Dosimetric comparison of TomoDirect, helical tomotherapy, VMAT, and ff-IMRT for upper thoracic esophageal carcinoma

Background: The new TomoDirect (TD) modality offers a nonrotational option with discrete beam angles. We aim to compare dosimetric parameters of TD, helical tomotherapy (HT), volumetric-modulated arc therapy (VMAT), and fixed-field intensity-modulated radiotherapy (ff-IMRT) for upper thoracic esophageal carcinoma (EC). Methods: Twenty patients with cT2-4N0-1M0 upper thoracic esophageal squamous cell carcinoma (ESCC) were enrolled. Four plans were generated using the same dose objectives for each patient: TD, HT, VMAT with a single arc, and ff-IMRT with 5 fields (5F). The prescribed doses were used to deliver 50.4 Gy/28F to the planning target volume (PTV50.4) and then provided a 9 Gy/5F boost to PTV59.4. Dose-volume histogram (DVH) statistics, dose uniformity, and dose homogeneity were analyzed to compare treatment plans. Results: For PTV59.4, the D-2, D-98, D-mean, and V-100% values in HT were significantly lower than other plans (all p < 0.05), and those in TD were significantly lower than VMAT and ff-IMRT (all p < 0.05). However, there was no significant difference in the D-2 and D-mean values between VMAT and ff-IMRT techniques (p > 0.05). The homogeneity index (HI) differed significantly for the 4 techniques of TD, HT, VMAT, and ff-IMRT (0.03 +/- 0.01, 0.02 +/- 0.01, 0.06 +/- 0.02, and 0.05 +/- 0.01, respectively; p < 0.001). The HI for TD was similar to HT (p = 0.166), and had statistically significant improvement compared to VMAT (p < 0.001) and ff-IMRT (p=0.003). In comparison with the 4 conformity indices (Cis), there was no significant difference (p > 0.05). For PTV50.4, the D-2 and D-mean values in HT were significantly lower than other plans (all p < 0.05), and those in TD were significantly lower than VMAT and ff-IMRT (all p < 0.05). However, there was no significant difference in the D-2 and D-mean values between VMAT and ff-IMRT techniques (p 0.05). No D-98 and V-100% parameters differed significantly among the 4 treatment types (p > 0.05). HT plans were provided for statistically significant improvement in HI (0.03 +/- 0.01) compared to TD plans (0.05 +/- 0.01, p=0.003), VMAT (0.08 +/- 0.03, p < 0.001), ff-IMRT (0.08 +/- 0.01, p < 0.001). The HI revealed that TD was superior to VMAT and ff-IMRT (p < 0.05). The CI differed significantly for the 4 techniques of TD, HT, VMAT, and ff-IMRT (0.59 +/- 0.10, 0.69 +/- 0.11, 0.64 +/- 0.09, and 0.64 +/- 0.11, respectively; p = 0.035). The best CI was yielded by HT. We found no significant difference for the V-5, V-10, V-15, V-30, and the mean lung dose (MLD) among the 4 techniques (all p > 0.05). However, the V-20 differed significantly among TD, HT, VMAT, and ff-IMRT (21.50 +/- 7.20%, 19.50 +/- 5.55%, 17.65 +/- 5.45%, and 16.35 +/- 5.70%, respectively; p = 0.047). Average V-20 for the lungs was significantly improved by the TD plans compared to VMAT (p = 0.047), and ff-IMRT (p = 0.008). The V-5 value of the lung in TD was 49.30 +/- 13.01%, lower than other plans, but there was no significant difference (p > 0.05). The D-1 of the spinal cord showed no significant difference among the 4 techniques (p = 0.056). Conclusions: All techniques are able to provide a homogeneous and highly conformal dose distribution. The TD technique is a good option for treating upper thoracic EC involvement. It could achieve optimal low dose to the lungs and spinal cord with acceptable PTV coverage. HT is a good option as it could achieve quality dose conformality and uniformity, while TD generated superior conformality. (C) 2018 The Authors. Published by Elsevier Inc. on behalf of American Association of Medical Dosimetrists.