The reproducibility of organ position using active breathing control (ABC)during liver radiotherapy

Purpose: To evaluate the intrafraction and interfraction reproducibility of liver immobilization using active breathing control (ABC). Methods and Materials: Patients with unresectable intrahepatic tumors who c ould comfortably hold their breath for at least 20 s were treated with foca l liver radiation using ABC for liver immobilization. Fluoroscopy was used to measure any potential motion during ABC breath holds. Preceding each rad iotherapy fraction, with the patient setup in the nominal treatment positio n using ABC, orthogonal radiographs were taken using room-mounted diagnosti c X-ray tubes and a digital imager. The radiographs were compared to refere nce images using a 2D alignment tool. The treatment table was moved to prod uce acceptable setup, and repeat orthogonal verification images were obtain ed. The positions of the diaphragm and the liver (assessed by localization of implanted radiopaque intra-arterial microcoils) relative to the skeleton were subsequently analyzed. The intrafraction reproducibility (from repeat radiographs obtained within the time period of one fraction before treatme nt) and interfraction reproducibility (from comparisons of the first radiog raph for each treatment with a reference radiograph) of the diaphragm and t he hepatic microcoil positions relative to the skeleton with repeat breath holds using ABC were then measured. Caudal-cranial (CC), anterior-posterior (AP), and medial-lateral (ML) reproducibility of the hepatic microcoils re lative to the skeleton were also determined from three-dimensional alignmen t of repeat CT scans obtained in the treatment position. Results: A total of 262 fractions of radiation were delivered using ABC bre ath holds in 8 patients. No motion of the diaphragm or hepatic microcoils w as observed on fluoroscopy during ABC breath holds. From analyses of 158 se ts of positioning radiographs, the average intrafraction CC reproducibility (sigma) of the diaphragm and hepatic microcoil position relative to the sk eleton using ABC repeat breath holds was 2.5 mm (range 1.8-3.7 min) and 2.3 mm (range 1.2-3.7 mm) respectively. However, based on 262 sets of position ing radiographs, the average interfraction CC reproducibility (sigma) of th e diaphragm and hepatic microcoils was 4.4 mm (range 3.0-6.1 mm) and 4.3 mm (range 3.1-5.7 mm.), indicating a change of diaphragm and microcoil positi on relative to the skeleton over the course of treatment with repeat breath holds at the same phase of the respiratory cycle. The average population a bsolute intrafraction CC offset in diaphragm and microcoil position relativ e to skeleton was 2.4 mm and 2.1 mm respectively; the average absolute inte rfraction CC offset was 5.2 mm. Analyses of repeat CT scans demonstrated th at the average intrafraction excursion of the hepatic microcoils relative t o the skeleton in the CC, AP, and NIL directions was 1.9 mm, 0.6 mm, and 0. 6 mm respectively and the average interfraction CC, AP, and ML excursion of the hepatic microcoils was 6.6 mm, 3.2 mm, and 3.3 min respectively. Conclusion: Radiotherapy using ABC for patients with intrahepatic cancer is feasible, with good intrafraction reproducibility of liver position using ABC. However, the interfraction reproducibility of organ position with ABC suggests the need for daily on-line imaging and repositioning if treatment margins smaller than those required for free breathing are a goal. (C) 2001 Elsevier Science Inc.