Evaluation of the role of 18FDG-PET/CT in radiotherapy target definition in patients with head and neck cancer

Abstract

Background and purpose. As techniques for radiotherapy delivery have developed, increasingly accurate localisation of disease is demanded. Functional imaging, particularly PET and its fusion with anatomical modalities, such as PET/CT, promises to improve detection and characterisation of disease. This study evaluated the impact of ¹⁸FDG-PET/CT on radiotherapy target volume definition in head and neck cancer (HNC). Materials and methods. The PET/CT scans of patients with HNC were used in a radiotherapy planning (RTP) study. The gross tumour volume (GTV), clinical target volume (CTV) and planning target volume (PTV) were defined conventionally and compared to those defined using the PET/CT. Data were reported as the median value with 95% confidence intervals. Results. Eighteen patients were consented, 9 had known primary tumour site, 9 presented as unknown primary. In nine cases where the primary site was known, the combined primary and nodal GTV (GTVp+n) increased by a median of 6.1cm³ (2.6, 12.2) or 78% (18, 313), p=0.008 with CTV increasing by a median of 10.1cm³ (1.3, 30.6) or 4% (0, 13) p=0.012. In 9 cases of unknown primary the GTVp+n increased by a median 6.3cm³ (0.2, 15.7) or 61% (4, 210), p=0.012, with CTV increasing by a median 155.4cm³ (2.7, 281.7) or 95% (1, 137), p=0.008. Conclusion. ¹⁸FDG-PET revealed disease lying outside the conventional target volume, either extending a known area or highlighting a previously unknown area of disease, including the primary tumour in 5 cases. We recommend PET/CT in the RTP of all cases of unknown primary. In patients with a known primary, although the change in volume was statistically significant the clinical impact is less clear. ¹⁸FDG-PET can also show areas within the conventional target volume that are hypermetabolic which may be possible biological target volumes for dose escalation studies in the future.