To the Editor,

ROS1 gene rearrangements occur in less than 3 % of primary lung cancers (1.2–2.6 % of adenocarcinomas and 0–1.4 % of squamous cell carcinomas) and define a new molecular subtype of lung cancer that is sensitive to therapy with specific inhibitors such as the ALK/MET inhibitor crizotinib [1, 2]. We investigated the ROS1 gene status in a large series of lung cancer brain metastases (BM), which represent a common and serious disease manifestation for which new treatment options are needed.

All patients who underwent surgery for lung cancer BM at the Medical University of Vienna, Austria, between March 1990 and February 2011 were eligible for this retrospective study. A histologically confirmed primary lung cancer had to be evident for inclusion in this study. Institutional review board approval was obtained. Tissue micro arrays were constructed using two 1.5 mm spots per BM specimen, 3–5 micrometer thick sections were cut from the tissue micro arrays [3].

ROS1 gene status was investigated by fluorescent in situ hybridization (FISH) using a commercially available probe containing a double color break apart probe (Cytocell, Cambridge, UK). FISH was performed and analyzed according to the manufacturer´s instructions, two hundred cells were investigated in each case. ROS1 amplification was defined if 6 or more gene copies/nucleus were observed, in analogy to HER2-FISH in breast cancer.

We successfully investigated by FISH a total of 153 samples of lung cancer BM (99 adenocarcinomas, 11 squamous cell cancers, 3 adenosquamous carcinomas, 4 large cells carcinomas, 1 large cell neuroendocrine carcinoma, 35 small cell cancers). We found one case of adenocarcinoma BM with ROS1 translocation (0.6 % of all samples, 1 % of adenocarcinomas, Fig. 1) and one case of squamous cell carcinoma with ROS1 amplification (0.6 % of all samples), while 151 BM (98.7 % of all BM cases) samples showed neither ROS1 translocation nor ROS1 amplification. In the squamous cell lung cancer BM with the ROS1 amplification 6 gene copies/nucleus were observed.

Fig. 1
figure 1

FISH findings in the lung adenocarcinoma brain metastasis (green signal probes flanking the ROS1 gene on the telomere-side; red signal probes flanking the ROS1 gene on the centromere-side). a 66/200 of tumor cells showed 2 normal fusion signals. b 76/200 tumor cells showed two normal fusion signals and 1–2 additional green signals in close vicinity to one fusion signal: in these cells a rearrangement of ROS1 occurred with a break in ROS1 and a suspected rearrangement within the same chromosome. c 58/200 tumor cells showed an additional green signal randomly distributed within the nucleus, presumably resembling a subclone with an additional rearrangement with another chromosome

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Our data show that translocations and amplifications of ROS1 gene occur at a similarly low rate in lung cancer BM as reported for primary tumors. Thus, ROS1 gene alterations seem not to represent a risk factor for BM development in lung cancer. Nevertheless, ROS1 fusion proteins might represent feasible therapeutic targets in selected lung cancer patients with brain-metastatic disease. Our findings should be validated in further studies.