Assessment of organ doses from head and neck cone-beam computed tomography (CBCT) in adaptive radiation therapy: A phantom study

The purpose of this study was to estimate accumulated organ doses from cone-beam computed tomography (CBCT) in adaptive radiation therapy (ART) of head and neck cancer treated with Ethos linear accelerators (linacs) using the nanoDot detectors in the rando phantom. The nanoDot dosimeter was placed on the Rando phantom for experimental measurement. The measurement was performed during kilovoltage cone-beam computed tomography (kV-CBCT) image acquisition in Ethos linac (Varian Medical Systems, Pala Alto, CA, USA). Consequently, the absorbed dose at organ was estimated using depth correction factor and mass attenu-ation coefficient ratio between each organ and air ((mu en rho)tissue (mu en rho)air). The result has showed that an absorbed dose at lens, thyroid gland, and parotid glands was lower than the dose limit of ICRP 103 guideline. The range of absorbed doses was 0.36-1.17 mGy for left eye lens, 0.36-1.63 mGy for right eye lens, 3.00-3.20 mGy left lobe thyroid, 2.69-2.87 mGy for right lobe thyroid, 2.19-2.32 mGy for left lobe parotid, and 1.92-2.01 mGy for right lobe parotid. The result of accumulated dose estimation was determined for 33 fractions which was 35.1, 32.3, 46.0, 39.8, 63.2, and 56.7 mGy for left lens, right lens, left parotid, right parotid, left lobe thyroid, and right lobe thyroid, respectively. Although the organ doses received from daily CBCT images were less than those from radiation treatment, it should be a concern when the dose at OARs is near the limitation in the treatment planning process.