A Dosimetric Analysis of Locoregional Failure Using Deformable Image Registration in Hypopharyngeal Cancer After Sequential-Boost Intensity-Modulated Radiotherapy

Abstract

<h3>Purpose/Objective(s)</h3> Sequential boost (SQB) is an IMRT approach that is considered comparable to simultaneous-integrated boost. Dosimetric analysis in SQB using two planning CTs requires advanced dose-accumulation techniques, such as a deformable image registration (DIR) workflow. The purpose of this study was to assess patterns of failure and identify dosimetric features for hypopharyngeal cancer patients treated with SQB-IMRT. <h3>Materials/Methods</h3> Between 2013 and 2019, 102 hypopharyngeal cancer patients were treated with definitive SQB-IMRT. In short, both the PTV-high risk, including the primary tumor and metastatic lymph nodes, and the PTV-low risk, including elective nodal regions, received 46 Gy at 2 Gy/fraction using the first planning CT. The PTV-high risk sequentially received an additional 24 Gy using a second planning CT. The doses for the first and second plans were accumulated and registered onto the second planning CT using the DIR workflow. All locoregional recurrences were contoured on the follow-up CT and registered with the second planning CT. Recurrences were classified as in-field (more than 95% of the recurrence volume (Vrec) received 95% of the prescribed dose), marginal (20-95%), or out-of-field (less than 20%). The toxicity was also evaluated using CTCAE ver4.0. <h3>Results</h3> The median age was 66 years. Stage I, II, III and IV diseases were diagnosed in 3, 23, 20 and 56 patients, respectively. Eighty-three patients (81%) received concurrent chemoradiotherapy, and 19 patients (19%) received radiation therapy alone. The median total dose was 70 Gy. The median follow-up period was 22 months for all patients and 24 months for survivors. The 2-year overall survival rate and locoregional control rate were 79% and 57%, respectively. Thirty-four (33%) locoregional recurrences were observed. All locoregional failures were defined as in-field, and there was no marginal or out-of-field recurrence. The median mean dose and of the Vrec were 72.5 Gy (range, 71.6-74.1). In the patients who experienced locoregional failure, the median mean dose to the elective nodal region was assessed to be 60.1 (range, 55.4-68.4) Gy, and no recurrence occurred in the area. Late toxicities 6 months after RT were evaluated in 34 patients, and 24%, 3%, 25% and 2% of the patients experienced grade 2 and 3 dysphagia, grade 2 dry mouth and G2 dysgeusia, respectively. There were no grade 4/5 late toxicities. <h3>Conclusion</h3> In the detailed dosimetric analysis using DIR, all locoregional relapses after SQB-IMRT were in-field recurrences in the high-dose region. There was no recurrence in the prophylactic dose area, in which the accumulated dose reached as high as 60 Gy. More aggressive treatment for gross tumors and lower prophylactic doses may contribute to improved treatment outcomes and reduced side effects.