(P138) Dosimetric Evaluation of Parotid Dose in Whole-Brain Radiation Plans Covering C1 vs C2

April 30, 2015

Clinicians who choose to cover C2 with WBRT should be aware of the increased radiation dose delivered to the parotid glands. NTCP modeling predicts an increased rate of treatment-associated xerostomia as a result.

Andrew W. Orton, MD, John D. Gordon, MS, DABR, Tyler P. Vigh, Allison E. Tonkin, MD, George M. Cannon, MD; Huntsman Cancer Institute; Intermountain Medical Center

BACKGROUND AND PURPOSE: Dosimetric analysis has demonstrated a higher-than-anticipated dose to the parotid glands during whole-brain radiation therapy (WBRT). Even in the computed tomography (CT) planning era, WBRT fields continue to be designed based on anatomic landmarks identified on digitally reconstructed radiographs (DRRs) of the skull. The dose of radiation delivered incidentally to the parotid glands is influenced by the location of the inferior border of the treatment field. The purpose of this study is to compare the dose delivered to the parotid glands and associated normal tissue complication probabilities (NTCPs) in plans covering the C1 vs C2 vertebral level.

MATERIALS AND METHODS: A total of 15 patients underwent CT simulation of the brain, followed by dosimetric evaluation of parotid dose. Two treatment plans were produced for each patient using the CT simulation images: the first extended the inferior field border to the inferior edge of the C1 vertebra, and the second extended the field to the inferior edge of C2. Two dose prescriptions were compared: 30 Gy and 37.5 Gy. NTCPs were estimated using the Lyman-Burman-Kutcher model with parameters obtained from studies published by Eisbruch, Emami, and Roesink. Mean dose to the parotids and NTCPs were compared between the two groups, and statistical significance was determined using a patient-matched two-sided t-test.

RESULTS: The mean parotid dose was significantly higher when C2 was covered. For the 30-Gy prescription, coverage to C2 increased the parotid dose from 14.3 Gy to 18.3 Gy (P < .01). For the 37.5-Gy plan, coverage to C2 increased the parotid dose from 18.5 Gy to 23.4 Gy (P < .01). NTCPs for the parotid gland were also higher when covering C2 vs C1. At 30 Gy, NTCP estimates for C1 coverage ranged from 0.0 to 0.08 compared with a range of 0.0 to 0.12 in plans covering C2. At 37.5 Gy, C1 coverage resulted in predicted rates of 0.0 to 0.12 vs 0.01 to 0.21 in plans covering C2.

CONCLUSIONS: Clinicians who choose to cover C2 with WBRT should be aware of the increased radiation dose delivered to the parotid glands. NTCP modeling predicts an increased rate of treatment-associated xerostomia as a result.

Proceedings of the 97th Annual Meeting of the American Radium Society - americanradiumsociety.org