Endobronchially Implanted Real-Time Electromagnetic Transponder Beacon-Guided, Respiratory-Gated SABR for Moving Lung Tumors: A Prospective Phase 1/2 Cohort Study

Abstract

Purpose: Endobronchial electromagnetic transponder beacons (EMT) provide real-time, precise positional data of moving lung tumors. We report results of a phase 1/2, prospective, single-arm cohort study evaluating the treatment planning effects of EMT-guided SABR for moving lung tumors.

Methods and materials: Eligible patients were adults, Eastern Cooperative Oncology Group 0 to 2, with T1-T2N0 non-small cell lung cancer or pulmonary metastasis ≤4 cm with motion amplitude ≥5 mm. Three EMTs were endobronchially implanted using navigational bronchoscopy. Four-dimensional free-breathing computed tomography simulation scans were obtained, and end-exhalation phases were used to define the gating window internal target volume. A 3-mm expansion of gating window internal target volume defined the planning target volume (PTV). EMT-guided, respiratory-gated (RG) SABR was delivered (54 Gy/3 fractions or 48 Gy/4 fractions) using volumetric modulated arc therapy. For each RG-SABR plan, a 10-phase image-guided SABR plan was generated for dosimetric comparison. PTV/organ-at-risk (OAR) metrics were tabulated and analyzed using the Wilcoxon signed-rank pair test. Treatment outcomes were evaluated using RECIST (Response Evaluation Criteria in Solid Tumours; version 1.1).

Results: Of 41 patients screened, 17 were enrolled and 2 withdrew from the study. Median age was 73 years, with 7 women. Sixty percent had T1/T2 non-small cell lung cancer and 40% had M1 disease. Median tumor diameter was 1.9 cm with 73% of targets located peripherally. Mean respiratory tumor motion was 1.25 cm (range, 0.53-4.04 cm). Thirteen tumors were treated with EMT-guided SABR and 47% of patients received 48 Gy in 4 fractions while 53% received 54 Gy in 3 fractions. RG-SABR yielded an average PTV reduction of 46.9% (P < .005). Lung V5, V10, V20, and mean lung dose had mean relative reductions of 11.3%, 20.3%, 31.1%, and 20.3%, respectively (P < .005). Dose to OARs was significantly reduced (P < .05) except for spinal cord. At 6 months, mean radiographic tumor volume reduction was 53.5% (P < .005).

Conclusions: EMT-guided RG-SABR significantly reduced PTVs of moving lung tumors compared with image-guided SABR. EMT-guided RG-SABR should be considered for tumors with large respiratory motion amplitudes or those located in close proximity to OARs.

Figure 1

CONSORT (Consolidated Standards of Reporting Trials) diagram.

Figure 2

Dosimetric outcomes: (A) waterfall plot of percentage planning target volume reduction comparing EMT beacon guided, respiratory-gated, SABR with standard image-guided SABR. Red striped bar represents the mean reduction of 47.2%, P < .005. (B) Lung dose metric(V5Gy, V10Gy, V20Gy) comparison of standard image-guided SABR (black columns) with EMT beacon guided, respiratory-gated, SABR. *P < .005.

Figure 3

Illustrative cases. (A) Sixty-two-year-old male with a solitary pulmonary metastasis from colorectal cancer. Top: standard IG-SABR was not safe/feasible to deliver because of PTV (orange contour) was overlapping with heart (pink contour) and heart maximum dose of 6182 cGy. Bottom: Prone deep-inspiration breath hold SABR using Calypso EMT guidance with maximum heart dose of 2523 cGy. The heart (pink contours) and PTV (orange contours) no longer overlap. (B) Twenty-two-year-old female with metastatic synovial sarcoma with 2 pulmonary metastases in close proximity. Both tumors were treated concurrently using 1 set of EMT beacons. Red contours: standard IG-SABR PTVs. Blue contours: EMT guided, respiratory-gated, SABR PTVs. PTVs were reduced by 50.6% (anterior tumor) and 54.7% (posterior tumor), respectively, using EMT-guided respiratory-gated SABR as compared with IG-SABR. Abbreviations: EMT = electromagnetic transponders; IG = image guided; PTV = planning target volume.