Radiation Exposure of Parotid Gland during Whole-Brain Radiation Therapy

Abstract

Whole-brain radiation therapy (WBRT) is used for patients with brain metastases (BM), primary central nervous system (CNS) lymphoma, or CNS prophylaxis from cancer. The parotid gland is located adjacent to skull base and cervical vertebrae, therefore WBRT portals include a certain amount of parotid volume. The goal of this study was to measure the amount of parotid gland irradiated during WBRT. Fifty-nine patients who underwent conventional WBRT between January 2006 and December 2009 were selected for this retrospective study. Each brain was fully contoured along with the spinal cord to the bottom of the C1 or C2 vertebral body according to the location of disease. The parotid gland was delineated on existing the WBRT plan. Traditional parallel-opposed lateral treatment plans were generated with inferior portals margin to C1 or C2 vertebral body (WBRT-C1 and WBRT-C2) in each patients. Additionally, intensity-modulated radiation therapy (IMRT) plan was constructed for 10 patients. For both techniques, the nominal prescribed dose was 30 Gy in 10 fractions using 6-MV photons. Comparative dose-volume histograms were analyzed for each patient. Threshold of radiation-induced xerostomia was defined as 20 Gy (biologically equivalent dose of 25 Gy in 1.8 Gy fraction size) in this study. Indications for WBRT were of the following; 49 BMs (83%), 4 CNS lymphomas (7%), and 6 CNS prophylaxis (10%). Median age of patients was 58 years. Mean parotid dose (parotid Dmean) of 59 patients was 18.4±3.6 Gy with WBRT-C1 and 24.4±3.1 Gy with WBRT-C2 (p < 0.001). Parotid Dmean of 45 patients (76%) was less than 20 Gy with WBRT-C1, while 53 patients' (90%) parotid Dmean was more than 20 Gy of with WBRT-C2. Parotid Dmean increased from 17.4±1.8 Gy (WBRT-C1) to 24.3±2.0 Gy (WBRT-C2) in 39 patients (66%) (p < 0.001). Fourteen patients (24%) received more than 20 Gy of parotid Dmean regardless of the level of inferior portal margin (22.6±1.6 Gy with WBRT-C1 and 26.6±1.8 Gy with WBRT-C2). IMRT plans for ten out of those 14 patients revealed 53% reduction of parotid Dmean (12.7±1.0 Gy with IMRT and 26.6±1.8 Gy with WBRT-C2). These data suggest that the parotid gland receives a considerable amount of radiation dose during conventional WBRT. Special cautions are needed to spare parotid gland, especially WBRT with inferior portals margin lower than C1 vertebral body. Using an IMRT plan for WBRT may be of benefit for selected patients. The study of the clinical impact of conventional WBRT on xerostomia is ongoing.