COMPARATIVE ANALYSIS OF [¹⁸F]-FDG PET-CT AND CONTRAST-ENHANCED CT FOR TARGET VOLUME DELINEATION IN RADIOTHERAPY PLANNING OF HEAD AND NECK CANCERS

Background: Target volume delineation in radiotherapy planning (RTP) for head and neck cancers is conventionally based on contrast-enhanced computed tomography (CECT). [¹⁸F]-FDG PET-CT adds functional (metabolic) information to anatomical imaging and may alter both the delineated tumour volume and the clinical stage. The objective is to compare and quantify the gross tumour volume (GTV) delineated on [¹⁸F]-FDG PET-CT with that delineated on CECT in head and neck cancers, and to assess the impact of PET-CT on TNM staging and RT planning. Materials and Methods: In this prospective observational study, 52 patients with biopsy-proven or clinically suspected head and neck carcinoma planned for radiotherapy (15 nasopharyngeal, 23 oropharyngeal/oral cavity, 14 laryngopharyngeal/laryngeal) underwent simulation CECT followed by PET-CT in the treatment position. Primary tumour and nodal volumes were contoured separately on CECT and on PET-CT (qualitative visual method) using a treatment planning system, and GTVs were compared using paired-sample statistics. Results: Of 52 patients (37 men, 15 women; mean age 58 ± 13.3 years), mean primary tumour volume was significantly smaller on PET-CT than CECT (22.50 ± 17.42 cc vs 32.39 ± 24.70 cc; mean difference 9.88 ± 10.73 cc, p < 0.001), and mean nodal volume was likewise smaller on PET-CT (15.49 ± 9.72 cc vs 18.08 ± 10.01 cc; mean difference 2.59 ± 2.01 cc, p < 0.001). The reduction was more pronounced for nasopharyngeal tumours (mean difference 13.41 cc) than for oropharyngeal/oral cavity (8.43 cc) or laryngopharyngeal/laryngeal tumours (8.49 cc). PET-CT changed TNM stage in 22 of 52 patients (42.3%), including T-downstaging in 12 patients (23.1%), N-upstaging in 12 patients (23.1%; recorded within the same 22), and new detection of distant metastases in 3 patients (5.8%). Conclusion: [18F]-FDG PET-CT-based contouring yields significantly smaller, more metabolically accurate primary and nodal GTVs than CECT-based contouring, and meaningfully alters TNM stage and management in a substantial minority of patients. Incorporating PET-CT into RT planning may allow more precise dose delivery to tumour while sparing adjacent organs at risk.