HS also formerly known as “true” histiocytic lymphoma is a rare neoplasm showing evidence of histiocytic differentiation. It often appears in the skin, lymph nodes, and intestinal tract . Although CNS involvement is rare, there are a few cases reported in the literature [1–5] and other cases diagnosed in animals, such as cats, dogs, and camels [6–9]. The cases involving the CNS occurred by continuation of the disease except the case reported by Torres et al.  which appeared in the subarachnoid space.
HS has often been classified as being similar to diseases such as malignant histiocytosis, histiocytic lymphoma, and reticular sarcoma, based on morphological criteria . However, advances in molecular and biological markers have improved immunohistochemistry findings, allowing for differentiation between HS and B or T cells and non-Hodgkin lymphomas. These diseases have a prognosis and treatment different from sarcomas.
HS is composed of mononuclear tumor cells with large cytoplasmic vacuoles. Some cells present with phagocytosis phenomena. The diagnosis is based on immunohistochemistry markers of histiocytic cells such as positive CD68, lysozyme, α1-antitrypsin and CD163, which is regarded as a specific HS marker . However, HS presents negative markers of epithelial (epithelial membrane antigen or cytokeratin), melanocytic (CD34 or myeloperoxidase) and lymphoid (CD3 or CD20) neoplasms as in our case. Markers of Langerhans cell (CD1a), follicular dendritic cells, B cells, T cells, and myeloid cells are negative. Lack of expression of CD30 and ALK-1 excludes anaplastic large cell lymphoma, which is an important differential diagnosis. The focal staining for S-100 protein is not uncommonly seen in the true histiocytic lymphomas as reported in the literature .
Radiological findings have not been established due to the low number of cases. The HS image study reported in the literature addressing the brain is MRI , which describes the extent of the disease. The images point towards neoplastic disease, but there are no specific images suggesting HS. Some MRI studies in dogs are inconclusive because the image presented could be either a benign process (such as encephalitis) or a malignant process (such as meningioma) .
The standard treatment for these sarcomas is surgery. The best option for treating these tumors is unclear. Furthermore, when the tumor is located in the CNS the therapeutic options are reduced. Brain sarcomas have an aggressive course with an overall survival rate of around 4 to 5 months, regardless of treatment. There are no studies where the benefit of radiotherapy or chemotherapy for HS has been established.
In our case, the patient received concomitant chemoradiotherapy after surgery. We decided to administer temozolomide with the radiotherapy because the patient was young and she presented with a sarcoma affecting the CNS. It is also known that this type of chemotherapy crosses the blood–brain barrier and it has shown benefit over radiotherapy alone in other aggressive tumors . She had a good tolerance and currently lives without disease (disease-free interval of 42 months). The cases reported before were treated with different schedules of chemotherapy by the vein or with intrathecal treatment but none received concomitant radiotherapy. All the cases had an aggressive course except one case that relapsed after 3 years . In the literature, some cases of extranodal HS treated with surgery and some type of chemotherapy or radiotherapy had a good outcome. These reported patients lived varying months after treatment . Recently, a radiotherapy-induced HS has been treated with radiation alone and the patient remained free of recurrent disease after 1.5 years . Radiation alone could be beneficial but we do not know whether radiation-induced HS differ in their biological behavior.