The Tolerance of Brainstem in Reirradiation With Intensity Modulated Radiation Therapy in Recurrent Nasopharyngeal Carcinoma

Abstract

To explore the tolerance of brainstem in reirradiation with IMRT and to show a method of acquiring actual cumulative dose of reirradiation. Materials/Methods: We retrospectively reviewed the archives of 448 patients with locally advanced NPC who underwent IMRT in our hospital from March 2005 to April 2012. Fifteen patients who received reirradiation were included in this study. Acquisition of cumulative dose of the brainstem: 1) Define a series of new OARs in the reirradiation plan, such as brainstem35Gy, brainstem40Gy, brainstem45Gy, which means the parts of the brainstem within the isodose curves of 35Gy, 40Gy, 45Gy of initial treatment plan, respectively. 2) Collect the dosimetric information of new OARs of brainstem in the reirradiation plan. 3) The equivalent dose in 2 Gy fractions (EQD 2) was calculated according to the LQ model. a/b value Z 2.5Gy. 4) Cumulative EQD 2 was additive of the EQD 2 of isodose curve in the initial irradiation and the EQD 2 in the reirradiation. The final cumulative dose was the maximum value of the new OARs of brainstem. Multiple dosimetric parameters were acquired, including D max , D0.1cc, D 0.5cc , D 1.0cc. Severe late toxicity was defined as grade 3 brainstemassociated complication according to CTCAE 4.0. A software package was used for statistical analysis. Results: The median age was 51.0 (range, 30-71) years. The median time to reirradiation was 20.8 (range, 12.6-49.5) months. The median follow-up time after reirradiation was 14.0 (range, 4.2-29.0) months. In addition, no grade 3 brainstem-associated toxicity has been observed. We put the initial dose and reirradiation dose into scatter plots. Patients who had received initial radiation with D max of 40Gy in the brainstem can at least tolerate reirradiation with D max of 39Gy (11/14 cases). Patients with initial D 0.1cc of 40Gy in the brainstem can at least tolerate reirradiation with D 0.1cc of 34Gy (10/12 cases). Patients with initial D 0.5cc of 39Gy can at least tolerate reirradiation with D 0.5cc of 28Gy (7/8 cases). Patients with initial D 1.0cc of 38Gy can at least tolerate reirradiation with D 1.0cc of 29Gy (5/6 cases). We put the cumulative dose and interval to reirradiation into scatter plots and found that 11/15 (73.3%) cases can tolerate cumulative D max of 79Gy, D 0.1cc of 71Gy, D 0.5cc of 65Gy, and D 1.0cc of 60Gy. In addition, when interval to reirradiation increased to more than 17 months, 9/11 (81.8%) cases can tolerate the cumulative dose (D max 79Gy, D 0.1cc 71Gy, D 0.5cc 65Gy, and D 1.0cc 60Gy) in brainstem. Conclusion: With conservative estimation, when the interval was more than 12 months, patients who had received initial D max of 40Gy in the brainstem can tolerate reirradiation with D max of 39Gy. Brainstem can tolerate cumulative D max of 79Gy, D 0.1cc of 71Gy, D 0.5cc of 65Gy, and D 1.0cc of 60Gy. Method in this study would be a precise one for cumulative dose acquisition.