Dosimetric evaluation between megavoltage cone-beam computed tomography and body mass index for intracranial, thoracic, and pelvic localization.

The aim of this study was to evaluate radiation dose for organs at risk (OAR) within the cranium, thorax, and pelvis from megavoltage cone-beam computed tomography (MV-CBCT). Using a clinical treatment planning system, CBCT doses were calculated from 60 patient datasets using 27.4 × 27.4 cm(2) field size and 200° arc length. The body mass indices (BMIs) for these patients range from 17.2-48.4 kg/m(2). A total of 60 CBCT plans were created and calculated with heterogeneity corrections, with monitor units (MU) that varied from 8, 4, and 2 MU per plan. The isocenters of these plans were placed at defined anatomical structures. The maximum dose, dose to the isocenter, and mean dose to the selected critical organs were analyzed. The study found that maximum and isocenter doses were weakly associated with BMI, but linearly associated with the total MU. Average maximum/isocenter doses in the cranium were 10.0 (± 0.18)/7.0 (± 0.08) cGy, 5.0 (± 0.09)/3.5 (± 0.05) cGy, and 2.5 (± .04)/1.8 (± 0.05) cGy for 8, 4, and 2 MU, respectively. Similar trends but slightly larger maximum/isocenter doses were found in the thoracic and pelvic regions. For the cranial region, the average mean doses with a total of 8 MU to the eye, lens, and brain were 9.7 (± 0.12) cGy, 9.1 (± 0.16) cGy, and 7.2 (± 0.10) cGy, respectively. For the thoracic region, the average mean doses to the lung, heart, and spinal cord were 6.6 (± 0.05) cGy, 6.9 (± 1.2) cGy, and 4.7 (± 0.8) cGy, respectively. For the pelvic region, the average mean dose to the femoral heads was 6.4 (± 1.1) cGy. The MV-CBCT doses were linearly associated with the total MU but weakly dependent on patients' BMIs. Daily MV-CBCT has a cumulative effect on the total body dose and critical organs, which should be carefully considered for clinical impacts.