Dosimetric Evaluation of Two-step Method as Adaptive Replanning Strategies for Head and Neck IMRT

Abstract

To avoid the risk of changes in the dose distribution during IMRT, we adopted a two-step IMRT method for head and neck cancers. The aim of this study was to show the benefit of this method by examining the geometric and dosimetric changes along the treatment course. We retrospectively selected 20 patients (10 nasopharynx ; 6 oropharynx ; 4 hypopharynx) treated with the two-step IMRT combined with cisplatin (80 mg/m2,q3w). In an initial step, a dose of 46-50 Gy was delivered to the whole neck, followed by a boost to the high-risk clinical target volume to a total of 70 Gy/2Gy. First, the geometric changes were evaluated in the primary tumors (GTV-p), the largest metastatic lymph node (GTV-n), and the parotid glands between CT-1 for the initial and CT-2 for the boost plans. Second, initial plans were applied to CT-2, and dose distributions were recalculated. These transferred plans were compared with the corresponding initial plans for the dosimetric parameters to targets and risk organs. The mean volumes of GTV-p and GTV-n on CT-2 were decreased by 62.9% and 52.4% of those on CT-1, respectively (p = 0.002 and p = 0.081), whereas those of parotid glands was also decreased by 18.1% with significant difference (p < 0.0001). In addition, the lateral faces of parotid glands shifted medially with an average of 4.2 mm (p < 0.0001) on CT-2. Despite of anatomical changes of GTV-p and GTV-n , D98 was unchanged. The mean dose of parotid glands in initial, transferred and boost plans were 25.2Gy, 30.5Gy and 20.0Gy per 70Gy, respectively. D2 of the spinal cord were 37.1Gy, 39.2Gy and 36.8Gy per 70Gy, respectively. The mean dose of parotid glands significantly reduced in boost plans compared with transferred plans. D2 of the spinal cord reduced in boost plans compared with transferred plans. Grade of xerostomia 1-2 years after the start of IMRT could be evaluated for 15 patients, and 10 patients were grade 0 and 5 patients were grade 1. The two-step method can provide preservation of both target coverage and dose constrains for risk organs. Especially the benefit of replanning is improved sparing of the parotid glands.