Organ dose measurements using an adult anthropomorphic phantom and risk estimation of cancer incidence from CBCT exposures

Abstract

Cone-beam CT (CBCT) has become an essential tool for pre-treatment verification of the patient's position and for targeting the tumor volume localization in Image Guided Radiotherapy (IGRT). CBCT imaging is employed generally on a daily-basis, for each treatment fraction, and several times per patient, to ensure the correct patient's set-up. This leads to cumulative imaging doses to the tissues surrounding the exposed target-organs. The objective of this work is to determine the patient organ doses from a thorax CBCT scan in order to estimate risk of cancer incidence due to CBCT exposures.A thorax CBCT scan was performed in an anthropomorphic phantom of an adult male (CIRS ATOM) and the organ doses were assessed from point measurements using Thermo Luminescent Detectors (TLDs). The measurements were performed using a CBCT imaging system mounted on a LINAC (EdgeTM, Varian Medical Systems). The lifetime attributable risk (LAR) of cancer incidence was determined using the BEIR risk models. Considering a single thorax CBCT scan, the highest organ doses were calculated for heart and left lung which registered values of 5.84 ± 0.99 mGy and 4.90 ± 0.83 mGy, respectively. In contrast, the lowest organ dose was determined for right lung, with an absorbed dose of 3.07 ± 0.52 mGy. Regarding risk estimation, after a complete course of IGRT treatment for lung cancer (24 fractions) and assuming that, at least, one CBCT scan is performed per fraction, the LAR of cancer incidence varies between 27 to 309 cases per 100.000 exposed persons, depending on the organ evaluated. This work highlights the need to determine radiation induced cancer risks arising from CBCT repeated exposures in order to optimize the selected scanning protocols and consequently the radiological protection of patient. Furthermore, accurate organ dose calculation is fundamental to reduce the uncertainties associated with radiological risk estimation in imaging procedures.