Background and Purpose To analyze the recurrence pattern in relation to target volumes and 18F-fluorodeoxyglucose (FDG) uptake on positron emission tomography in head and neck squamous cell carcinoma (HNSCC) patients treated with definitive chemoradiation. Material and Methods 520 patients received radiotherapy for HNSCC from 2005–2009. Among 100 patients achieving complete clinical response and a later recurrence, 39 patients with 48 loco-regional failures had a recurrence CT scan before any salvage therapy. The estimated point of origin of each recurrence was transferred to the planning CT by deformable image co-registration. The recurrence position was then related to the delineated target volumes and iso-SUV-contours relative to the maximum standard uptake value (SUV). We defined the recurrence density as the total number of recurrences in a sub-volume divided by the sum of that volume for all patients. Results 54% (95% CI 37–69%) of recurrences originated inside the FDG-positive volume and 96% (95% CI 86–99%) in the high dose region. Recurrence density was significantly higher in the central target volumes (P<0.0001) and increased with increasing FDG avidity (P=0.036). Conclusions The detailed pattern-of-failure data analysis suggest that most recurrences occur in the FDG PET positive areas or the solid tumor.

Significant differences between methods were found which may affect strategies to improve radiotherapy based on pattern of failure analyses.

The interobserver delineation variation was very small in this systematic cross-sectional analysis, although significantly larger in the CC direction than in the transversal plane, stressing that anisotropic margins should be applied. This study is the first to make a systematic cross-sectional analysis of delineation variation for peripheral lung tumours referred for SBRT, establishing the evidence that interobserver variation is very small for these tumours.