The central nervous system (CNS) is surrounded by specialized immune interfaces, including the meninges and the olfactory bulb, which can serve both as permissive routes for metastatic colonization and as entry points for anti-tumor immune responses. Understanding how immune cell dynamics evolve within these structures during metastatic progression is essential for identifying mechanisms that either support or restrict tumor growth in the brain. This protocol details the establishment of a breast cancer brain metastases (BCBM) mouse model through intracranial injection of E0771 breast cancer cells into C57BL/6 mice. After tumor development, the meninges and olfactory bulb are carefully dissected, enzymatically digested, and processed into single-cell suspensions. These populations are then analyzed using multiparametric spectral flow cytometry to comprehensively characterize lymphoid, myeloid, and stromal cell subsets within CNS-associated immune niches. This workflow enables comparative analyses between tumor-bearing and non-tumor controls, identifying alterations within CNS immune interfaces that may underlie tumor immune evasion and represent potential therapeutic targets. Overall, the protocol provides a reproducible approach to study BCBM and interrogate immune regulation at CNS-peripheral immune system interfaces. It can be adapted for additional cancer cell lines, immunotherapies, or genetic mouse models to investigate immune resistance mechanisms and therapeutic response within the brain microenvironment.

Leptomeningeal metastases (LM) is a rare but clinically important subtype in central nervous system (CNS) metastasis. Multicentre data was reviewed to detail the epidemiology, clinicopathological associations among patients with positive cerebrospinal fluid cytology (CSF) cytology. Patients with CNS metastasis and clinical data were identified through the Hong Kong Hospital Authority Clinical Data Analysis and Reporting System (CDARS). In total, through 1st January 2000 to 31st December 2024, 38,893 patients with CNS metastasis were retrieved, 1503 had CSF cytology. The most common primary malignancies were lung (54.16%), breast (11.18%) and gastrointestinal (5.66%; colonic: 3.33%, gastric: 1.86%, esophageal: 0.47%). There were 220 (14.64%) patients with positive CSF, they were younger (57.72 years vs. 60.03, p = 0.005), and likely with breast (20.84%, p = 0.016) and lung primaries (16.47%, p = 0.030). The median overall survival (OS) was 113 days. Positive CSF, age > 60 and gastrointestinal primary were correlated with shorter OS (p < 0.05). The inclusion of brain radiotherapy (260 day vs. 82, p < 0.001) and systemic therapy (496 days vs. 61, p < 0.001) for treatment of leptomeningeal metastases was independently associated with longer OS. Positive CSF cytology is a strong independent indicator of poor outcomes. Radiotherapy and systemic therapy improve overall survival for the medium term (6-12 months).

Antibody-drug conjugates (ADCs) are targeted therapies that employ monoclonal antibodies to deliver potent cytotoxic agents to cancer cells. The treatment of breast cancer with brain and leptomeningeal metastases has long been limited by the assumption that the blood-brain barrier (BBB) and blood-cerebrospinal fluid barrier (BCSFB) restrict the penetration of large molecules into the central nervous system (CNS). However, recent evidence from several trails (DESTINY-Breast12, DEBBRAH, TUXEDO-3, etc.) challenges this paradigm. In these studies, trastuzumab deruxtecan, a HER2-targeted ADC, demonstrated notable intracranial responses in patients with active CNS disease, suggesting that CNS barriers are dynamically remodeled within the tumor microenvironment to permit drug access. These findings have renewed interest in understanding how macromolecular ADCs penetrate the CNS and exert anti-tumor effects. By integrating emerging clinical evidence with key pharmacological determinants and tumor-associated microenvironmental changes, this review delineates the mechanisms governing ADC activity in brain metastases and identifies critical factors underlying intracranial response. Collectively, these insights provide a mechanistic framework to guide the rational design of next-generation ADCs and optimize therapeutic strategies for patients with advanced CNS involvement.

