Patient doses in image-guided radiotherapy: Status in Europe for CBCT imaging in the pelvic region

Patient doses in pelvic IGRT in Europe

Abstract

Background. Cone-beam CT (CBCT) imaging doses are often neglected in image-guided radiation therapy (IGRT). However, frequent imaging for patient positioning can result in significant and unrecorded additional radiation exposure. This study aims to evaluate clinical imaging doses from kV-CBCT and assess if they are optimized and how,  in prostate and pelvic patient positioning protocols across Europe.

Materials and methods. Data collected from a survey distributed across Europe on IGRT practices were compiled and analysed. A representative set of imaging protocols were simulated using Monte Carlo based ImpactMC software to assess absorbed doses in various organs in ICRP standard phantom. Absorbed doses to red bone marrow were estimated with a three-parameter mass-energy absorption coefficient method. The simulations were validated against measurements with MOSFET detectors and radiochromic film.

Results. Simulated prostate doses ranged from 12mGy to 34 mGy per imaged fraction for pelvic protocols, and 4 mGy to 26 mGy for prostate protocols. The selected length of the imaging region influenced doses to the femur and sacral red bone marrow. Overall, 74% of treatments involved positioning imaging at every fraction, indicating substantial cumulative doses from kV-CBCT imaging.

Conclusions. The results of this study suggest that clear guidelines and standardized protocols for CBCT imaging in IGRT are lacking. There is significant potential to optimize the imaging doses. Given that most clinics already perform regular quality assurance for imaging equipment, including dosimetry and positioning accuracy verification, establishing diagnostic reference levels for CBCT imaging in IGRT could help promote further dose optimization.