Combination therapy for melanoma brain metastases (MM) using stereotactic radiosurgery (SRS) and immune checkpoint-inhibition (ICI) or targeted therapy (TT) is currently of high interest. In this collective, time evolution and incidence of imaging findings indicative of pseudoprogression is sparsely researched. We therefore investigated time-course of MRI characteristics in these patients.
Data were obtained retrospectively from 27 patients (12 female, 15 male; mean 61 years, total of 169 MMs). Single lesion volumes, total MM burden and edema volumes were analyzed at baseline and follow-up MRIs in 2 months intervals after SRS up to 24 months. The occurrence of intralesional hemorrhages was recorded.
17 patients (80 MM) received ICI, 8 (62 MM) TT and 2 (27 MM) ICI + TT concomitantly to SRS. MM-localization was frontal (n = 89), temporal (n = 23), parietal (n = 20), occipital (n = 10), basal ganglia/thalamus/insula (n = 10) and cerebellar (n = 10). A volumetric progression of MM 2-4 months after SRS was observed in combined treatment with ICI (p = 0.028) and ICI + TT (p = 0.043), whereas MMs treated with TT showed an early volumetric regression (p = 0.004). Edema volumes moderately correlated with total MM volumes (r = 0.57; p < 0.0001). Volumetric behavior did not differ significantly over time regarding lesions' initial sizes or localizations. No significant differences between groups were observed regarding rates of post-SRS intralesional hemorrhages.
Reversible volumetric increases in terms of pseudoprogression are observed 2-4 months after SRS in patients with MM concomitantly treated with ICI and ICI + TT, rarely after TT. Edema volumes mirror total MM volumes. Medical treatment type does not significantly affect rates of intralesional hemorrhage.

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