ENDOBRONCHIAL IMPLANTED REAL-TIME RADIOFREQUENCY (RF) TRANSPONDER BEACON GUIDED, RESPIRATORY-GATED, STEREOTACTIC BODY RADIOTHERAPY FOR MOVING LUNG TUMOURS: INTERIM ANALYSIS OF A PROSPECTIVE PHASE I/II COHORT STUDY

Purpose/Objective(s)

Endobronchially implanted real-time tracking system (technology company) radiofrequency (RF) transponder beacons provide real-time, high precision, positional data of moving lung tumors. We report interim results of a Phase I/II prospective single arm prospective cohort study evaluating feasibility and dosimetric impact of real-time tracking system beacons for patients undergoing SBRT for moving lung tumors.

Materials/Methods

Eligible patients were adults, ECOG 0-2, with T1-T2N0 Non-small cell lung cancer (NSCLC) or pulmonary metastasis ≤4cm of the right middle/lower lobe, or lingula. Upper lobe tumors were eligible if they moved ≥5mm. Three real-time tracking system beacons were endobronchially implanted at prespecified peritumoral locations using SuperDimension™ navigational bronchoscopy (Medtronic, USA). Four-dimensional free-breathing CT simulation scans were obtained and end-exhalation phases were selected to define a gating window (GW). A 3mm expansion of the ITV_GW defined the PTV. Real-time tracking system-guided, respiratory phase-gated, SBRT (GW-SBRT) was delivered using 10MV VMAT photon arcs at 2400MU/min to total doses of 54Gy/3# or 48Gy/4#. For each GW-SBRT plan, a corresponding 10-phase plan was generated for comparison to standard image-guided (IG)SBRT. PTV, and OAR metrics were tabulated and differences between GW-SBRT and IG-SBRT plans analyzed using non-parametric Wilcoxon Signed-Rank pair test. Treatment related toxicity was graded using the Common Terminology Criteria for Adverse Events (CTCAE) version 4.0.

Results

From November 2017 to December 2021, 41 patients were screened and 17 patients consented to participate. Two patients did not undergo beacon implant: one had tumor progression; another withdrew consent. Of the 15 patients with beacon implants, median age was 73, with 7 females. 47% were T1 NSCLC, 40% were M1 and remaining 13% were T2 NSCLC. Median tumor dimension was 1.9cm. 73% of targets were located peripherally, and 27% centrally with a median respiratory tumor motion of 1.25 cm (range 0.53 cm to 4.04 cm). No beacon migration was observed between CT simulation scan and the completion of SBRT treatments. Of the N=12 tumor targets were treated with GW-SBRT, 47% of patients received 48Gy/4# and 53% received 54Gy/3#. GW-SBRT yielded an average relative reduction of PTV of 46.9% (p<0.005) compared with IG-SBRT. Lung V5, V10, V20 and D2 had mean relative reductions of 11.3%, 20.3%, 31.1% and 15.5% respectively (p<0.005). Dose to OARs was significantly reduced (p<0.05) except for spinal cord. At 6 months follow-up, mean radiographic tumor volume reduction was 53.5% (p<0.005); No treatment related grade 3 toxicity was observed.

Conclusion

Endobronchial RF beacons are safe and effective at reducing SBRT treatment volumes for patients with moving lung tumors and can be considered for tumors with large motion amplitude or those located in close proximity to organs at risk. This study has the ClinicalTrials.gov identifier NCT03322072.