Brain metastases constitute the most prevalent intracranial malignancies. Despite recent therapeutic advances, patient prognoses remain poor, largely due to significant treatment resistance and a highly restrictive tumor microenvironment. The rediscovery of meningeal lymphatic vessels (MLVs) has provided new insights into how the brain interacts with the immune system. MLVs are responsible for cerebrospinal fluid drainage, waste clearance, and immune cell trafficking, and their dysfunction is linked to many neurological diseases. In brain metastases, MLVs play a dynamic, dual role based on the evolving tumor microenvironment. Under physiological conditions or in early stages, MLVs promote anti - tumor immunity by draining tumor antigens to cervical lymph nodes and supporting T-cell activation. However, as the tumor progresses, excessive tumor - derived factors like VEGF-C cause pathological MLV remodeling. This structural change is a critical switch: pathologically dilated MLVs facilitate tumor dissemination and their drainage dysfunction creates a local immunosuppressive niche, leading to immune evasion. This "friend and foe" character makes MLVs a potential therapeutic target. Enhancing MLV drainage may improve immunotherapy and drug delivery, while inhibiting tumor-driven lymphangiogenesis may help limit metastatic spread. In this review, we summarize current knowledge on MLV biology, their interactions with brain metastases, and discuss potential strategies and challenges for targeting MLVs in future therapies.

Leptomeningeal carcinomatosis (LMC) is extremely rare in gastric cancer, as is recurrence >10 years after gastrectomy. Herein, we report an autopsy case of LMC that developed 29 years after gastrectomy for gastric signet ring cell carcinoma. A 74-year-old man presented with multiple progressive cranial neuropathies involving cranial nerves III, V, VI, VII, IX, X, and XII. CSF analysis revealed pleocytosis and markedly elevated protein levels; however, cytological examination of five consecutive samples was negative for malignant cells. Multiplex PCR analysis excluded infectious etiologies. MRI demonstrated contrast enhancement of cranial nerves III, V, VII, and XII. Despite antiviral, antituberculous, and corticosteroid therapy, the patient's condition deteriorated, and he ultimately died three months after symptom onset. Autopsy revealed extensive infiltration of signet ring cell carcinoma in the leptomeninges and cranial nerves. Immunohistochemical analysis of this tissue revealed cytokeratin 7 positivity and cytokeratin 20 negativity, identical to the gastric carcinoma resected 29 years earlier. No recurrence was observed in the remnant stomach or other organs. This case suggests reactivation of microscopic leptomeningeal metastases following an exceptionally prolonged period of tumor dormancy. Signet ring cell carcinoma is thought to be prone to dormancy owing to its weak intercellular adhesion, leading to reduced proliferative signaling and activation of stress response pathways. This experience suggests that LMC should be considered when meningitis-like neurological symptoms develop, even in patients who remain recurrence-free for decades postoperatively.

Breast cancer (BC) is the second leading cause of brain metastases (BMs). The incidence of BMs has increased over the last decades as results of both the improvement in imaging detection power and the longer life expectancy of patients with metastatic disease. In human epidermal growth factor receptor 2-positive (HER2 +) advanced BC, the incidence of BMs is especially high and is associated with poor prognosis. The central nervous system (CNS) has historically been considered a sanctuary site since the blood-brain barrier prevents the penetration of most therapeutic agents. Therefore, loco-regional therapies, such as surgery or radiotherapy, have largely represented the only effective strategies for treating intracranial disease. However, the development of several anti-HER2 agents with proven CNS efficacy has resulted in unprecedented disease control, raising the question of the optimal strategy to adopt in the management of both extracranial and intracranial disease. Here, we provide a comprehensive narrative review of the therapeutic management of HER2 + BMs and leptomeningeal disease, highlighting recent advances in the efficacy of both locoregional and systemic strategies and briefly discuss brain imaging screening and biomarkers for BMs risk stratification. Building on this evidence, we propose an updated, integrated treatment algorithm designed to optimize clinical decision-making.

Enteric-type thymic adenocarcinomas are rare histological variants with limited treatment options. We report the case of a 56-year-old woman with stage IVB enteric-type thymic adenocarcinoma who was treated with lenvatinib. The patient underwent chemoradiotherapy and subsequently received lenvatinib for local recurrence. Despite the patient receiving a low dosage owing to side effects, the size of the recurrent lesions decreased. However, 5 months later, new brain metastases and carcinomatous meningitis developed, and the patient eventually succumbed to cancer. This case suggests that lenvatinib may be an effective treatment for enteric-type adenocarcinomas. However, at low doses, its efficacy may be limited to intracranial lesions.

