Exploratory Study of Low-Dose Whole-Brain Radiation Therapy With HeLaXON 1X in Patients With Mild Alzheimer's Disease Dementia

Inclusion Criteria: 1. Male or female participants aged 60 to 85 years, inclusive. 2. Mild dementia (Stage 4) according to the NIA-AA (New Diagnostic Criteria for Alzheimer's Disease) criteria. 3. At screening, participants must meet all of the following criteria: * K-MMSE-2 score of 20 to 26, inclusive. * CDR Global Score of 1, or CDR Global Score of 0.5 or 1 with a CDR-SB score of 4.5 to 9.0, inclusive. * Memory Box Score of at least 0.5. 4. If receiving a cholinesterase inhibitor (donepezil, galantamine, or rivastigmine) for symptomatic treatment, the participant must have been maintained on a stable dose for at least 12 weeks before the screening visit. 5. Positive cerebral amyloid accumulation confirmed by amyloid PET. 6. Able to undergo the cognitive assessments and imaging examinations required by the study. 7. Has a caregiver who can provide information regarding the participant's overall condition and changes in cognitive and functional status. 8. Written informed consent for study participation must be obtained from a legally authorized representative. If, based on the investigator's assessment of decision-making capacity, the participant is considered able to understand the purpose and procedures of the study and voluntarily express his or her wishes, written informed consent will also be obtained from the participant. Exclusion Criteria: 1\. History of prior radiation therapy to the brain. 2. History of seizure within 10 years before the screening date. 4. Presence of a scalp skin disorder. 5. History of hypersensitivity to Vizamyl™ (flutemetamol F 18 injection) or any of its major excipients, including polysorbate 80. Contraindication to MRI, including but not limited to a pacemaker, metallic implant, or severe claustrophobia that prevents MRI examination. 6\. Diagnosis of a malignant tumor within 5 years before the screening visit or currently receiving treatment for a malignancy. 7\. Pregnant or breastfeeding. 8. Previous treatment with an anti-amyloid monoclonal antibody without an interval of at least 5 half-lives before the screening visit. 9\. Cognitive impairment attributable to a condition other than Alzheimer's disease, including drug-related causes or other neurological or neurodegenerative conditions such as substance abuse, vitamin B12 deficiency, thyroid dysfunction, stroke or other cerebrovascular disease, Lewy body dementia, frontotemporal dementia, or traumatic brain injury. 10\. Clinically significant unstable psychiatric disorder, including uncontrolled depression, schizophrenia, or bipolar disorder, diagnosed within 6 months before screening. 11\. Any of the following findings on brain MRI: * Acute or subacute hemorrhage. * Prior macrohemorrhage greater than 1 cm in diameter on T2-weighted imaging, or prior subarachnoid hemorrhage, unless it can be documented that the finding is not due to an underlying structural or vascular abnormality and does not suggest a risk of recurrent hemorrhage. * More than 4 microhemorrhages. * Cortical infarct greater than 1.5 cm in diameter, irrespective of anatomical location. * More than 1 lacunar infarct greater than 1.5 cm in diameter. * Superficial siderosis. * Diffuse white matter disease defined as a score of 3 on the age-related white matter changes scale. * Any MRI finding that, in the opinion of the investigator, may contribute to the participant's dementia, pose a risk to the participant, or prevent satisfactory MRI assessment for safety monitoring. 12\. Participation in another interventional clinical trial at screening with receipt of an investigational drug or application of an investigational medical device. 13\. Any other clinically significant finding that, in the investigator's judgment, makes the participant unsuitable for study participation. 14\. Unwilling or unable to use at least one medically acceptable method of contraception throughout the study.