SU‐E‐T‐795: Measuring Uncertainty in Dose Delivered to the Cochlea Due to Setup Error during External Beam Treatment of Patients with Cancer of the Head & Neck

Abstract

Purpose: To understand the relationship between hearing loss and cochlea dose in patients treated for cancers of the head and neck with external beam IMRT, uncertainties in planned and delivered cochlea doses must be known. We use treatment cone beam scans to determine the positional and dosimetric uncertainty for the cochlea. Methods: Data from 6 head & neck patients with 6 planning CT and 29 Cone Beam CTs taken at time of treatment are used in this study. Patients were treated 70 Gy to the primary target, and between 50–60 Gy to nodal and sub‐clinical disease using IMRT dose painting. We use 6 dimensional rigid body registration to find translational and rotational setup error between the planning CT and Cone Beam CT scans. The Planning CT is rotated and translated so that the cochlea positions match those seen in the cone beam scans and cochlea doses recalculated. Results: We found most probable value of the magnitude of mean error in cochlea position is 0.250.02cm, but it can increase to 7mm in some patients. Uncertainty in the dose to cochlea depends on each treatment plan and relative positions of the cochlea and target volumes. Most of differences in doses to cochlea in our study are around 2% of planned dose. But for some patients this may be as large as 5–9%. Conclusions: Patient setup error introduces uncertainty in the position of the cochlea. Consequent uncertainties in dose heavily depends on the relative position of cochlea w.r.t. the target volumes and the individual treatment plan. Results from this study will be used to model the uncertainty in the dose response of hearing loss.This research is supported in part by NIH under RO1‐CA129182.