Background: POLE-mutated endometrial carcinomas are associated with exceptionally favorable outcomes, forming the basis for treatment de-escalation in early-stage disease. Nevertheless, rare adverse clinical courses have been reported. This study describes an unusual case of late metastatic recurrence in a POLE-mutated tumor and provides a review of similar cases in the literature. Methods: We present a detailed clinical, radiological, pathological, and molecular description of a patient who developed metastatic recurrence 16 years after initial surgery. A systematic literature search was conducted to identify reports of recurrence, progression, or cancer-related death in POLE-mutated endometrial carcinoma, with extraction of recurrence patterns, genomic features, treatment, and outcomes. Results: The patient experienced sequential pulmonary, cerebral, and leptomeningeal metastases despite harboring a canonical POLE hotspot mutation, proficient mismatch repair status, wild-type TP53, no additional known driver mutation beyond PTEN alterations. The literature review identified a small number of similarly adverse cases. Reported recurrences were heterogeneous, though distant and occasionally central nervous system involvement were noted. Conclusions: While POLE-mutated tumors overall retain an excellent prognosis, rare cases may follow an atypical and aggressive course. Improved molecular annotation and integrated risk-stratification models are needed to better identify this minority of higher-risk patients.

Intracranial sarcomas can arise secondarily from primary brain tumors, including gliomas and meningiomas, either spontaneously or following radiotherapy. The current WHO classification recognizes sarcomatous transformation in several tumor entities; however, sarcomas arising from meningiomas remain poorly characterized and are regarded as a possible histological manifestation within the spectrum of anaplastic meningiomas. We analyzed nine matched meningioma-sarcoma pairs using integrated histopathological assessment and molecular profiling, including DNA methylation analysis, next-generation sequencing, copy number profiling, and proteomics. Although recurrent sarcomatous tumors were clonally related to their meningioma precursors-sharing identical NF2 alterations and overlapping chromosomal aberrations-they demonstrated pronounced divergence at the histological, immunophenotypic, and epigenetic levels. Importantly, sarcomatous transformation occurred in four cases without prior radiotherapy. Sarcomatous recurrences exhibited loss of meningothelial markers and acquired expression of cytokeratin and myogenic markers. DNA methylation profiling revealed a shift away from canonical meningioma signatures toward profiles resembling non-meningothelial mesenchymal tumors. Proteomic analysis showed consistent upregulation of SOX2 in sarcomatous tumors compared with their primary counterparts, suggesting acquisition of stem-like features during lineage divergence. Clinically, these tumors were associated with aggressive growth, early recurrence, and extracranial metastases, resembling malignant sarcomas more closely than anaplastic meningiomas. In addition, analysis of an institutional cohort of NF2-mutant intracranial tumors (n = 316) suggests that sarcomas with inactivating NF2 mutations may originate from meningiomas even in the absence of a clinically recognized precursor. Together, these findings suggest that sarcomatous transformation represents a rare evolutionary endpoint in NF2-mutant meningiomas, marked by clonal continuity but pronounced biological divergence. These results highlight limitations of morphology-based classification and emphasize the value of integrated molecular diagnostics in distinguishing these tumors from conventional high-grade meningiomas. Given their sarcoma-like behavior despite a meningioma ancestry, these tumors may not be adequately captured by current meningioma grading schemes.

Leptomeningeal disease (LMD) or leptomeningeal metastases (LM) occurs when tumor cells invade the leptomeningeal and cerebrospinal fluid (CSF) space. Melanoma is the most common cancer causing LMD (M-LMD). Its biology and pathophysiology are very poorly understood. Research efforts are being made to explore the mechanisms underlying the immune landscape and antitumor response in LMD. The significant advances in immunocellular and targeted therapies in unresectable or metastatic melanoma in the last decade have not translated to patients with LMD and the prognosis for these patients remains dismal. While several reviews in LMD have provided an overview including challenges in diagnosis and treatment, herein we highlight evidence-based findings with a focus on immunocellular and radiation therapies, as well as the challenges that we need to overcome to help breach the existing gap in research and treatments in M-LMD. This could potentially guide additional research avenues and a better understanding of the needs and challenges of M-LMD to find novel therapies and improve patient survival.