Doses délivrées par l’imagerie de contrôle en radiothérapie externe guidée par l’image

[Management of image guidance doses delivered during radiotherapy]

Image-guided radiotherapy (IGRT) has become a standard irradiation technique to improve the clinical outcome of patients in terms of toxicity and local control due to better targeting of radiation during the irradiation fraction. Positioning imaging systems, whether embedded or not, such as kV for 2×2D acquisitions and especially kVCBCT for 3D acquisitions are however irradiating in a large volume including the target volume but also healthy tissue, with a theoretical risk of increased toxicity and second cancer. It therefore appears very important both to optimize the absorbed dose due to IGRT practice but also to report it, especially in case of kVCBCT. The AAPM report published in 2018 (« Image guidance doses delivered during radiotherapy: Quantification, management, and reduction ») proposes a management of image guidance doses delivered during radiotherapy. This report is the basis of this focus article that aims at giving orders of magnitude and proposing a management of image guidance doses delivered during radiotherapy in clinical practice. The dose delivered per kVCBCT is about 0.5 to 2 cGy at isocenter according to treatment site. As long as the calculation algorithms are not available in the treatment planning systems, it seems appropriate to use at least the published dose orders of magnitude. This estimate should ultimately allow the clinician to decide on the therapeutic strategy in the event of accumulation of positioning imaging sessions.

[Image-guided radiotherapy]

The IGRT is described in its various equipment and implementation. IGRT can be based either on ionizing radiation generating 2D imaging (MV or kV) or 3D imaging (CBCT or MV-CT) or on non-ionizing radiation (ultrasound, optical imaging, MRI or radiofrequency). Adaptive radiation therapy is then presented in its principles of implementation. The function of the technicians for IGRT is then presented and the possible dose delivered by the on-board imaging is discussed. The quality control of IGRT devices is finally described.

[Image-guided radiotherapy: Overview of devices and practice in France in 2015]

Image-guided radiation therapy consists in acquiring in-room images to improve patient and mainly tumour set up accuracy. Many devices based on ionising or non-ionising radiations were designed in recent years. The use of such devices is of major importance in the management of patient radiotherapy courses. Those imaging sessions require to clearly define procedures in each radiotherapy department (image modality, acquisition frequency, corrective action, staff training and tasks). A quick review of the different existing image-guided radiation therapy devices is presented. In addition, the results of a French national survey about image-guided radiation therapy are presented: the survey is about both equipment and procedures. A total of 57 radiotherapy departments have participated, representing more than 160 treatment devices. About three linear accelerators out of four are equipped with an image-guiding device. The most common equipment is the CBCT system. Most centres have set up training sessions for the technicians to allow them to analyse online daily images. The management of in-room imaging dose is still under investigation, but many centres use an accounting scheme. While the devices are used to adjust the positioning of patients, in more than half of the centres, the practice had an impact on the choice of clinical and planning target volume margins. This survey led to an inventory in 2015, and could be renewed in some years.

[Evaluation and choice of imaging protocols on the Elekta XVI(®) kilovoltage cone-beam computed tomography imaging system]

PURPOSE

This work proposes an evaluation of the Elekta XVI(®) kilovoltage cone-beam computed tomography imaging system. The average dose delivered for each acquisition protocol proposed by default by the manufacturer was measured with several detectors and compared to theoretical dose values given by Elekta. At the same time, an evaluation of image quality for pelvic protocols correlated to dose measurements in homogeneous and heterogeneous mediums allowed to optimize the use of the XVI(®) system.

MATERIALS AND METHODS

The dose was measured for each acquisition protocol (varying filters, FOV and collimations) with four detectors (CT pencil ion chamber, 0.3 and 0.125 cm(3) cylindrical ion chambers, radiothermoluminescent dosimeters) in a CTDI phantom. The dose evaluation in a heterogeneous medium was performed in an experimental anthropomorphic phantom simulating a male pelvis. Image quality was assessed with a Catphan(®) 600 phantom.

RESULTS

The average dose measured in a homogeneous medium was about 17 mGy and 25 mGy per acquisition for Pelvis and Prostate protocols and about 17 mGy and 1 mGy for Lung and Head protocols. The study performed with different detectors showed that doses obtained were of the same order of magnitude (± 10 %) and agreed with those supplied by the manufacturer. The evaluation of image quality correlated to the average dose measured allowed to optimize the use of XVI(®) acquisition protocols. Measurement results in a heterogeneous medium showed a dose decrease by a factor 1.5 for bone and by a factor 2 for titanium.

CONCLUSION

The study showed that theoretical values proposed by the manufacturer could be used to estimate the average dose delivered to the patient by the kV-CBCT imaging system. The analysis of all the results led to the implementation of a procedure allowing to optimize and account for the dose delivered to the patient by the CBCT imaging system and to report it in the patient folder.