Diaphragm and lung dose in liver high-dose-rate interstitial brachytherapy: A dosimetry and toxicity report

Purpose: The aim of this study was to retrospectively analyze and report on dose-volume and clinical toxicity of liver high-dose-rate interstitial brachytherapy (HDR-IBT) used in diaphragm and lung tissue. Material and methods: Computed tomography (CT)-based liver HDR-IBT using Oncentra Brachy treatment plan-ning system (TPS) plans of patients with malignant liver tumor (MLT) from September 2018 to June 2023 were re-viewed to identify patients, whose diaphragm and lung tissue were within 100% prescription isodose. These organs at risk (OARs) were contoured in axial CT slices. Maximum point dose (Dmax), dose to 0.2 cc, 0.5 cc, 1 cc (D0.2cc, D0.5cc, D1cc), and volume receiving 30 Gy and 50 Gy (V30Gy and V50Gy) were analyzed. Toxicity data of these patients were retrieved from hospital electronic records. Results: The analysis included 27 patients with 43 and 36 MLTs, whose 100% prescription isodose of liver HDR-IBT plan was within diaphragm and lung tissue. Median prescription dose was 25 Gy (range, 15-25 Gy) in single-fraction. Median Dmax, D0.2cc, D0.5cc, and D1cc of the diaphragm were 302 Gy (range, 54-396 Gy), 68 Gy (range, 38-234 Gy), 48 Gy (range, 32-128 Gy), and 35 Gy (range, 27-88 Gy), while for the lung, 90 Gy (range, 39-295 Gy), 55 Gy (range, 32-207 Gy), 44 Gy (range, 29-117 Gy), and 34 Gy (range, 25-79 Gy), respectively. Median V30Gy and V50Gy for the diaphragm were 1.1 cc (range, 0-5.8 cc) and 0.2 cc (range, 0-2.5 cc), while for the lung, 0.8 cc (range, 0-10.1 cc) and 0.1 cc (range, 0-2.3 cc), receptive-ly. Two patients with repeated HDR-IBT sessions received cumulative Dmax diaphragm of 698 Gy and 792 Gy. At median follow-up of 23 months, no patient reported any suspicious symptom of radiation-induced diaphragm or lung injury. Conclusions: This is the first publication reporting diaphragm and lung tissue dose-volume and clinical toxicity in liver HDR-IBT. Small volume of diaphragm and lung tissue tolerated extreme high radiation doses [5 times of stereotactic body radiotherapy (SBRT) range in single fraction] without clinically significant toxicity. A standardized reporting for diaphragm and lung dose volume is needed for future liver HDR-IBT studies. The results of the current study can be employed in future for expanded indication of brachytherapy, such as CT-guided trans-thoracic lung brachytherapy.