The use of active breathing control (ABC) to reduce margin for breathing motion

Purpose: For tumors in the thorax and abdomen, reducing the treatment margi n for organ motion due to breathing reduces the volume of normal tissues th at will be irradiated. A higher dose can be delivered to the target, provid ed that the risk of marginal misses is not increased. To ensure safe margin reduction, we investigated the feasibility of using active breathing contr ol (ABC) to temporarily immobilize the patient's breathing. Treatment plann ing and delivery can then be performed at identical ABC conditions with min imal margin for breathing motion. Methods and Materials: An ABC apparatus is constructed consisting of 2 pair s of flow monitor and scissor valve, 1 each to control the inspiration and expiration paths to the patient. The patient breathes through a mouth-piece connected to the ABC apparatus. The respiratory signal is processed contin uously, using a personal computer that displays the changing lung volume in real-time. After the patient's breathing pattern becomes stable, the opera tor activates ABC at a preselected phase in the breathing cycle. Both valve s are then closed to immobilize breathing motion. Breathing motion of 12 pa tients were held with ABC to examine their acceptance of the procedure. The feasibility of applying ABC for treatment was tested in 5 patients by acqu iring volumetric scans with a spiral computed tomography (CT) scanner durin g active breath-hold. Two patients had Hodgkin's disease, 2 had metastatic liver cancer, and 1 had lung cancer. Two intrafraction ABC scans were acqui red at the same respiratory phase near the end of normal or deep inspiratio n. An additional ABC scan near the end of normal expiration was acquired fo r 2 patients. The ABC scans were also repeated 1 week later for a Hodgkin's patient. In 1 liver patient, ABC scans were acquired at 7 different phases of the breathing cycle to facilitate examination of the liver motion assoc iated with ventilation. Contours of the lungs and livers were outlined when applicable. The variation of the organ positions and volumes for the diffe rent scans were quantified and compared. Results: The ABC procedure was well tolerated in the 12 patients. When ABC was applied near the end of normal expiration, the minimal duration of acti ve breath-hold was 15 s far 1 patient with lung cancer, and 20 s or more fo r all other patients. The duration was greater than 40 s for 2 patients wit h Hodgkin's disease when ABC was applied during deep inspiration. Scan arti facts associated with normal breathing motion were not observed in the ABC scans. The analysis of the small set of intrafraction scan data indicated t hat with ABC, the liver volumes were reproducible at about 1%, and lung vol umes to within 6%. The excursions of a "center of target" parameter for the livers were less than 1 mm at the same respiratory phase, but were larger than 4 mm at the extremes of the breathing cycle. The inter-fraction scan s tudy indicated that daily setup variation contributed to the uncertainty in assessing the reproducibility of organ immobilization with ABC between tre atment fractions. Conclusion: The results were encouraging; ABC provides a simple means to mi nimize breathing motion. When applied for CT scanning and treatment, the AB C procedure requires no more than standard operation of the CT scanner or t he medical accelerator. The ABC scans are void of motion artifacts commonly seen on fast spiral CT scans. When acquired at different points in the bre athing cycle, these ABC scans show organ motion in three-dimension (3D) tha t can be used to enhance treatment planning. Reproducibility of organ immob ilization with ABC throughout the course of treatment must be quantified be fore the procedure can be applied to reduce margin for conformal treatment. (C) 1999 Elsevier Science Inc.