Robustness Analysis of Reference Surfaces for Surface Guided Radiation Therapy of the Breast

Abstract

Surface guided radiation therapy (SGRT) implements an optical imaging system in radiation therapy for positioning and motion management. This system projects visible light onto a patient and the reflected light is used to generate 3D positional information so that clinicians can accurately reproduce body positions. Patient setup shifts are calculated with six degrees of freedom by a registration algorithm comparing a reference surface (RS) of the patient to a live surface map of the patient on treatment day. SGRT has been an effective tool in daily localization for the treatment of breast cancer patients. It is common for patients to have multiple RS throughout the course of their treatment to account for anatomical variation between fractions. We sought to evaluate the robustness of reference surfaces and vendor specific algorithms used for SGRT. At our institution, positional shift data for five patients treated for right-sided breast cancer were retrospectively analyzed. SGRT performance was compared between RS using bilateral breasts or a single ipsilateral breast. Shift parameters were calculated over the entire treatment course for all patients with a vendor supplied software tool that offers rigid and deformable registration algorithms. The deformable algorithm was used for treatment setups, with the treatment RS encompassing both breasts plus a margin. Two robustness tests were carried out: 1) a trimmed down RS encompassing just the ipsilateral breast and 2) a comparison of deformable vs rigid registration of the clinically used RS. After obtaining translational and angular shift data, the absolute mean differences between shifts were calculated to compare differences between RS size and algorithm performance. On average, 1.4 new RS were created per patient guided by weekly radiographic imaging to adjust for anatomical changes. The absolute value of the average of the discrepancies between shifts using the clinical RS subtracted from the trimmed external (89 fractions) were <1mm and 1° and the maximum differences were: Lateral: 2.6mm, Longitudinal: 1.4mm, Vertical: 1.1mm, Yaw: 1.1°, Roll: 1.5°, Pitch: 1.7°. Discrepancies between tracking algorithms (83 fractions) were <1.5mm and 1° and the maximum differences were: Lateral: 3.4mm, Longitudinal: 3.5mm, Vertical: 2.0mm, Yaw: 2.4°, Roll: 2.7°, Pitch: 1.9°. Clinically negligible mean discrepancies were observed for both robustness tests showing that neither the reference surface size nor the algorithms investigated caused systematic variations in the shifts for this group of patients. Maximum discrepancies of up to 3 mm and 3° were found between the algorithms, which indicate some variation, but within clinical tolerance. Overall, different selection of reference surfaces and algorithms had a minor effect on clinical shifts for SGRT of the breast.