Deglutition-Induced Dose Deformations in Head-and-Neck Cancer IMRT Treatments: Volume Shuttle Imaging Analysis

Abstract

The 4D respiration-correlated imaging using external metric system is ideal way to capture range of motion for moving targets. However momentary apneic phase during swallowing leads to sorting errors and image artifacts thus deterring this modality from being used to track motion displacements in head and neck region. A special Imaging tool used for CT-Angio/joint motion study called Dynamic helical Volume shuttle (DHVS) fills this void. DVHS is continuous bidirectional scan with extended 'z' coverage and improved temporal sampling. Present study aims to assess the deglutition induced displacement and dose deformations in patients with head and neck cancer using DHVS imaging. Twenty patients of treatment naive oropharyngeal cancers following rigid immobilization were setup in treatment position on a 128 slice multidetector CT Scanner and underwent DHVS imaging (2 mts scan). Image data for single patient was acquired during voluntary effortful swallowing act (once in 20 seconds) at 2 alternating table positions with the table shuttling back and forth. Around 3000 images/per pt. were reformatted using Dynamic Pitch Cone Beam Reconstruction (RetroRecon) for 12 phases @ 1.25 mm thickness and cine looped to quantify the range of motion during swallowing. Deglutition-induced GTV and Spinal cord displacements were quantified based on position change during deglutition relative to pre-swallow structure location for anterior (A), posterior (P), superior (S), and inferior (I) directions. The image datasets in extreme bins (peak expiratory and peak inspiratory bins) were backprojected on to the original plan in QA mode (to preserve the plan integrity) and dose deformations happening consequent to swallowing were analyzed. Deglutition-induced maximal GTV displacements ranged from 3 mm to 18 mm with mean and standard deviation of 3.5 +/- 2, 2.50 +/- 1.51, 1.75 +/- 4.0, and 9.60 +/- 7.7 mm in the A, P, I, and S directions respectively. The SC displacement ranged from 2.8 mm of the prescribed dose would still be delivered to the tumor even if the tumor partly vacated the PTV during the entire swallowing event for 15 of the 20 patients during the VHS scan. However, for the other 5 patients’ cohort, which had frequent swallowing, the target coverage failed to reach 80%. This is the first study reported in literature using DHVS for deglutition displacements quantification. Asymmetric IMs and PRV margins, derived from directional displacement, using DVS imaging should be employed to account for tumor motion for high precision treatments in HNC.