Abstract
This study proposes a respiratory compensating system which is mounted on the top of the treatment couch for reverse motion, opposite from the direction of the targets (diaphragm and hemostatic clip), in order to offset organ displacement generated by respiratory motion. Traditionally, in the treatment of cancer patients, doctors must increase the field size for radiation therapy of tumors because organs move with respiratory motion, which causes radiation‐induced inflammation on the normal tissues (organ at risk (OAR)) while killing cancer cells, and thereby reducing the patient's quality of life. This study uses a strain gauge as a respiratory signal capture device to obtain abdomen respiratory signals, a proposed respiratory simulation system (RSS) and respiratory compensating system to experiment how to offset the organ displacement caused by respiratory movement and compensation effect. This study verifies the effect of the respiratory compensating system in offsetting the target displacement using two methods. The first method uses linac (medical linear accelerator) to irradiate a 300 cGy dose on the EBT film (GAFCHROMIC EBT film). The second method uses a strain gauge to capture the patients' respiratory signals, while using fluoroscopy to observe in vivo targets, such as a diaphragm, to enable the respiratory compensating system to offset the displacements of targets in superior‐inferior (SI) direction. Testing results show that the RSS position error is approximately 0.45 ~ 1.42 mm, while the respiratory compensating system position error is approximately 0.48 ~ 1.42 mm. From the EBT film profiles based on different input to the RSS, the results suggest that when the input respiratory signals of RSS are sine wave signals, the average dose (%) in the target area is improved by 1.4% ~ 24.4%, and improved in the 95% isodose area by 15.3% ~ 76.9% after compensation. If the respiratory signals input into the RSS respiratory signals are actual human respiratory signals, the average dose (%) in the target area is improved by 31.8% ~ 67.7%, and improved in the 95% isodose area by 15.3% ~ 86.4% (the above rates of improvements will increase with increasing respiratory motion displacement) after compensation. The experimental results from the second method suggested that about 67.3% ~ 82.5% displacement can be offset. In addition, gamma passing rate after compensation can be improved to 100% only when the displacement of the respiratory motion is within 10 ~ 30 mm. This study proves that the proposed system can contribute to the compensation of organ displacement caused by respiratory motion, enabling physicians to use lower doses and smaller field sizes in the treatment of tumors of cancer patients.PACS number: 87.19. Wx; 87.55. Km
Highlights
In cancer treatment, in vivo organ movements caused by respiratory motion would result in a considerable impact for physicians in clinical diagnosis and radiation therapy
The physician must rely on other technologies, including four-dimensional computed tomography (4D CT), to determine the location of the tumor, as well as the field and size for radiation therapy.[1,2,3,4,5,6,7] A number of research teams have attempted to use fluoroscopy, magnetic resonance imaging (MRI), intensity-modulated radiation therapy (IMRT), and 4D CT approaches to analyze the displacements of different tumors, organs, and areas caused by respiratory motion.[8,9,10,11,12,13,14,15,16,17] These studies found that, the superior–inferior (SI) organ displacement is greater than the anterior– posterior (AP) and medial–lateral (ML) displacement, and the displacement is greater when closer to the diaphragm.[18]
If the movement distance is greater, the dose received in the neighboring regions is greater, which increases damage to the organs surrounding the tumor of the patient under radiation therapy
Summary
In vivo organ movements caused by respiratory motion would result in a considerable impact for physicians in clinical diagnosis and radiation therapy. In 2006, D’Souza and McAvoy[30] used an existing treatment couch (Hexapod) to study the couch dynamics, and created an internal model controller to simulate feedback control of respiration-induced motion. Their motion data were obtained by using a skin marker placed on the abdomen of the patient to track the tumor motion. They proposed a real-time couch compensation system by using a stereoscopic infrared camera system interfaced to a commercial robotic couch (Hexapod) to address the tumor motion problem. In 2012, Haas et al[27] presented a couch-based active motion compensation system
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