Tradeoffs of integrating real-time tracking into IGRT for prostate cancer treatment

Abstract

This study investigated the integration of the Calypso real-time tracking system, based on implanted ferromagnetic transponders and a detector array, into the current process for image-guided radiation treatment (IGRT) of prostate cancer at our institution. The current IGRT process includes magnetic resonance imaging (MRI) for prostate delineation, CT simulation for treatment planning, daily on-board kV and CBCT imaging for target alignment, and MRI/MRS for post-treatment assessment. This study assesses (1) magnetic-field-induced displacement and radio-frequency (RF)-induced heating of transponders during MRI at 1.5 T and 3 T, and (2) image artifacts caused by transponders and the detector array in phantom and patient cases with the different imaging systems. A tissue-equivalent phantom mimicking prostate tissue stiffness was constructed and implanted with three operational transponders prior to phantom solidification. The measurements show that the Calypso system is safe with all the imaging systems. Transponder position displacements due to the MR field are minimal (<1.0 mm) for both 1.5 T and 3 T MRI scanners, and the temperature variation due to MRI RF heating is <0.2 degrees C. The visibility of transponders and bony anatomy was not affected on the OBI kV and CT images. Image quality degradation caused by the detector antenna array is observed in the CBCT image. Image artifacts are most significant with the gradient echo sequence in the MR images, producing null signals surrounding the transponders with radii approximately 1.5 cm and length approximately 4 cm. Thus, Calypso transponders can preclude the use of MRI/MRS in post-treatment assessment. Modifications of the clinical flow are required to accommodate and minimize the substantial MRI artifacts induced by the Calypso transponders.

Respiratory-Induced Prostate Motion

“…Such transponders, e.g., the Calypso system (Calypso 4D Localization System, Calypso Medical, Seattle, WA), allow the tracking of the prostate position during irradiation, but also have drawbacks like an invasive insertion procedure and severe artifacts in subsequent MRI examinations which limit diagnostic options in patient aftercare [31]. Patients who most likely will not benefit from such a procedure could be spared, thus, making the therapy more adequate and more cost effective.…”

Clinical development of a failure detection‐based online repositioning strategy for prostate IMRT—Experiments, simulation, and dosimetry study

“…The solution using prostate position tracking with electromagnetic transponders 27 has the drawback that MR-based post-treatment assessment is impeded because the large metallic transponders produce severe MRI artifacts. 7,8 Stereoscopic fluoro-kV imaging [9][10][11] induces undesirably large imaging dose, 15 whereas paired MV-kV imaging 12-14 uses a single kV source, which reduces the imaging dose. Using single-source continuous MV or kV imaging to estimate 3D marker position has also been studied.…”

“…7,8 Real-time simultaneous acquisition of two x-ray images from different viewing angles for 3D target localization has also been implemented with submillimeter accuracy, e.g., fluoroscopic imaging using two kV systems [9][10][11] and MV-kV imaging using cine-MV and fluoroscopic kV systems. [12][13][14] Stereoscopic x-ray imaging with one or two kV sources, however, poses the problem of accumulating excessive patient imaging dose.…”

Expanding the use of real‐time electromagnetic tracking in radiation oncology

“…( 1 ) The risk of pneumothorax is greatly reduced with transbronchial fiducial placement in the lung, even in peripheral lesions under fluoroscopic guidance. ( 1 ) Also, when taking into consideration follow‐up imaging studies of patients, the RF transmitter's relatively large ferrite core creates significant artifact on MRI or even kilovoltage cone beam CT. ( 52 ) It was reported that implanted transponder displacements due to the MR field were minimal; however, the null signal reported due to image artifacts were up to 1.5 cm in radius and 4 cm in length, thus creating significant issues with post‐treatment imaging with patient follow‐up. ( 52 ) A reduction in the sizes of RF devices would be beneficial, but would not prevent these image artifacts from occurring.…”