Effect of replacing cochlea contour with inner ear contour on cochlea dose-volume calculations in conventional 2 dimensional and conformal 3 dimensional radiotherapy of brain

Sensorineural hearing loss is a radiotherapy complication in patients with brain tumors.The auditory system of patients with brain tumors often is placed inside of radiation field and receives a significant amount of radiation dose resulting in hearing loss.The purpose of this study was to compare contouring and delivery dose to cochlea and inner ear in two techniques of 3D conformal radiotherapy vs 2D conventional radiotherapy in patients with brain tumors.
Methods and materials:Computerized tomography (CT) scan of43 patients(23male and20 female) were used.Patients were treated for brain tumor at the radiotherapy unit of Isfahan Milad Hospital 2016.Contouring of cochlea and inner ear were done on patients CT.by Radiation oncologist.using TiGRTtreatment planning system.Dose volume histograms (DVHs) for methods of 2D (two parallel and opposite fields) and 3D (two parallel and opposite field and a vertex field using wedge and Multileaf collimators(MLC)) were compared.
The mean dose in 2D technique for the cochlea was 2624 ± 338.7and for the inner ear was 2718.3 ± 341.3,and in 3D radiotherapy technique for the cochlea 2507±294.6,and for inner ear was 2581± 295.6.The mean dose of inner ear and cochlea in 3D was 5.3% and 4.6%lower than 2D,respectively.
The study of dosimetric parameters of the auditory system in treatment planning of two methods of radiation therapy showed that there was no significant difference between the doses of cochlea and inner ear in 2D and 3D methods (Pvalues=0.86,0.85).Therefore in treatment planning of these two radiotherapy techniques, contour of cochlea can be replaced by inner ear.