Study of inter-fraction movements of tongue during radiation therapy in cases of tongue malignancy using volumetric cone beam computed tomography (CBCT) imaging

Abstract

Original ArticleAbstract Purpose: Tongue is a mobile organ in head and neck region predisposing it for geographic miss during the course of fractionated radiotherapy for tongue malignancy. This study analyses movement of tongue during the course of radiotherapy using volumetric KV-cone beam computed tomography (KV-CBCT) imaging for patients of tongue malignancy treated without using tongue bite. Methods: We analysed 100 KV-cone beam CTs performed on 10 patients with carcinoma of tongue undergoing fractionated radiotherapy. All the patients underwent thermoplastic mask immobilisation and CT simulation. During the course of radiotherapy, all patients underwent volumetric KV-CBCT imaging to assess the movements of tongue. Five arbitrary reference points were used to analyse the movements of tongue in 3-dimensions: 1) Point A: Tip of tongue; 2) Point B: Point over right lateral border, 4 cm posterior to the tip of tongue; 3) Point C: Point over left lateral border, 4 cm posterior to the tip of tongue; 4) Point D: Point over superior most part (dorsum) of tongue, 4 cm posterior to the tip of tongue; 5) Point E: Point over the surface of base of tongue at the level of tip of epiglottis. Results: Mean movements of point A: +0.21 cm (SD: 0.12) and -0.23 cm (SD: 0.14), point B: +0.14 cm (SD: 0.04) and -0.19 cm (SD: 0.1), point C: +0.12 cm (SD: 0.05) and -0.14 cm (SD: 0.06), point D: +0.15 cm (SD: 0.07) and -0.29 cm (SD: 0.22) and point E: +0.23 cm (SD: 0.15) and -0.23 cm (SD: 0.14). Conclusion: Organ movement is one of the great challenges encountered during radiotherapy. Tongue is one such organ in head and neck region. Concept of internal target volume (ITV) margin which takes into account the internal organ movements should be considered for tongue malignancies. ITV to PTV margin will depend on the setup accuracy, immobilization device and imaging modality utilised for setup verification. In an IGRT (Image Guided Radio Therapy) setup, a PTV margin of 0.3 to 0.5 cm from ITV would be safe.