Can Bronchoscopically Implanted Anchored Transponders be Used As a Surrogate to Accurately Track Lung Tumors During Deep Inspiration Breath Hold Radiation Therapy?

Abstract

Implanted EMTs provide a means to position and track, in real time, tumors during deep inspiration breath hold (DIBH) radiotherapy. However, during a course of RT, the local anatomy of a lung tumor can change, possibly reducing the geometric accuracy of the EMTs. We investigate whether correcting for such changes prior to each treatment is clinically feasible. 14 patients with lung tumors accrued to an IRB approved protocol were each implanted with 3 anchored EMTs placed in small airways (< 2mm) near or within the tumor. The EMT is investigational; the study was conducted under an IDE. Cone beam scans were acquired prior to every SBRT treatment, and once weekly for conventional fractionated patients. Dual registrations: tumor to tumor and EMT to EMT were done for each scan. Any discrepancy was used to correct the EMT tracking. Correction was verified using CBCT and EMT tracking. Retrospectively, 5 registrations of each DIBH CBCT with the planning scan were made; features registered were: the tumor, each EMT, and 3-4 vertebrae at tumor level. The displacement of each EMT with respect to the tumor was computed. From the CBCT scans, the mean and std. dev. of the distributions of the absolute differences of the change in each EMT position with respect to the tumor were (in mm) 1.0 ± 0.9 Left-Right, 1.0±1.1 Post-Ant, and 1.1±1.3 Sup-Inf. Individual EMT displacements of more than 2 and 3 mm with respect to the tumor occurred in 76% and 59% of 66 CBCT scans. The EMT centroid exhibited fewer large displacements, centroid displacements of more than 2 and 3 mm occurring in 30% and 15% of scans. Thus deformation of the EMT - tumor anatomy occurs. Examination of small lung features in the volume around the transponders showed no evidence of their migration. The variation in EMT position with respect to the tumor appears to arise from the dynamic nature of the tumor and lung tissue surrounding it. Tumor shrinkage in conventionally fractionated patients, occasional tumor associated atelectasis, and differences in the diaphragm shape during breath hold were all observed. In 3/14 patients, EMT tracking coordinates were altered immediately prior to treatment based on dual CBCT registrations. Anchored EMTs, combined with CBCT scans, can be used as part of a clinically robust procedure to accurately track a lung tumor throughout a course of DIBH RT.