The growth pattern and site of tumor are complex, and the radiation therapy irradiation field should include all tumor tissues and lymphatic drainage area as well as a certain range of peripheral margins, also called safety margins. In order to achieve the requirement of consistent shape of radiation volume and target volume, while avoiding unnecessary irradiation to normal tissues, most of the irradiation fields are irregular in shape, and in the past clinical radiotherapy practice, the low soluble point lead block technique was generally used to implement radiotherapy with irregular irradiation fields.
In the 1940s, semi-automatic primitive multileaf grating (MLC) technology or low-solution lead block was applied to implement the most primitive conformal radiotherapy with multiple irregular fields under the guidance of two-dimensional radiotherapy planning, and this technology has been used in the clinic for half a century. Due to advances in computer technology, radiation physicists have replaced the hand-made lead baffles with more advanced multileaf gratings for radiation shaping purposes, using computer-controlled multileaf gratings to control the shaping of the multileaf gratings, which can be fully automated by changing the orientation of the blades as the gas pedal frame rotates to adjust the field shape according to the shape of the target volume at different views.
It raises the conformal radiotherapy technology to a new level. In recent years, computer processing of diagnostic imaging images has enabled three-dimensional reconstruction of radiotherapy target areas and adjacent important tissues and organs in the human body, thus realizing three-dimensional conformal radiotherapy under the guidance of three-dimensional radiotherapy planning in clinical practice. It is now used in more and more hospitals and oncology treatment centers worldwide for clinical practice of radiation oncology and is gradually being incorporated into routine applications.
Compared with the radiotherapy techniques for head and neck tumors, the physiological movements of the thorax and abdomen affect the 3D reconstruction of images and the accuracy of radiotherapy planning.
Therefore, the requirements for 3D conformal radiotherapy techniques for tumors of the trunk are relatively high. the ICRU50 report provides a detailed description of tumor volume, clinical target volume, planned target volume, and standardization of treatment prescription. Broadly speaking, radiotherapy in which the radiation dose volume is consistent with the shape of the target volume based on three-dimensional image reconstruction and implemented under the guidance of three-dimensional treatment planning should be called three-dimensional conformal radiotherapy. However, the equipment and apparatus used to perform 3D conformal radiotherapy for head tumors with the stereotactic radiosurgery [SRS] system are different from those used for 3D conformal radiotherapy for trunk tumors, and there are also some differences in operation techniques. In fact, SRS, FSRT, SRT, 3D-CRT and stereotactic brachytherapy should all belong to the category of stereotactic radiotherapy.
The implementation of three-dimensional conformal radiotherapy is mainly supported by the following four technical supports.
[1] multileaf grating system MLC, which has various types, including manual, semi-automatic and fully automatic. The uses of the MLC system are: replacing lead blocks; simplifying the shaping process of irregular irradiation fields so that the number of fields can be increased to improve shielding of normal organ structures; applying a static irradiation field of the multileaf grating and a single rack angle can be used to adjust beam flatness; the blades can be moved during rack rotation to accommodate dynamic adjustment of irregular tumor shape.
The [2] 3D radiotherapy planning system, which is mainly characterized by treatment display based on 3D reconstruction of CT images. Beameyeview (BEV), for example, can display the degree of conformity between the irradiation field shape and tumor shape at any angle of radiation incidence and the shielding of adjacent critical structures, which is a key function to realize “conformal irradiation”. The Room-view (RV) function, which displays the treatment in any orientation within the treatment room, compensates for the lack of beam-view display of BEV, especially when setting the center depth of the beam, and allows for appropriate geometric adjustment of the treatment technique. The dose-volume histogram display [DVH] function shows the rationality of the treatment plan, the isodose curve including the treatment volume status, and an evaluation of the entire protocol.
The [3] computer-controlled radiation therapy machines, the new generation of linear gas pedals, some of the high block cobalt 60 treatment machines and post-mounted treatment machines are computer-controlled.
[4] Positioning fixation and validation systems, mainly body fixation frames for increasing the accuracy of repetitive positioning, head and neck fixation frames, thermosorbable masks, vacuum pads and devices for limiting visceral movement; confirmation images of the irradiation field and some validation equipment. Although the clinical application of 3D conformal radiotherapy technique obtains a uniform distribution of high dose radiation within the target area while minimizing the irradiation of normal tissues; theoretically it can greatly improve the local control rate of tumors, an important problem encountered in clinical practice is: how to determine the extent of the treatment volume? The recognition and determination of the treatment volume margin depends largely on the imaging technology and the operator’s level of image reading. Therefore, in 3D conformal radiotherapy, the accuracy of the determination of the treatment volume is closely related to the recognition of the tumor extent. Obviously, modern diagnostic imaging technology has a crucial role in the implementation of 3D conformal radiotherapy.
Intensity-modulated radiotherapy [IMRT] is the abbreviation of three-dimensional conformal radiotherapy.
The advantages of IMRT over conventional radiotherapy are.
[1] precise body fixation and stereotactic positioning techniques are used.
It improves the positioning accuracy, positional accuracy and irradiation accuracy of radiotherapy.
[2] The use of precise treatment planning.
Inverse calculation, i.e., the physician first determines the maximum optimized planning result, including the irradiation dose to the target area and the tolerated dose to the sensitive tissues around the target area, and then the computer gives the method and parameters to achieve this result, thus achieving automatic optimal optimization of the treatment plan.
IMRT can satisfy the radiotherapist’s desire for the “four most”: maximum dose to the target area, maximum dose to the surrounding tissue, and maximum dose to the target area. IMRT can meet the “four most” desires of radiologists: the highest dose to the target area, the lowest dose to the surrounding normal tissue, the most accurate positioning and irradiation of the target area, and the most uniform dose distribution in the target area. The clinical results are: significantly increase the local control rate of tumor and reduce the radiation damage to normal tissues.
The main implementations of IMRT include.
[1] two-dimensional physical compensator for intensity modulation, and
[2] multileaf collimator static intensity modulation, and
[3] multileaf collimator dynamic intensity modulation, [4] tomographic intensity modulation
[4] tomographic intensity modulation of radiotherapy, [5] electromagnetic scan intensity modulation, [6] electromagnetic scan intensity modulation
[5] electromagnetic scanning intensity modulation radiotherapy, etc. At present, the more common clinical application is the electric multileaf grating intensity modulation technology. Zelefsky et al. used IMRT and 3D-CRT to treat patients with prostate cancer, and the dose distribution in the target area was significantly better than that of 3D-CRT at the same prescribed dose [81 Gy]; the incidence of radiation damage in early and late stages of rectal cancer was also significantly lower in the IMRT group than in the 3D-CRT group. The incidence of radiation damage in the IMRT group was also significantly lower than that in the 3D-CRT group. The use of IMRT for the treatment of head and neck tumors can not only better protect the parotid gland, brain stem and other vital organs, but also further improve the efficacy if the small field additional dose [SIB] technique is used. The use of IMRT technology for post-breast-conserving radiotherapy for breast cancer can improve the dose distribution in the target area and provide better protection for the lung and heart. Several units in China have used IMRT technology for radiotherapy of nasopharyngeal, breast, esophageal, and lung cancers, with positive preliminary findings. There is no doubt that IMRT will become the mainstream modality of radiation therapy in the future.