A prospective study on thoracic radiation therapy with deep inspiration breath hold guided by implanted electromagnetic transponders

Purpose: Respiratory motion poses a challenge to thoracic radiation therapy (RT). Electromagnetic lung transponders implanted bronchoscopically in airways near the tumor can serve as an internal fiducial to track tumor motion in real time. Here we present our investigator-initiated prospective trial assessing the feasibility of transponder-guided deep inspiration breath hold (DIBH) thoracic RT.

Methodology: We enrolled patients with primary lung cancer or lung metastases who were candidates for thoracic RT using DIBH and navigational bronchoscopy for endobronchial transponder implantation near the target tumor. Three transponders were implanted, followed by ≥3 days of observation and then simulation with DIBH, free-breathing, and 4D-CT as backup. At treatment, DIBH was monitored using an antenna positioned over the patient’s chest. A gating window of +/-5 mm was used for the first 25 patients. We then opened a second cohort undergoing stepwise reduction of the gating window by 1mm after each additional 5 patients to determine the smallest feasible gating window. The primary endpoint was feasibility, defined as completion of RT using transponder-guided DIBH. Patients were followed every 3 months for 1 year for assessment of transponder- and RT-related toxicity per CTCAE v4.0.

Results: We enrolled 48 patients (35 with primary lung cancer, 13 with lung metastases) with a median age of 69 years. A total of 144 transponders were implanted with a median distance of 1.6 cm (IQR 0.6–2.8 cm) to the gross tumor volume. One patient lost one transponder likely implanted in an airway too wide for implantation. Transponder-guided DIBH was feasible in all but 2 patients (96% feasible), who failed because the distance between transponders and antenna was >19cm. Among the remaining 46 patients, 6 were treated prone to keep the transponders within 19cm of the antenna, and 40 were treated supine. Thirty-five patients received stereotactic body RT (SBRT), 4 received hypofractionated RT (8 or 15 fractions), and 7 received conventionally fractionated RT. The gating window was successfully reduced to +/-2mm. Thirty-nine (85%) patients completed one year of follow-up. Toxicity at least possibly related to transponder implantation was grade 2 in 6 patients (cough and hemoptysis), grade 3 in 5 patients (cough, dyspnea, pneumonia, and supraventricular tachycardia), and grade 4 pneumonia in 1 patient (pneumonia a few days after implantation but recovered fully and completed RT). Toxicity at least possibly related to RT was grade 2 in 41 patients (most commonly cough, fatigue, and pneumonitis) and grade 3 in 7 patients (most commonly pneumonia), and no patient had grade 4 or higher toxicity.

Conclusions: Electromagnetic transponder–guided DIBH lung RT is feasible and safe, allowing for precise tumor targeting and reduced normal tissue exposure. Transponder-antenna distance was the most common challenge due to limited antenna range, which could sometimes be circumvented by prone positioning.

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