Advances in navigation and intraoperative imaging for intraoperative electron radiotherapy (2017)

This thesis is framed within the field of radiotherapy, specifically intraoperative electron radiotherapy (IOERT). This technique combines surgical resection of a tumour and therapeutic radiation directly applied to a post-resection tumour bed or to an unresected tumour. The high-energy electron beam is collimated and conducted by a specific applicator docked to a linear accelerator (LINAC). Dosimetry planning for IOERT is challenging owing to the geometrical and anatomical modifications produced by the retraction of structures and removal of cancerous tissues during the surgery. No data of the actual IOERT 3D scenario is available (for example, the applicator pose in relation to the patient’s anatomy or the irregularities in the irradiated surface) and consequently only a coarse approximation of the actual IOERT treatment administered to the patient can be estimated. Intraoperative computed tomography (CT) images of the actual scenario during the treatment would be useful not only for intraoperative planning but also for registering and evaluating the treatment administered to the patient. This information is essential for prospective trials.
In this thesis, the feasibility of using a multi-camera optical tracking system to obtain the applicator pose in IOERT scenarios was firstly assessed. Results showed that the accuracy of the applicator pose was below 2 mm in position (mean error of the bevel centre) and 2o in orientation (mean error of the bevel axis and the longitudinal axis), which are within the acceptable range proposed in the recommendation of Task Group 147 (commissioned by the Therapy Committee and the Quality Assurance and Outcomes Improvement Subcommittee of the American Association of Physicists in Medicine [AAPM] to study the localization accuracy with non-radiographic methods such as infrared systems in external beam radiation therapy). An important limitation of this solution is that the actual pose of applicator is superimposed on a patient’s preoperative image. An intraoperative image would provide updated anatomical information and would allow estimating the 3D dose distribution.
The second specific study of this thesis evaluated the feasibility of acquiring intraoperative CT images with a CT simulator in real IOERT scenarios. There were no complications in the whole procedure related to the transport step using the subtable and its stretcher or the acquisition of intraoperative CT images in the CT simulator room. The acquired intraoperative studies were used to evaluate the improvement achieved in the dose distribution estimation when compared to that obtained from preoperative CT images, identifying the dominant factor in those estimations (air gap and the surface irregularities, not tissue heterogeneities). 
Finally, the last specific study focused on assessing several kilovoltage (kV) CT technologies other than CT simulators to acquire intraoperative images for estimating IOERT dose distribution. That would be necessary when a mobile electron LINAC was available in the operating room as transferring the patient to the CT simulator room could not be approved. Our results with an abdominal phantom revealed that a portable CT (BodyTom) and even a LINAC with on-board kV cone-beam CT (TrueBeam) would be suitable for this purpose.