Pseudoprogression in Koos grade 4 vestibular schwannomas following stereotactic radiosurgery: temporal dynamics and...

To gain insight into the volumetric temporal response dynamics following primary stereotactic radiosurgery (SRS) and to evaluate possible MRI-derived predictors at time of treatment for pseudoprogression in sporadic Koos grade 4 vestibular schwannomas (VS).

Patients treated with Gamma Knife radiosurgery for a unilateral sporadic Koos grade 4 VS, larger than 4 cm³ at time of treatment, were included. In tumors with cystic components, separate annotations of the solid and cystic components were additionally obtained. Uni- and multivariable logistic analyses were performed to evaluate possible predictors for pseudoprogression.

A total of 258 patients with a median tumor volume of 6.8 cm³ (IQR 5.1-8.9) were included in the study. Pseudoprogression was observed in 75 tumors (29%) with a median time-to-peak of 8 months (IQR 6-14) and a median relative increase of 23% (IQR 15-34) at peak. In 19 out of 39 macrocystic tumors, the solid versus cystic components exhibited distinct response dynamics. Multivariable logistic regression analyses revealed that only the presence of microcystic components was significantly inversely associated with pseudoprogression with an adjusted OR of 0.15 (95% CI 0.05-0.46; P < 0.001). Ultimately, 30 patients (12%) required 31 additional interventions after a median of 36 months (IQR 21-60).

Pseudoprogression rates in selected Koos grade 4 VS appear comparable to those of smaller tumors. Tumors with microcystic components exhibited significantly lower pseudoprogression rates, which may render them more suitable candidates for SRS. In contrast, macrocystic tumors demonstrated more variable dynamics, warranting careful monitoring in these tumors.