Diagnostic and Therapeutic Impact of FDG‐PET/CT Following MRI Staging in Anal Cancer: A Systematic Review and Meta‐Analysis

INTRODUCTION: Accurate staging is essential in anal cancer to guide therapy and prognostication. While MRI remains the modality of choice for local staging, its limitations in assessing nodal and distant metastases have prompted evaluation of FDG PET/CT as an adjunct. The American College of Radiology recommends FDG-PET/CT as a complementary modality for initial staging, particularly for nodal assessment. METHODS: A systematic search of PubMed, EMBASE, Web of Science and Scopus was conducted up to August 2025 following PRISMA guidelines (PROSPERO ID: CRD1149778). Studies included reported adult patients with biopsy-proven anal squamous cell carcinoma who underwent both MRI and FDG-PET/CT for initial staging. Primary outcomes included per-patient sensitivity/specificity for metastasis, changes in TNM staging and therapeutic outcomes, including management modification. RESULTS: Six studies (n = 246) met the inclusion criteria. Five studies reported on staging changes, where FDG-PET/CT altered staging in 22.5% (95% CI: 12.3-34.7) of patients, more commonly through upstaging than downstaging (16.2% [95% CI: 10.7-22.5] vs. 6.3% [95% CI: 1.5-14.2]). Upstaged patients were predominantly nodal (74.6% [95% CI: 63.2-83.1]). Previously occult metastases were identified with FDG PET/CT in 3% (95% CI: 1.1-6.9) of patients. Management changes occurred in 20.7% (95% CI: 14.9-27.4), predominantly through radiotherapy field expansion or dose modifications. CONCLUSION: FDG-PET/CT following MRI provides incremental diagnostic and therapeutic value in anal cancer staging, through refining nodal and metastatic staging and influencing radiotherapy planning, supporting its routine integration to optimise staging accuracy and management decisions. TRIAL REGISTRATION: PROSPERO: CRD42023446290.