ESTRO 2021 Abstract Book

S844

ESTRO 2021

1 University Hospital Zurich and University of Zurich, Department of Radiation Oncology, Zurich, Switzerland

Purpose or Objective The aim of this study was to evaluate volumetric changes and migration of parotid glands and submandibular glands in head-and-neck cancer (HNC) patients treated with magnetic resonance imaging (MRI)-guided

adaptive radiotherapy. Materials and Methods

Patients with advanced HNC treated in a prospective protocol on a ViewRay MRIdian Linac were included (NTC03972072). All patients received 35 fractions of 2 Gy daily, resulting in a total of 70 Gy to the macroscopic tumor, 60 Gy to the involved nodal levels and 50 Gy for elective nodal irradiation. Weekly offline MR-guided adaptive re-planning was performed based on the actual MR-setup image. These weekly MRIs were used to evaluate salivary glands’ volume over the course of radiotherapy. Additionally, inter-gland distances were evaluated using the geometrical centers. Wilcoxon signed-rank test was used to evaluate the values with respect to baseline. P-values below 0.05 were considered significant. Results Twelve patients were included in the analysis. The mean [range] change in parotid volume was -9.2% [-29.7 – 7.2] after one week of treatment (p<0.0001), and -32.4% [-53.4 - -14.5] after five weeks (p<0.0001) (Figure 1A), corresponding to -8.3 mL [-13.4 - -3.7]. The average volume reduction was comparable for the ipsilateral and contralateral parotids (33.9% vs. 32.3%, NS). For the submandibular glands, the volume change was -11.2% [-29.1–10.4] after one week (p=0.0002) and -29.3% [-51.4–9.3] after five weeks (p<0.0001) (Figure 1B). In absolute numbers, the change was -2.3 mL [-6.3–0.6]. Linear regression showed an average decrease of 0.21 mL/day and 0.06 mL/day for parotids and submandibular glands, respectively. The relative gland volume was significantly correlated with cumulative dose for with slopes of -1.1%·Gy -1 (R 2 =0.60) for ipsilateral parotids, - 1.8%·Gy -1 (R 2 =0.74) for contralateral parotids, -0.6%·Gy -1 (R 2 =0.43) for ipsilateral submandibular glands, and - 0.9%·Gy -1 (R 2 =0.63) for contralateral submandibular glands.

Inter-parotid distance changed on average by -5.5% [-11.9% - -1.0%] after five weeks (p=0.0005), corresponding to -6.6 mm [-14.9 - -1.0] (Figure 2). The inter-submandibular gland distance did not change significantly (p=0.08). For relative gland volume, a statistically significant linear correlation with relative patient weight was observed for both parotids (p<0.0001, R 2 =0.14) and submandibular glands (p=0.0002, R 2 =0.08), but this was not observed for relative inter-gland distance (p=0.14 and p=0.40, respectively).