Amyloid PET Guidance for Atypical Dementia Presentations

Imaging Findings and Diagnostic Uncertainty

Frontotemporal cortical atrophy is atypical for Alzheimer’s disease, which more commonly shows medial‑temporal (hippocampal) and posterior‑parietal volume loss; this pattern raises suspicion for frontotemporal dementia or mixed pathology. 1
Co‑existing small‑vessel ischemic white‑matter changes may represent vascular contributions, mixed dementia, or age‑related changes that can confound clinical interpretation. 3

The 2024 Society of Nuclear Medicine guideline recommends amyloid PET for patients with dementia who have atypical clinical features (e.g., non‑amnestic presentation, unusual progression, or mixed etiologies). This is a guideline‑level recommendation. 5
Patients with prominent frontotemporal atrophy are identified as a subgroup that particularly benefits from amyloid PET because typical AD‑related MRI patterns are absent. 7

In cohorts with uncertain dementia etiology, amyloid PET leads to a change in diagnosis in 25 %–67 % of cases, increasing diagnostic confidence and often altering management plans. 1
A positive amyloid PET confirms the presence of AD pathology (which may coexist with vascular or frontotemporal changes) and therefore justifies initiation of donepezil and makes the patient eligible for anti‑amyloid monoclonal antibodies (e.g., lecanemab, donanemab) when other criteria are met. 1
A negative amyloid PET effectively excludes AD as the primary driver of cognitive decline, prompting redirection of the work‑up toward frontotemporal dementia, primary progressive aphasia, or other non‑AD dementias. 3
Positive amyloid PET also provides prognostic information for patients and families regarding disease trajectory. 4

Step 1 – Perform amyloid PET: The scan should be ordered promptly in the setting of atypical presentation; interpretation must be performed by a dementia specialist who considers pre‑test probability, age, and clinical context. 9
Step 2 – Interpret results: Recognize that asymptomatic amyloid deposition is common in adults > 75 years and may not fully explain symptoms; integrate PET findings with MRI atrophy patterns and vascular imaging. 3
Step 3 – Treatment decision:
- If PET is positive → start donepezil (FDA‑approved for AD) and evaluate eligibility for disease‑modifying anti‑amyloid therapies.
- If PET is negative → avoid donepezil (risk of harm) and consider further imaging such as FDG‑PET to differentiate frontotemporal from other dementias. 1

Small‑vessel disease identified on MRI should trigger aggressive vascular‑risk‑factor management regardless of amyloid status, as it may independently contribute to cognitive decline. 4

Do not initiate donepezil until amyloid PET results are available; the predominant frontotemporal atrophy creates sufficient diagnostic uncertainty that premature cholinesterase‑inhibitor therapy (approved only for AD) is not justified when definitive biomarker testing is pending. PET results will either confirm AD (supporting donepezil and possible anti‑amyloid therapy) or exclude AD (guiding alternative diagnostic and therapeutic pathways). 6