Correlation of Salivary Gland Dose to Volume Changes Noted During Adaptive Replanning of Head and Neck Radiation Therapy

Abstract

Salivary gland shrinkage is known to occur with radiation therapy (RT) for head and neck cancer. Adaptive Radiation Therapy (ART) is commonly performed during the course of Intensity-Modulated Radiation Therapy (IMRT) due to weight loss and/or tumor shrinkage. Our goal was to investigate changes in parotid (PG) and submandibular gland (SMG) volumes measured on initial and replanning CT. We hypothesize that dose received by gland at the time of replan predicts for quantitative gland volume reduction. Replanning thus has the potential to reduce this effect which ultimately may reduce severity of xerostomia. We analyzed 100 patients who underwent adaptive re-planning (200 plans) between December 2014 to August 2017. Volumetric Modified Arc Therapy (VMAT) based IMRT was used in all. All patients had an initial CT simulation scan (Plan 1) used for radiation planning and a subsequent new CT simulation for adaptive replan (Plan 2). Dose-volume histograms were used to measure mean and maximum doses to right and left PG and SMG. We then correlated these doses to PG and SMG volume changes noted at the time of replan (PG1-PG2; SMG1-SMG2). Univariate and multivariate analyses were performed to identify factors contributing to gland volume changes. Tumor site included 9 oral cavity, 50 oropharynx, 14 Larynx, 7 Nasopharynx, and 20 others. Treatments were definitive chemo-RT in 67%, adjuvant chemo-RT in 20%, or adjuvant RT alone in 10%. Chemo used was Cisplatin-based 36%, Carboplatin-based 22%, or Cetuximab-based in 28%. Median total RT dose was 70 Gy (60-70 Gy in 30-35 daily fractions). Median time to performing replan was at fraction #21 (42 Gy) and around day 37 of therapy (13-66 days). Mean weight loss at time of replan was -4.3% (-21.3 to +17.5%). Analysis included 181 PG and 116 SMG. Mean dose received at time of replan with corresponding volume change: RPG = 1548 cGy, -13.6%; LPG = 1647 cGy, -14.6%; RSMG = 3359 cGy, -12.7%; LSMG = 3366 cGy, -14.4%. The correlation values of dose received and volume change: RPG r = 0.17, LPG r = -0.21, RSMG r = -0.33, and LSMG r = -0.27 (all with P<.05 except RPG, P=.13). Anatomic changes during radiation can result in increased doses to normal tissues and increased risk of toxicity. Our results show a significant change in gland volume in 3 of 4 salivary glands at the time of replanning with a direct correlation to dose received to the gland. Adaptive replan has the potential to reduce salivary gland doses.