SU-E-J-125: Uncertainty in Accumulated Dose to the Cochlea Due to Setup Error during External Beam Treatment of Head & Neck Cancer Patients.

PURPOSE Treatment uncertainties are not included in modeling dose response of hearing loss. We determine uncertainty of accumulated dose to cochleas of head & neck cancer patients due to setup error during treatments of external beam IMRT. METHODS We studied 8 patients: 1 planning CT for each patient and total 40 cone beam CTs. Those patients were treated with 33 fractions. Treatments delivered 70 Gy to PTV, 50 to 60 Gy to nodal and sub-clinical disease using IMRT dose painting. Setup error was measured using 6-dimensional rigid registration between planning CT and cone beam scans obtained prior to treatment. Planning CTs were transformed and resliced to produce 'treatment CTs' according to the measured setup error matrix to simulate the cochlea position during treatment. We calculated dose delivered to cochleas at each treatment position. Treated dose to the cochlea from each fraction was accumulated on the planning CT using its registration with each 'treatment CT'. RESULTS The RMS value of set error of left cochlea is 0.48pm0.28cm, right cochlea is 0.47pm0.26cm. Mean values of left and right cochleas are ∼zero. Uncertainty in the dose to cochlea depends on each treatment plan and relative positions of the cochlea and target volumes. Average uncertainty of mean dose to cochlea is 5.0%, or 5.2cGy per fraction. CONCLUSION Patient setup error introduces uncertainty to the position of the cochlea and consequent uncertainties in accumulated dose. Our method calculates accumulated dose delivered to the same cochlea volume at different treatment positions due to setup error. We found uncertainty in cochlea doses was 5% of planned dose after 4 to 7 fractions. The largest uncertainty was 17 cGy per fraction. Our Results will be used to determine uncertainties in dose response of hearing loss in head and neck patients. NIH R01-CA129182.