Electromagnetic transponders localization and real-time tracking for prostate cancer radiotherapy: clinical impact of metallic hip prostheses.

Abstract Purpose To assess electromagnetic transponders (EMTs) ability to localize and track movements in prostate cancer (PCa) patients with metallic hip prostheses (MHP) treated with curative radiotherapy (RT). Methods and materials Datasets of eight PCa patients with MHP (3 bilateral and 5 unilateral) treated between 2016 and 2018 with RT and EMT tracking were retrospectively assessed. The distances between the 3 EMTs (apex to left, left to right, right to apex) and the isocenter were calculated both on planning computed tomography (CT) and cone beam CT (CBCT) at the first treatment fraction and compared with data reported by Calypso® (Varian Medical Systems, Palo Alto, CA). EMTs position and treatment interruptions triggered by Calypso® were analyzed for all evaluable treatment fractions (n=120). Localization accuracy was quantified by recording the geometric residual value (expected limit ≤ 0.2 cm) at the RT setup. Results The Calypso® system was able to localize and track prostate position without any detectable interference from MHP. For every treatment fraction, the agreement between the CBCT images and Calypso® guidance was optimal, with EMTs always within the defined tolerance (i.e., CT-Calypso or CBCT-Calypso measured differences in inter-EMT distances within 0.3 cm). Electromagnetic transponders to isocenter distances measured by Calypso® reproduced CT data and were confirmed on CBCTs. During RT, the EMTs centroid exceeded the threshold 24 times (20% of all fractions): 5 times in the left-right, 15 times in the anterior-posterior (AP), and 4 times in the superior-inferior directions. The largest motions recorded were in the AP axis: 0.6 cm anteriorly and 0.5 cm posteriorly in patients with unilateral and bilateral MHP, respectively. Conclusions Our study represents the first clinical experience assessing the localization and tracking accuracy of Calypso® EMTs during curative RT of PCa patients with of unilateral or bilateral MHP.