic Components

To the Editor: We appreciate the authors' comments1 on our article2 which bring up positive discussion. Despite the fact that pseudoprogression of vestibular schwannomas (VS) after radiosurgery is a commonly observed phenomenon, there was no consensus on definition and time course. Response assessment in neuro-oncology criteria,3 as mentioned by the authors, proposed the definition of pseudoprogression as a >20% increase in volume compared with the baseline and followed by shrinkage in 24–36 months. However, in previous literature, the definition of pseudoprogression of VS varies significantly. For instance, the threshold volume change ranges from 10% to 20%, and the time to peak and duration also varied. We strongly agree with the necessity of standard definition of pseudoprogression to allow comparability between studies and meta-analysis. On the other hand, response assessment in neuro-oncology criteria recommended thin-slice MRI with slice thickness less than 2 mm. However, in busy clinical practice, patients in most institutions received MRI with slices of 2–3.1 mm because of the concerns of scanning time. We totally understood that the accurate volumetry is based on thinner images. With the improvement of MRI machines, we will encourage to perform thinner images in the near future. Regarding the size of VS, although surgery is traditionally considered the primary approach for large VS (ie, more than 3 cm), data from our institution showed satisfactory tumor control rate and excellent preservation of nerve function after radiosurgery,4 among other studies with similar results. While the management of large VS remains challenging, more and more consider radiosurgery as a promising and effective alternative for large VS. It is an excellent suggestion that radiologic features from advanced MRI techniques may correlate with tumor response after radiosurgery and clinical factors including ethnicity and mass effects as mentioned by the authors. Future studies focusing on the relationship between clinical factors and different MRI sequences and histopathological features of tumor and response to treatment could be beneficial. We would like to thank the authors for reading our articles and pointing out several important aspects and limitations regarding our study. Further studies would be warranted to overcome these limitations.