Inadequate target volume delineation and local-regional recurrence after intensity-modulated radiotherapy for human papillomavirus-positive oropharynx cancer.

PURPOSE: To perform a spatial analysis of local-regional recurrences, in relation to quantitative dose distribution, among patients treated by intensity-modulated radiotherapy (IMRT) for human papillomavirus (HPV)-positive oropharyngeal cancer. METHODS AND MATERIALS: The records of 107 consecutive patients who presented for consideration of re-irradiation for local-regional recurrent disease occurring in a previously irradiated field were reviewed. The original IMRT plans were retrieved for those with HPV-positive disease originating from the oropharynx, and deformable image registration was used to fuse the magnetic resonance imaging (MRI) and positron emission tomography (PET) scans obtained at recurrence to the pre-treatment planning computed tomography (CT) dataset. The recurrent tumor volume (V(recur)) was subsequently identified on axial imaging, and the dose of radiation received by V(recur) was then calculated and analyzed using dose-volume histograms. RESULTS: A total of 83 recurrent lesions occurring in 50 oropharyngeal cancer patients were HPV-positive and met inclusion criteria. Using PET-defined V(recur), thirty-three lesions were classified as in-field recurrences (40%), 35 were marginal misses (41%), and 15 were true misses (18%). Using the MRI-defined V(recur), thirty-seven lesions were classified as in-field recurrences (45%), 32 were marginal misses (39%), and 14 were true misses (17%). CONCLUSION: A significant proportion of local-regional recurrences from HPV-positive oropharyngeal cancer represented geographical misses which possibly could have been prevented with more meticulous attention to IMRT planning. This finding has important implications with respect to ongoing attempts to de-escalate radiation dose for this disease. Our data highlight the importance of robust quality assurance with careful review of target volumes prior to the initiation of IMRT.