With the rapid development of science and technology, accurate positioning has gradually been integrated into our lives. Through this technology, takeaway ordering, self-driving travel, etc. have become extremely convenient. When precise localization extends to the field of radiation therapy in medical technology, we have to mention the "golden eye of fire" in precision radiation therapy - IGRT image-guided radiation therapy.
▲ Radiotherapy center registration
Image-guided radiation therapy (IGRT) is an imaging technology that uses a variety of imaging devices to monitor a patient's tumor and normal organs in real time during treatment, and then adjust the irradiation range in time. Many factors such as human respiratory movements, gastrointestinal peristalsis movements, positioning errors of each radiotherapy session, and shrinking of the tumor target area can cause the distribution of the actual radiation therapy dose to be very different from when the radiotherapy plan was formulated. IGRT can accurately control the displacement error between the movement of anatomical tissues during treatment and fractional treatment, and can synchronously adjust the treatment conditions according to the changes in the position of the patient's organs, so that the irradiation field can closely "follow" the target area, so that the treatment can be accurate in the true sense.
At present, a number of equipment manufacturers have launched various types of image-guided radiotherapy technologies, from electron field imaging system (EPID) to KV-level cone beam CT (CBCT); from In-room CT combination direct addition to Exactrac X-ray; from MV-level CBCT to ultrasound-guided radiotherapy; from MR image-guided radiotherapy to future PET/CT image-guided radiotherapy.
In the face of a variety of image guidance methods, we must not only consider the accuracy of its positioning, but also consider the balance of precise management of radiotherapy doses to meet the needs of patients for precision radiotherapy. At present, the main radiotherapy image guidance methods used in our hospital include electron field imaging system, KV-level cone beam CT, respiratory gating treatment system, etc. Our photon radiotherapy equipment includes the Varian TrueBeam linear accelerator, which integrates two imaging systems, EPID and KV-CBCT, and is also equipped with respiratory gating technology for detecting respiratory movements.
▲ Varian TrueBeam linear accelerator used by the radiotherapy department
Electronic field imaging system
The Electronic Portal Imaging Device (EPID) is one of the earliest imaging verification techniques and one of the most widely used imaging techniques available today. It is typically validated by 6 MV megawatt x-rays, with better image quality at smaller doses. It has the advantages of small size, high resolution, high sensitivity, and wide energy response range. Clinically, filming is simple and inexpensive. It can not only verify the size, shape, position and patient position of the field offline calibration, but also directly measure the dose in the field, which is a simple and practical two-dimensional image verification device. The disadvantage is that the contrast between the shoot field bone and the air is low, the soft tissue imaging is not clear, and it is more dependent on the subjective judgment of the operator.
KV grade conical beam CT
Cone-beam computed tomography (CBCT) imaging technology is the most widely used radiotherapy image guidance technology. It uses a large area of amorphous silicon digital X-ray detection board, and the frame can be rotated in one turn to acquire and reconstruct CT images within a certain volume range. The reconstructed 3D image model of ct images in this volume can be matched and compared with the patient model of the Treatment Planning System (TPS) and the parameters that need to be adjusted by the treatment bed are automatically calculated.
CbCT is small, lightweight, open architecture and can be integrated directly into linear accelerators. CBCT's image quality has high spatial resolution and is simple and fast to operate. Its volumetric imaging capability facilitates the rapid completion of in-line correction of treatment locations, so it is used clinically with high frequency. At the same time, it also has the functions of X-ray fluoroscopy and filming at the treatment location.
▲ CBCT images are registered with traditional CT images; KV quadrature X-ray images are registered with traditional digitally reconstructed X-ray images
Respiratory gating technology
The implementation of Respiratory gating for scanners (RGSC) requires the acquisition of images of patients when they are positioned in CT simulations and the analysis of the association of these images with respiratory movements. When implementing radiation therapy, the technology will automatically trigger the radiation beam and closing beam of the treatment machine within the set allowable movement range according to the cycle of the target movement of the patient's lesion, or control the movement of the treatment bed to ensure the spatial consistency of the irradiation field and the tumor target area, and realize real-time dynamic and accurate radiotherapy.
The TrueBeam system's respiratory gating technology can accurately target targets by tracking the patient's respiratory rate, reducing the side effects of radiation therapy and achieving better treatment results. The main indications of this technology are lung, liver, breast, abdominal tumors and other organs and tumors that are greatly affected by breathing. At present, the radiotherapy department of our hospital has used this technology to carry out more than a dozen cases of deep inspiration breath-hold (DIBH) precision radiotherapy in the left breast. In the future, all left breast cancer patients admitted to the radiotherapy department of our hospital can use DIBH technology to reduce the dose of cardiac exposure and reduce the incidence of various complications after treatment.
▲ In DIBH radiotherapy, the patient's thoracic movement waveform
▲ During the whole course of multiple radiotherapy, there was a significant difference in the accuracy of treatment without IGRT (top) and between IGRT (bottom).
It is important to note that in order to make each radiation therapy more accurate, the radiotherapy department performs CBCT or KV orthogonal imaging before each treatment. Although this mode of work brings a lot of work to therapists and doctors, colleagues work tirelessly to maximize the benefits of patients.
Written by: Zhou Qiaomin, Department of Radiotherapy, Guangzhou Taihe Cancer Hospital
Reviewer: Guangzhou Taihe Cancer Hospital Radiotherapy Department Yuan Taize