Adaptive Radiation Therapy in Adults With Nasopharyngeal Carcinoma: A Retrospective Study at 1 Institution

Abstract

Purpose/Objective(s): Curative radiation therapy (RT) for locally advanced nasopharyngeal carcinoma (NPC) is based on the gross tumor volume (GTV), but the magnitude and timing of GTV changes during combined modality therapy remain unclear. This study analyzes GTV changes at phases of induction chemotherapy and sequential concurrent chemoradiation therapy (CRT) in patients with locally advanced NPC. Materials/Methods: Subjects included 13 patients with newly diagnosed stage III-IV NPC who underwent treatment between 2011 and 2014. Criteria for eligibility included 2 cycles of neoadjuvant chemotherapy, at least 5 cycles of concurrent chemotherapy and 3 magnetic resonance imaging (MRI) scans at specific phases of treatment (T0: before treatment, T1: postinduction, and T3: 3 months after CRT). The induction phase consisted of 2 cycles of gemcitabine and cisplatin. The CRT phase consisted of weekly cisplatin and RT delivered using volumetric modulated arc therapy (VMAT). The total dose was 70 Gy over 35 daily fractions administered 5 days/week. A subset of 3 patients received an additional MRI 4 to 5 weeks into CRT (T2). Primary gross tumor volume (GTVp) was defined as the GTV and adjacent involved retropharyngeal lymph nodes. Tumor volumes were delineated on gadolinium-enhanced fatsaturation T1 weighted MRIs by 2 observers. Mean values are reported +/one standard deviation. Results: Preliminary analysis included 6 (out of 13) subjects. The mean initial GTVp was 62.7 +/-32.8 mL. The mean GTVp response after induction phase was 21.4% +/-12.3% with a mean rate of volume change of 0.31 +/-0.19 mL/day which corresponded to a 0.56% +/-0.35% daily reduction in tumor volume. The total mean GTVp response after completion of treatment (T3) was 77.6% AE 21.6%. Subgroup analysis of subjects who underwent an additional MRI showed a mean GTVp reduction of 42.5% AE 22.6% and a mean rate of volume change of 0.87 AE 0.08 mL/day which corresponded to a 1.7% AE 0.93% daily reduction in tumor volume (from T1 to T2). Conclusion: Preliminary results suggest that the GTVp progressively diminishes following both induction chemotherapy and CRT. The mean GTVp response after 4 to 5 weeks of CRT exceeded the response observed after induction chemotherapy by a factor of 2. The mean rate of volume change at 4 to 5 weeks of CRT was threefold the rate seen during induction chemotherapy. These observations may support the optimal timing of imaging for replanning in the context of adaptive field RT. Analysis of the full NPC patient dataset is ongoing and will be reported.