The goal of this clinical trial is to evaluate the efficacy and safety of a four-drug combination regimen as first-line treatment for adults aged 18 years and older with newly diagnosed primary central nervous system lymphoma (PCNSL). The main questions it aims to answer are:

Does the combination of pirtobrutinib, sintilimab, rituximab, and high-dose methotrexate achieve a higher complete response rate than standard treatment for newly diagnosed PCNSL? What is the safety and tolerability profile of this four-drug combination regimen? Researchers will compare the experimental four-drug combination to investigator-selected standard-of-care regimens (all based on high-dose methotrexate) to see if the experimental regimen improves complete response rate, progression-free survival, and overall survival while maintaining an acceptable safety profile.

Participants will:

Be assigned to either the experimental group or the standard treatment group based on their personal preference Receive 6 cycles of induction therapy (21 days per cycle) with their assigned treatment regimen Undergo regular clinical assessments, including contrast-enhanced brain MRI scans, blood tests, and cerebrospinal fluid examinations Complete the EORTC QLQ-C30 quality-of-life questionnaire at baseline, mid-treatment, end of treatment, and follow-up visits Receive optional consolidation or maintenance therapy based on their response to induction treatment Be followed for up to 2 years after completing treatment to monitor for disease progression and long-term outcomes

Primary Central Nervous System Lymphoma (PCNSL) is a rare extranodal non-Hodgkin lymphoma with poor prognosis, characterized by MYD88 L265P/CD79B mutations and PD-L1/PD-L2 overexpression. Current first-line therapies based on high-dose methotrexate (HD-MTX) have limitations including high recurrence rates, poor blood-brain barrier penetration, and significant toxicity. Pirtobrutinib, a highly selective reversible BTK inhibitor, exhibits superior CNS penetration and safety profiles compared to covalent BTK inhibitors. Sintilimab (anti-PD-1) enhances anti-tumor immunity by blocking PD-1/PD-L1 axis. This study evaluates the efficacy and safety of the quadruple combination (methotrexate+rituximab + sintilimab + pirtobrutinib ) in treatment-naive PCNSL, with a concurrent control cohort providing comparative evidence.

Inclusion Criteria:

1. Age >= 18 years.
2. Voluntarily signed informed consent.
3. ECOG Performance Status 0-3.
4. Expected survival > 3 months.
5. Histopathologically confirmed Diffuse Large B-Cell Lymphoma (DLBCL) restricted to the CNS or eyes (PCNSL).
6. Measurable lesion on contrast-enhanced MRI (>10x10 mm) or positive CSF cytology for leptomeningeal disease.
7. No prior systemic treatment for lymphoma (corticosteroids excepted).
8. Adequate bone marrow and organ function (ANC >=1.5x10^9/L, PLT >=80x10^9/L, Hb >=80 g/L; Bilirubin <=1.5xULN, AST/ALT <=2.5xULN; Creatinine <=1.5xULN or CrCl >=60 mL/min) .
9. Stable controlled comorbidities allowed (e.g., hypertension with blood pressure <=160/100 mmHg, type 2 diabetes with HbA1c <=8%, mild coronary heart disease without myocardial infarction in the past 6 months).
10. Basic communication ability to complete PROs questionnaires (no severe cognitive impairment).
11. Reproductive-aged females and males with childbearing potential: No pregnancy plans during the study and 3 months after treatment discontinuation; use effective contraception (abstinence, physical contraception, or hormonal contraceptives initiated >=3 months before first dose). Males prohibited from donating sperm during treatment and 3 months after discontinuation.
12. For Observational Cohort (Palliative Care Subgroup only): Pathologically confirmed DLBCL restricted to the CNS or eyes; Follow-up available for efficacy assessment (at least one CR evaluation) .

Exclusion Criteria:

1.Prior treatment with PD-1/PD-L1 inhibitors or CTLA4 monoclonal antibodies. Uncontrolled active infection. 2.Uncontrolled or significant cardiovascular diseases: 3.Congestive heart failure (NYHA class III/IV),

1. myocardial infarction, unstable angina within 6 months before first dose; arrhythmia requiring treatment; LVEF <50%.
2. Primary cardiomyopathy.
3. History of clinically significant QTc prolongation, second-degree type II/third-degree atrioventricular block, or QTc interval (Fridericia method) >470 msec (females) / >480 msec (males).
4. Atrial fibrillation (EHRA grade ≥2b).
5. Refractory hypertension. 4.Active hepatitis B/C infection (HBV-DNA ≥ detection limit, HCV RNA positive) or syphilis. (Exceptions: HBV-DNA < detection limit, cured HCV).

5.HIV infection. 6.Prior organ transplantation or allogeneic stem cell transplantation. 7.Pregnant or lactating females. 8.Prior/current pulmonary fibrosis, interstitial pneumonia, pneumoconiosis, or radiation pneumonitis (unsuitable for study per investigator).

9.Autoimmune diseases requiring systemic treatment within 2 years. 10.For Observational Cohort (Palliative Care Subgroup only): Incomplete clinical data (e.g., no pathological report, inability to perform MRI/PET-CT assessment).

While the participants and clinical investigators are aware of the treatment assignment (Open Label), Independent radiologists (from the central imaging group) will be completely blinded to patient grouping, clinical judgment, and laboratory results when evaluating the primary endpoint (Complete Remission).