Dynamic MRI analysis of tumor and organ motion during rest and deglutition and margin assessment for radiotherapy of head-and-neck cancer

Purpose: To quantify swallowing frequency and tumor and normal structure displacements during deglutition using dynamic magnetic resonance imaging (MRI) and to determine planning target volume (PTV) margins to account for resting and deglutition-induced displacements in patients with head-and-neck cancer (HNC).

Methods and materials: Twenty-two patients with HNC were imaged in the treatment position using dynamic MRI. Sagittal images were acquired. Two-dimensional displacement was analyzed using contours of normal structures and GTV drawn for one swallowing event. Deglutition-induced displacements were quantified based on position change during deglutition relative to preswallow structure location for anterior (A), posterior (P), superior (S), and inferior (I) directions. Additional long-time MRI series were obtained from a subset of 11 patients while they were resting in order to determine swallowing frequency and duration. PTV margins to account for setup error, frequency and duration of deglutition, and resting and deglutition-induced GTV motion were calculated.

Results: Mean maximum resting displacements ranged from 1.5 to 3.1 mm for combined GTV subsites. Mean maximum swallowing GTV displacement for combined subsites ranged from 4.0 to 11.6 mm. Swallowing was nonperiodic, with a frequency ranging from 0 to 19 swallows over 12.8 min and mean swallow duration of 3.5 s. Based on the average swallowing characteristics in this cohort, the average PTV margins to account for setup error and tumor motion are estimated to be 4.7 mm anteriorly, 4.2 mm posteriorly, 4.7 mm inferiorly, and 6.0 mm superiorly.

Conclusions: The measurable mean maximum resting displacement for the GTV indicates that tumor motion occurs even when the patient is not swallowing. Nonuniform margins should be used as a standard PTV margin that accounts for setup error and tumor motion in radiotherapy of HNC unless adaptive radiotherapy with respect to intrafraction tumor motion is performed. The PTV margin can be individualized to a single patient's swallowing characteristics or calculated as an average based on the swallowing data from the cohort.