PSMA PET/CT-Guided SBRT Plus Darolutamide in mHSPC

Inclusion Criteria: * Male participants aged ≥18 years and \<85 years. * Histologically confirmed prostate adenocarcinoma. * No neuroendocrine carcinoma, ductal adenocarcinoma, small-cell carcinoma, signet-ring cell carcinoma, or sarcomatoid carcinoma component. * Metastatic prostate cancer confirmed by imaging and/or pathological evidence. * Baseline PSMA PET/CT assessment demonstrates a total of ≤20 metastatic lesions. * No prior radical prostatectomy or radiotherapy for prostate cancer. * No prior systemic anti-tumor therapy that may affect the efficacy assessment of this study, including androgen receptor signaling inhibitors (ARSIs; e.g., abiraterone, enzalutamide, apalutamide, rezvilutamide, or darolutamide), chemotherapy, PARP inhibitors, radionuclide therapy, or other such treatments. * Short-term androgen deprivation therapy (ADT) and/or first-generation antiandrogen therapy prior to enrollment is permitted, provided that the interval from the first initiation of any such treatment to the date of enrollment does not exceed 3 months (≤12 weeks). * Expected survival \>12 months. * Able to understand the study and voluntarily sign written informed consent. * Able and willing to comply with study visits and protocol procedures. * Willing to provide tumor tissue, blood, and other biological samples as required by the study protocol. * Absolute neutrophil count ≥1.5 × 10\^9/L. * Platelet count ≥100 × 10\^9/L. * Hemoglobin ≥90 g/L. * Total bilirubin ≤1.5 × upper limit of normal. * Alanine aminotransferase and aspartate aminotransferase ≤2.5 × upper limit of normal. * Serum albumin ≥20 g/L. * Serum creatinine ≤1.5 × upper limit of normal or creatinine clearance ≥50 mL/min. * Eastern Cooperative Oncology Group performance status ≤2. Exclusion Criteria: * Current or prior history of another primary malignancy. * History of another malignancy within 3 years that differs from the study cancer in primary site or histology. * History of papillary thyroid carcinoma is allowed if it is well controlled. * History of basal cell carcinoma of the skin is allowed if it is well controlled. * History of squamous cell carcinoma of the skin is allowed if it is well controlled. * History of cervical carcinoma in situ is allowed if it is well controlled. * Prior radical prostatectomy. * Prior external beam radiotherapy. * Prior radical or ablative local therapy for prostate cancer. * Prior treatment with an androgen receptor signaling inhibitor, including abiraterone, enzalutamide, apalutamide, rezvilutamide, or darolutamide. * Prior chemotherapy for prostate cancer. * Major surgery within 4 weeks before enrollment. * Serious trauma within 4 weeks before enrollment. * Contraindication to radiotherapy. * Spinal cord compression. * Active enteritis. * Severe pelvic infection. * Inability to maintain the required body position for radiotherapy. * History of allergy to PET/CT tracer. * Nuclear medicine assessment showing super bone imaging. * Marked discordance between PSMA PET/CT and FDG PET/CT findings. * Disease progression during the 6-month run-in period of androgen deprivation therapy plus darolutamide before randomization. * Unacceptable toxicity during the 6-month run-in period of androgen deprivation therapy plus darolutamide before randomization. * No active tumor lesion on PSMA PET/CT after the 6-month run-in period. * Allergy to any component of the study treatment. * Active or poorly controlled serious infection. * Human immunodeficiency virus infection. * Acute or chronic active hepatitis B infection, defined as positive hepatitis B surface antigen with hepatitis B virus DNA \>1 × 10\^3/mL. * Acute or chronic active hepatitis C infection, defined as positive hepatitis C virus antibody with hepatitis C virus RNA \>15 IU/mL. * Active pulmonary tuberculosis. * Other serious infectious disease. * New York Heart Association class III or IV congestive heart failure. * Persistent symptomatic arrhythmia. * Uncontrolled atrial fibrillation. * Left ventricular ejection fraction below the lower limit of normal on repeated echocardiographic assessments. * Uncontrolled hypertension, defined as systolic blood pressure ≥160 mmHg or diastolic blood pressure ≥100 mmHg. * Arterial thrombotic, embolic, or ischemic event within 6 months before enrollment. * Myocardial infarction within 6 months before enrollment. * Unstable angina within 6 months before enrollment. * Cerebrovascular accident within 6 months before enrollment. * Transient ischemic attack within 6 months before enrollment. * Medical condition requiring warfarin or coumarin anticoagulation therapy. * Uncontrolled hypercalcemia, defined as ionized calcium \>1.5 mmol/L, total calcium \>12 mg/dL, or corrected serum calcium above the upper limit of normal. * Symptomatic hypercalcemia requiring continuous bisphosphonate therapy. * Uncontrolled adrenal insufficiency. * History of abdominal fistula within 6 months before enrollment. * History of gastrointestinal perforation within 6 months before enrollment. * History of intra-abdominal abscess within 6 months before enrollment. * Severe non-healing wound. * Severe non-healing ulcer. * Gastrointestinal disease that may impair absorption. * Active peptic ulcer disease. * Uncontrolled nausea. * Uncontrolled vomiting. * Uncontrolled diarrhea. * History of small bowel resection that may impair drug absorption. * Other acute or chronic disease, psychiatric disorder, or laboratory abnormality that may increase the risk associated with study participation or study treatment. * Other acute or chronic disease, psychiatric disorder, or laboratory abnormality that may interfere with interpretation of study results. * Any other medical or psychological condition that, in the investigator's judgment, may affect participant safety, compliance, or study integrity. * Any condition that, in the investigator's judgment, makes the participant unsuitable for this study.