Quantitative Assessment of Volumetric Changes Using Fan Beam and Cone Beam Computed Tomography During Head and Neck Image Guided Radiotherapy

Abstract

Purpose/Objectives: To develop a standardized methodology for cranial nerves IX-XII contouring among patients treated with intensitymodulated radiotherapy (IMRT) for head and neck cancer (HNC). Materials/Methods: Using anatomic texts, radiologic data, and T1/2 magnetic resonance imaging (MRI), a standardized method for delineating the cranial nerves IX-XII on computed tomography (CT) was performed. A neuroradiologist assisted with identification of the cranial nerves IX-XII and adjacent structures (i.e. Midbrain, Pons, and Medulla Oblongata). These organs at risk were then contoured on 5 consecutive patients undergoing IMRT for locally advanced HNC (i.e. base of skull, nasopharyngeal and Paranasal-sinus cancer). Dosevolume histogram (DVH) curves were generated by applying the proposed cranial nerves contour to the initial treatment plan. Due to anatomical nature and proximity of cranial nerves IX, X and XI, they were contoured as one structure while cranial nerve XII was contoured individually. Results: The median total dose to the planning target volume (PTV) was 70 (ranged from 66–70 Gy). The median cranial nerves (IX-XI) and (XII) volumes were 20 cm3 (15–25) and 18 cm3 (15–22) respectively. The median V50, V60, V66, and V70 of the cranial nerves (IX-XI) and (XII) volumes were (85, 77, 71, 65) and (88, 80, 74, 64) respectively. The maximal dose to the cranial nerves (IX-XI) and (XII) were 72 (66–77) and 71 Gy (64–78) respectively. Conclusions: The proposed methodology provides a highly reproducible, defined and standardized way for delineating the cranial nerves IX-XII organ at risk on IMRT based CT planning. Our dosimetric analysis should serve as pilot data for prospective studies for patients undergoing IMRT for HNC to establish a limiting dose for these structures at risk.