Helical tomotherapy for head and neck squamous cell carcinoma: Dosimetric comparison with linear accelerator-based step-and-shoot IMRT

Background: Linear Accelerator-based Intensity Modulated Radiation Therapy (IMRT), either as step-and shoot (SS) or in dynamic mode, is now considered routine in the definitive management of head and neck squamous cell carcinoma (HNSCC). Helical TomoTherapy (HT) is a new platform to deliver IMRT. This study aims to compare step-and-shoot Intensity Modulated Radiation Therapy (SS IMRT) with dynamic Helical TomoTherapy (HT) dosimetrically in patients with head and neck squamous cell carcinoma (HNSCC). Materials and Methods: Twelve patients with HNSCC, previously treated with SS IMRT, were re-planned on HT using the same CT dataset. Plans were compared for target coverage and organs-at-risk (OARs) sparing. Sparing of parotids was assessed after stratifying for side (contralateral vs. ipsilateral) and site of disease (laryngopharynx vs. oropharynx). Normal tissue complication probabilities (NTCP) were also compared for the parotid glands. Results: All HT plans showed improvement in target coverage and homogeneity, and reduction in OAR doses as compared to SS IMRT plans. For PTV 66, the mean V(99) improved by 14.65% (P = 0.02). Dose Homogeneity (D(10-90)) was significantly better in the HT plans (mean 2.07Gy as compared to 4.5Gy in the SS IMRT plans, P = 0.02). HT resulted in an average reduction of mean parotid dose of 12.66Gy and 18.28Gy for the contralateral and ipsilateral glands (P = 0.003) respectively. This translated into a 24.09% and 35.22% reduction in Normal Tissue Complication Probability (NTCP) for the contralateral and ipsilateral parotids respectively (P < 0.01). Site of disease (laryngopharynx vs. oropharynx) did not have any significant impact on parotid sparing between SS IMRT and HT. The maximum dose to the spinal cord showed a mean reduction of 12.07Gy in HT plans (P = 0.02). Conclusion: Helical Tomotherapy achieved better target coverage with improved OAR sparing as compared to SS IMRT. The significant reduction in mean parotid doses translated into meaningful reduction in NTCP, with potential clinical implications in terms of reduction in Xerostomia and improved quality of life in patients with HNSCC.