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

Abstract

ObjectivesThere 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.MethodsFifteen 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.ResultsThe 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.ConclusionsCoverage of C2 significantly increases the mean parotid dose and predicted NTCPs and results in more frequent violation of commonly accepted dosimetric planning constraints.

Assessment of Risk of Xerostomia After Whole-Brain Radiation Therapy and Association With Parotid Dose

“…In 1980, Cairncross et al described acute parotitis with accompanying hyperamylasemia in 4 patients. More recent dosimetric studies in the era of 3-D planning (that makes possible the delineation and avoidance of organs at risk) report parotid doses of 15 to 20 Gy during WBRT, but these studies did not record clinical outcomes . Therefore, the clinical significance of this parotid exposure is unknown, and the parotid glands are not routinely delineated or avoided during WBRT .…”

“…More recent dosimetric studies in the era of 3-D planning (that makes possible the delineation and avoidance of organs at risk) report parotid doses of 15 to 20 Gy during WBRT, but these studies did not record clinical outcomes. [15][16][17][18][19][20] Therefore, the clinical significance of this parotid exposure is unknown, and the parotid glands are not routinely delineated or avoided during WBRT. 25,28 In contrast, radiation therapy for head and neck cancer is well known to be associated with substantial and sometimes permanent xerostomia.…”

“…For head and neck radiation therapy, numerous studies over several decades have illustrated the importance of limiting radiation dose to the parotid glands to minimize dry mouth and improve QOL . However, the parotid glands are not typically delineated nor avoided during WBRT, despite receiving substantial doses …”

“…[9][10][11][12][13][14] However, the parotid glands are not typically delineated nor avoided during WBRT, despite receiving substantial doses. [15][16][17][18][19][20] We hypothesized that WBRT is associated with clinically significant xerostomia and that xerostomia is associated with parotid dose. To investigate this hypothesis, we conducted a prospective observational study of patients receiving routine WBRT.…”

Prospective Assessment of Patient-Reported Dry Eye Syndrome After Whole Brain Radiation

“…Standard WBRT techniques have not changed appreciably for decades, and the routine practice of designing radiation fields to encompass the upper cervical vertebrae and the cribiform plate results in dose to the parotid and lacrimal glands, respectively. 8,[18][19][20][21] Figure 4A shows the radiation fields used to treat a study patient who developed persistent dry eye symptoms. In Figure 4B, the radiation fields were re-designed to spare the lacrimal glands.…”