The effect of induction chemotherapy on tumor volume and organ-at-risk doses in patients with locally advanced oropharyngeal cancer

Background and purpose: To retrospectively report changes in gross tumor volume (GTV) and organ-at-risk (OAR) doses after induction chemotherapy (IC) in oropharyngeal cancer using different contouring strategies. Materials and methods: GTV and OARs were delineated on pre- and post-IC planning CT. Two post-IC GTV contours were made: (1) a 'consensus set' using published guidelines (GTVconsensus), and (2) 'visible set', delineating only visible post-IC GTV (GTVvisible). Pre-IC interactively optimized volumetric modulated arc therapy plans were generated. The pre-IC planning constraints served as the starting point for both post-IC plans. Results reflect pooled data from all 10 patients. Results: Mean reduction in volume post-IC was 24% and 47% for consensus and visible primary tumor and 57% and 60% for consensus and visible nodes. Compared to pre-IC plans, average mean OAR dose for post-IC GTVconsensus plans was significantly lower for CL parotid. For GTVvisible plans both parotids, upper/lower larynx, inferior pharyngeal constrictor and cricopharyngeal muscles were significantly lower. However reductions compared with post-IC GTVconsensus plans were modest (1.6/1.5/1.2/3.7/5.9/2.6 Gy, respectively). Conclusion: IC in patients with oropharyngeal carcinoma results in substantial reductions in GTVs. If post-IC GTVs are used, which is contrary to current consensus, statistically significant but relatively small OAR dose reductions are observed