Differences in Parotid Dosimetry and Expected Normal Tissue Complication Probabilities in Whole Brain Radiation Plans Covering C1 Versus C2

OBJECTIVES: There is no consensus standard regarding the placement of the inferior field border in whole brain radiation therapy (WBRT) plans, with most providers choosing to cover the first versus (vs.) second cervical vertebrae (C1 vs. C2). We hypothesize that extending coverage to C2 may increase predicted rates of xerostomia. METHODS: Fifteen patients underwent computed tomography (CT) simulation; two WBRT plans were then produced, one covering C2 and the other covering C1. The plans were otherwise standard, and patients were prescribed doses of 25, 30 and 37.5 gray (Gy). Dose-volume statistics were obtained and normal tissue complication probabilities (NTCPs) were estimated using the Lyman-Burman-Kutcher model. Mean parotid dose and predicted xerostomia rates were compared for plans covering C2 vs. C1 using a two-sided patient-matched t-test. Plans were also evaluated to determine whether extending the lower field border to cover C2 would result in a violation of commonly accepted dosimetric planning constraints. RESULTS: The mean dose to both parotid glands was significantly higher in WBRT plans covering C2 compared to plans covering C1 for all dose prescriptions (p<0.01). Normal tissue complication probabilities were also significantly higher when covering C2 vs. C1, for all prescribed doses (p<0.01). Predicted median rates of xerostomia ranged from <0.03%-21% for plans covering C2 vs. <0.001%-12% for patients treated with plans covering C1 (p<0.01), dependent on the treatment dose and NTCP model. Plans covering C2 were unable to constrain at least one parotid to <20 Gy in 31% of plans vs. 9% of plans when C1 was covered. A total parotid dose constraint of <25 Gy was violated in 13% of plans covering C2 vs. 0% of plans covering C1. CONCLUSIONS: Coverage of C2 significantly increases the mean parotid dose and predicted NTCPs and results in more frequent violation of commonly accepted dosimetric planning constraints.