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

https://doi.org/10.1016/S0360-3016(99)00056-5Get rights and content

Abstract

Purpose: For tumors in the thorax and abdomen, reducing the treatment margin for organ motion due to breathing reduces the volume of normal tissues that will be irradiated. A higher dose can be delivered to the target, provided that the risk of marginal misses is not increased. To ensure safe margin reduction, we investigated the feasibility of using active breathing control (ABC) to temporarily immobilize the patient’s breathing. Treatment planning and delivery can then be performed at identical ABC conditions with minimal margin for breathing motion.

Methods and Materials: An ABC apparatus is constructed consisting of 2 pairs 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 continuously, using a personal computer that displays the changing lung volume in real-time. After the patient’s breathing pattern becomes stable, the operator activates ABC at a preselected phase in the breathing cycle. Both valves are then closed to immobilize breathing motion. Breathing motion of 12 patients were held with ABC to examine their acceptance of the procedure. The feasibility of applying ABC for treatment was tested in 5 patients by acquiring volumetric scans with a spiral computed tomography (CT) scanner during active breath-hold. Two patients had Hodgkin’s disease, 2 had metastatic liver cancer, and 1 had lung cancer. Two intrafraction ABC scans were acquired at the same respiratory phase near the end of normal or deep inspiration. An additional ABC scan near the end of normal expiration was acquired for 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 associated with ventilation. Contours of the lungs and livers were outlined when applicable. The variation of the organ positions and volumes for the different 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 active breath-hold was 15 s for 1 patient with lung cancer, and 20 s or more for all other patients. The duration was greater than 40 s for 2 patients with Hodgkin’s disease when ABC was applied during deep inspiration. Scan artifacts associated with normal breathing motion were not observed in the ABC scans. The analysis of the small set of intrafraction scan data indicated that with ABC, the liver volumes were reproducible at about 1%, and lung volumes 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 study indicated that daily setup variation contributed to the uncertainty in assessing the reproducibility of organ immobilization with ABC between treatment fractions.

Conclusion: The results were encouraging; ABC provides a simple means to minimize breathing motion. When applied for CT scanning and treatment, the ABC procedure requires no more than standard operation of the CT scanner or the medical accelerator. The ABC scans are void of motion artifacts commonly seen on fast spiral CT scans. When acquired at different points in the breathing cycle, these ABC scans show organ motion in three-dimension (3D) that can be used to enhance treatment planning. Reproducibility of organ immobilization with ABC throughout the course of treatment must be quantified before the procedure can be applied to reduce margin for conformal treatment.

Introduction

Significant advances have been made in the development of external beam conformal therapy techniques to improve local control by escalating tumor dose, without a concomitant escalation in toxicity. Most noteworthy is the use of three-dimensional (3D) imaging information from x-ray computed tomography (CT) and other modalities to better localize the disease. A tighter treatment margin can be prescribed such that a higher dose can be delivered to the tumor without increasing deleterious complications. In practice, the treatment margin must account for the potential variation of the treatment geometry. In its Report 50 (1), the ICRU recommends that the concept of a planning target volume (PTV) be adopted when prescribing a treatment margin. The PTV includes the clinical target volume (CTV), accepting the uncertainty in its definition by the physician, and a circumscribing volume to allow for daily field placement or setup error, and for the inter-fraction and intra-fraction variation of the CTV. In addition, the beam aperture designed to cover the PTV must also be adequately widened (defined here as aperture offset) to compensate for the lowered dose in the beam penumbra.

The reduction of the PTV margin is generally desirable in radiotherapy, but must be accompanied by the assurance that the risk of geometric misses does not also increase. To understand these critical issues, a broad range of studies have been made to characterize geometric treatment variation and to properly compensate for their detrimental effects. Promising advances have been made in several areas. Taking advantage of the capabilities of electronic portal imaging devices (EPIDs), decision rules for setup adjustment have been developed to maintain the standard deviation (σ) of setup error to less than 3 mm 2, 3, 4. For prostate conformal therapy, information about inter-fraction organ variation from repeated CT scans has led to improved design of the PTV 5, 6. Recent studies with inverse planning show that by increasing photon fluence at the beam periphery, the beam penumbra can be sharpened, thus allowing the use of a smaller aperture offset to cover the PTV 7, 8.

Organ motion in the thorax and abdomen during breathing, on the other hand, remains problematic. Imaging studies using fluoroscopy and ultrasound have shown that tumors and organs can move by 10 mm, to more than 30 mm, during the breathing cycle 9, 10, 11, 12. The size of the PTV margin in the upper abdominal region can thus be quite large. Approximately 8 mm is needed for daily setup variation, given that the σ is about 4–5 mm; about 10–15 mm is needed to cover the range of organ motion due to breathing. In addition, about 8 mm is needed as aperture offset to ensure that the PTV is enclosed by the 95% isodose level (ignoring the effects of lung inhomogeneity). To the first approximation, these “geometric expansions” can be summed arithmetically, because the margin for organ motion and the aperture offset can be treated as deterministic quantities. The total beam aperture expansion can therefore be 2.5 cm or larger, resulting in significant risk for serious complications when the radiation dose is escalated. Respiratory organ motion also greatly compromises new treatment method utilizing dynamic intensity modulation. The interplay of physiological and machine motions can easily result in > 10% dose inhomogeneity 13, 14. Consequently, the application of advanced treatment techniques to deliver high tumor dose may be deemed unsuitable for malignancies in the thoracic and abdominal regions.

In this paper, we introduce the approach of active breathing control (ABC) to minimize the margin for breathing motion. Briefly, the patient’s breathing is monitored continuously with an ABC apparatus. At a preset lung volume during either inspiration or expiration, airflow of the patient is temporarily blocked, thereby immobilizing breathing motion. The duration of the active breath-hold is that which is comfortably maintained by each individual patient. Radiation will be turned on and off during this period. In this paper, results of the feasibility studies where a prototype ABC apparatus was used to reproducibly immobilize organ motion during breathing are presented.

Section snippets

Methods and materials

With ABC, both the inspiration and expiration paths of the patient’s airflow are temporarily closed at a predetermined flow direction and lung volume. This basic function was demonstrated by modifying a Siemens “servo” ventilator (Servo Ventilator 900C, Siemens Electromedical Group, Danvers, MA).

As shown in Fig. 1 , this ventilator has two separate flow monitors and two “scissor” valves to monitor and control inspiration and expiration independently. During normal operation, the valves are

Results

Figure 4 shows CT images of a patient with Hodgkin’s disease in the transverse, sagittal, and coronal views. The images shown in the vertical panel (a) were acquired during normal breathing. Panel (b) shows the corresponding images acquired with ABC applied for 43 s during deep inspiration. Panel (c) shows those 43 s breath-hold images acquired at the same ABC phase 30 min later. Motion artifacts highlighted by the arrows are clearly noticeable in the normal breathing scans (see panel [a]).

Discussion

Reduction of margin for organ motion associated with breathing in order to facilitate tumor dose escalation is particularly compelling for lung cancer, where the outcome with radiotherapy has been disappointing 15, 16, 17, 18, 19. It is also highly desirable for treatment of other tumors in the thoracic and upper abdominal regions. Higher dose can be used to treat primary and focal hepatic metastasis without inducing radiation hepatitis 9, 20. For radiation therapy of Hodgkin’s disease, less

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    Supported in part by a NCI grant R01 CA76182, and by Elekta Oncology Systems, Ltd.

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