IAC therapy is not yet well established and the heterogeneity of existing studies complicates the interpretation of its efficacy [4]. A similar study we could compare our results to is Fortin et al. (2007) [8]. In their study, among others four patients with BCBM were treated with IAC. The methodology is comparable to the present study, with the difference that Fortin et al. (2007) used blood-brain barrier disruption in some patients. Overall, they achieved a median OS of 13.5 months after study entry for all tumors and 8.1 months for patients with BCBM. With a median OS time of 8.2 months, we can report similar survival for our BCBM patients. Due to the limited literature data available, these results cannot be compared with further studies. Nonetheless, we consider the tumor response of our study with 8 out of 11 patients achieving local tumor control a success, given the highly progressive nature of this disease.

The median OS of this study for BCBM patients was 8.2 months, which almost aligns with previously published data reporting a median OS of 7.9 months [1]. It is important to note that values in the previous literature describe patient survival from the time of metastasis diagnosis, while our values were measured from the first IAC treatment session. In our study, we could only treat severe cases with palliative intent treatment. The patients were already in the stage of progressive disease with exhausted therapy options. This reduces comparability with the data in the literature.

Patients with progressive disease developed new metastases with simultaneously small changes to SLD. This leads to the hypothesis that IAC may effectively target established secondary brain tumors but may not be able to prevent the occurrence of further metastases. To avoid this, the regional concept of IAC may have to be combined with systemic chemotherapy. In this context, more super-selective catheterization strategies might theoretically increase local drug concentration within predefined vascular territories. However, we preferred a more distal approach in order to cover larger areas of the brain and due to the high tumor burden in our patients.

Like other recent studies, we did not experience any severe adverse events and believe IAC to be a safe method [12]. Although intra-arterial administration via the internal carotid artery results in exposure of the ophthalmic artery, no clinical signs of retinal toxicity were observed. This is consistent with prior reports indicating a low risk of ocular complications [12]. The risk profile of IAC is comparable with other forms of local therapy such as radiotherapy. A study investigating stereotactic radiosurgery in 3271 patients with brain metastases reported a rate of 2.9% for radiation-related complications within six months following the treatment [13].

This study has several limitations starting by its retrospective design with the low number of patients. We were also not able to create a direct comparison group. Therefore, only comparison with literature data was performed. Additionally, our retrospective data were missing detailed information regarding the previous type of radiotherapy such as local or whole-brain radiotherapy as well as the exact dosage. Furthermore, we had no information on the histological or molecular subtypes of the tumors included. Status for HER2 or hormone receptor has a significant influence on patients outcome. This further limits the interpretation of our reported results.