International Journal of Radiation Oncology*Biology*Physics
Physics ContributionsThe use of active breathing control (ABC) to reduce margin for breathing motion☆
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
References (33)
- et al.
High-precision prostate cancer irradiation by clinical application of an offline irradiation by clinical application of an offline patient setup verification procedure, using portal imaging
Int J Radiat Oncol Biol Phys
(1996) - et al.
A method of incorporating organ motion uncertainties into three-dimensional conformal treatment plans
Int J Radiat Oncol Biol Phys
(1996) - et al.
The value of nonuniform margins for six-field conformal irradiation of localized prostate cancer
Int J Radiat Oncol Biol Phys
(1995) - et al.
Analysis of movement of intrathoracic neoplasms using ultrafast computerized tomography
Int J Radiat Oncol Biol Phys
(1990) - et al.
ASTRO plenary: Effect of chemotherapy on locally advanced non-small cell lung carcinoma: A randomized study of 353 patients
Int J Radiat Oncol Biol Phys
(1991) - et al.
Medically inoperable lung carcinoma: The role of radiation therapy
Semin Radiat Oncol
(1996) - et al.
Irradiation synchronized with respiration gate
Int J Radiat Oncol Biol Phys
(1989) - et al.
Clinical experience with a system for pediatric respiratory gated radiotherapy
Int J Radiat Oncol Biol Phys
(1998) - et al.
Deep inspiration breath-hold technique for lung tumors: The potential value of target immobilization and reduced lung density in dose escalation
Int J Radiat Oncol Biol Phys
(1996) - et al.
How important is breathing in radiation therapy of the thorax?
Int J Radiat Oncol Biol Phys
(1982)
A comparison of intensity modulated conformal therapy with a conventional external beam stereotactic radiosurgery system for the treatment of single and multiple intracranial lesions
Int J Radiat Oncol Biol Phys
Adaptive modification of treatment planning to minimize the deleterious effect of treatment setup error
Int J Radiat Oncol Biol Phys
The influence of scatter on the design of the optimized intensity modulators
Med Phys
Intensity modulation optimization, lateral transport of radiation and margins
Med Phys
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Supported in part by a NCI grant R01 CA76182, and by Elekta Oncology Systems, Ltd.