Target motion compensation
Motivation
Currently the treatment performed in the therapy project is limited to immobilized tumor sites mainly in the head and neck and along the spinal cord. In the future we plan to extend treatment to periodically moving sites such as tumors in thorax or abdomen which are influenced by respiration. Clinical practice for radiotherapy of these sites is the expansion of the treated volume with large enough margins to cover the tumor volume in all motion states. This approach leads to increased involvement of normal tissue and does not work for scanned beams because the motion of the scanning process and the target motion lead to an interplay effect which destroys dose homogeneity. For a scanned beam application (rasterscanner) the three main compensation methods are:
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Because of the drawbacks of the first two methods (larger treatment volume and increased irradiation time) we focus on online motion compensation.
Material and Methods
Motion compensation has to be three-dimensional, e.g. the position of the Bragg maximum has to be changed at time of treatment in the lateral plane and in depth. The lateral position can be changed the scanning magnets. Depth changes require energy modulation which presently is not possible by the accelerator in the time frame of respiratory motion (periods 2-6s). Instead we currently use a pair of lucite wedges mounted on linear motors between isocenter and beam exit window (Fig. 1).
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| Grözinger, PhD, TU Darmstadt, 2004 |
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fig. 1
Compensation system: The position of the Bragg maximum can be changed laterally by adjusting the currents in the scanning magnets. Quick depth changes are possible by changing the wedge position using linear motors. |
To allow an online compensation the tumor position has to be detected during treatment. Several commercial products exist for detection of a breathing cycle surrogate such as height of the chest wall, air flow, or temperature of the breathed air. Alternatively the position of implanted radio-opaque markers can be detected by fluoroscopy. Presently we have not decided between the different approaches.
Finally dedicated treatment planning is necessary. According to the treatment geometry (patient position with respect to beam direction) the geometrical tumor movement has to be translated into the required geometrical lateral compensation and the longitudinal density change which leads to the required change in wedge positions. These calculations can not be performed in parallel to treatment since the treatment speed is much greater than the required calculation time. Instead we base treatment planning on the information from time resolved computed tomography (4DCT) and precalculate the required compensation vectors.
Results
The technical feasibility of motion compensation has been shown experimentally (PhD-Thesis SO Grözinger). Fig. 2 shows film responses for a) static irradiation b) irradiation with moving film, and c) irradiation with moving film and motion compensation. The comparison of a) and c) indicates that motion compensation can restore the static result for a moving target. The distorted response in b) displays the impact of interplay between film and scanning motion. The longitudinal compensation accuracy was successfully shown in experiments using the water-column setup also used for treatment quality assurance.
| a) static film (reference) | b) moving film without compensation | c) moving film with compensation |
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| Grözinger, PhD, TU Darmstadt, 2004 |
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fig. 2 Film responses for a) irradiation of a static film (as reference), b) irradiation of a sinusoidal moving film where the interplay between scanning and film motion leads to a distorted response pattern, and c) irradiation of a sinusoidal moving film with motion compensation. The response of the static reference a) is widely restored. |
The GSI treatment planning suite TRiP has been extended to optimize parameters for motion compensated treatment and for dose calculation in the presence of target motion (PhD-Thesis Christoph Bert). The dose calculation approach is not restricted to motion compensated irradiations, but can also calculate physical dose and film response for irradiations without additional means. Fig. 3 shows an animation for irradiation of a lung tumor with and without motion compensation. The time dependency of the dose deposition is modelled and shown along to the 4DCT. Fig. 4 shows simulated dose distributions for a lung tumor for online motion compensation and the use of increased margins, only.
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| Bert, PhD, TU Darmstadt, 2006 | |
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fig. 3 Intermediate steps of treatment planning. Tumor motion is sampled by 4DCT. For each phase the sub-dose is calculated and summed up in the reference motion phase at inhale (0%). Without motion compensation misdosage occurs. Motion compensation leads to a very homogeneous dose distribution within the target volume. |
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a) static irradiation at end-exhale |
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b) motion compensated |
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c) increased margins w/o compensation |
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| Bert, PhD, TU Darmstadt, 2006 | ||
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fig. 4 Sagittal view through CT and dose distribution of a lung tumor simulation (tumor volume contoured in white). a) As reference dose distribution the static irradiation of the 4DCT frame at end-exhale is used. b) Simulation of motion compensated irradiation of the moving tumor. c) Expansion of the tumor volume to cover all motion states and irradiation without compensation. Due to the interplay the deposited dose is distorted also within the tumor volume. |
contact: Christoph Bert