Cesium-131 Brachytherapy for Lung Cancer: Dosimetric, Safety Considerations and Initial Experience

Cesium-131 (131Cs) radioactive seed is FDA approved for permanent seed implant for all cancers including lung, head and neck and colorectal cancers. Cs-131 has approximately similar energy to I-125 and a higher dose rate that can potentially result in better local control rates for aggressive tumors with a low α/β ratio, such as non-small cell lung cancers. We describe the first clinical report of Cs-131 use in lung cancers. Seven patients received Cs-131 implant for lung cancer. Six patients had AJCC stage I non-small cell lung cancer and 1 patient was found to be pT3 because of pleural involvement. Patients were selected for sublobar resection if they were not considered candidates for a lobectomy because of poor pulmonary functions. Median age was 78 years (range, 53-86 years). All patients had biopsy proven adenocarcinomas. The implant was performed with Cs-131 suture seeds by either Mesh technique or Double suture method. Seed placements, dose prescriptions, were determined intraoperatively, and were dependent on the length of the resection margin. Four patients underwent a wedge resection and three underwent a segmentectomy. Radiation exposure was determined at the completion of the procedure. A post-implant dosimetry was performed on all patients within 1 week after implant. The median tumor size was 1.35 cm (range, 0.6-1.8 cm). The median number of seeds placed was 26 (range, 10-50). The prescription dose for Cs-131 was 60-80Gy and median seed activity was 2.41U. A nomogram was developed using the variseed software with source data from AAPM TG-43 report to calculate the number of seeds needed to cover the wedge line with the prescription dose of 60-80 Gy. For example, for a prescription dose of 80Gy covering a 10cm length and a 4 cm width would require 10 Cs-131 seeds with a 2.15U. The median volume of prescription isodose (80Gy) for Cs-131 was 14.57cc (range, 2.16-35.76cc). In a virtual comparison, equivalent median isodose prescription volume for I-125 was 18.91cc (range, 3.07-42.45cc). Median exposure rates at 1 m for Cs-131 in the operating room were 0.2 mrem/h and 1.5 mrem/h at 35cm from the patient. The local control of all patients with a median follow-up of 5 months is 100%. No RTOG grade 1-IV toxicity was ascribed to the implant. Our preliminary experience with Cs-131 lung brachytherapy has been very encouraging with low toxicity, high local control rates and safety. We found the radiation exposure to surgeons and radiation oncologists performing the procedure to be within the recommended limits of occupational exposure (5000mrem/year). However, we will recommend various safety measures such as lead aprons and gloves continue to be used while performing the procedures.