Carotid sparing hypofractionated tomotherapy in early glottic cancers: Refining image guided IMRT to improve morbidity

Objective: Carotid artery damage has been reported secondary to radiotherapy. We report the feasibility of implementing hypofractionated laryngeal irradiation using carotid sparing tomotherapy (HT) and analyze the image guidance (IG) policy. Materials and Methods: Five patients with early glottic cancer (EGC) had radiotherapy using 3D conformal technique (conf) while repeat treatment plans were produced with helical tomotherapy using carotid sparing techniques (cstomo). Inverse and forward planned dose volume histograms were analyzed. Three hundred and sixty four daily images of 14 patients having daily Megavoltage head and neck CT imaging prior to irradiation were analyzed to assess errors. Results: There was no significant difference in the maximum and mean dose to the PTV (P = 0.058, 0.66). The left / right carotid median doses were significantly less in the cstomo plans as compared to conf plans (P = 0.0001/ 0.026). Cstomo plans had significantly better PTV Conformity Index (CI) (P = 0.0006) with comparable Homogeneity Index. A CTV-PTV margin of 5.3, 4, 5.3 cm in the 3 axes were calculated using Van Herks formula. After average shifts from imaging for first 5 fractions (AS5) were applied to remaining fractions, the residual shifts in the calculated CTV-PTV margins reduced to 2.9, and 2.1 in the X, Y axes respectively allowing further adaptation of PTV margin from fraction 6. Conclusions: Carotid sparing was possible using cstomo plans with significantly better conformity. Applying AS5 could enable us to reduce the PTV (3 mm) margin in X, Y axes for the remaining 15 fractions.