Patient doses in image-guided radiotherapy: status in Europe for cone-beam CT imaging in the pelvic region.

BACKGROUND

Organ absorbed doses in cone-beam CT (CBCT) imaging 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 organ doses from kV-CBCT and assess if they are optimized and how, in prostate and pelvic patient positioning protocols across Europe. Status of quality assurance in IGRT CBCT imaging is assessed in general.

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 mean absorbed doses in various organs in the International Commission on Radiological Protection (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 absorbed doses ranged from 12 mGy 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. Quality assurance was performed by 90% of responders, but good practice guides and national protocols do not exist.

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 patient doses resulting from imaging. 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.