Diagnostic Accuracy of PET/CT and Diffusion-weighted MRI in Detecting Residual Oropharyngeal Carcinoma after Chemoradiotherapy.

Abstract

Purpose To compare diffusion-weighted (DWI) MRI and PET/CT for diagnosing local-regional residual disease after curative-intent chemoradiation therapy (CRT) in oropharyngeal squamous cell carcinoma (OPSCC), including evaluation of DWI for clarifying equivocal PET/CT findings. Materials and Methods In this prospective study, consecutive participants with OPSCC treated with curative-intent CRT were enrolled between October 2018 and September 2021. DWI was added to the routine PET/CT protocol 3-3.5 months after treatment for local-regional residual disease detection. Reference standards were histopathologic confirmation or unequivocal progression or resolution at follow-up imaging. During qualitative evaluation, imaging findings were classified as negative, equivocal, or positive for residual disease; equivocal findings were considered positive for analysis. Sensitivity, specificity, positive predictive value, negative predictive value, and accuracy were calculated, with differences between modalities assessed using the McNemar test. As a secondary analysis, a sequential imaging strategy using PET/CT and DWI was evaluated. Results A total of 95 participants (mean ± SD age, 61.3 years ± 9.3; 72 male, 85 p16-positive) were included, of whom eight (8.4%) had local-regional residual disease. Sensitivity and negative predictive value for local-regional residual disease detection were 100% for both PET/CT and DWI (eight of eight and 61 of 61 at PET/CT; eight of eight and 72 of 72 at DWI). DWI demonstrated higher specificity (83% [72 of 87] vs 70% [61 of 87]; P < .05) and positive predictive value (35% [eight of 23] vs 24% [eight of 34]; P < .05) than PET/CT. In the sequential imaging analysis, DWI resolved 14 of 34 positive or equivocal PET/CT findings, whereas PET/CT was negative in three of 23 positive or equivocal DWI cases. Conclusion Both PET/CT and DWI demonstrated excellent sensitivity for detecting local-regional residual disease after CRT in OPSCC, as no residual primary tumors or nodal metastases were missed by either modality. DWI showed higher specificity and positive predictive value than PET/CT and demonstrated potential to clarify equivocal PET/CT findings. Keywords: PET/CT, MR-Functional Imaging, MR-Diffusion Weighted Imaging, Head/Neck, Comparative Studies Supplemental material is available for this article. © RSNA, 2026.