Tumour therapy in four dimensions
Tumour in internal organs, e. g. the lung or the liver, are in constant movement due to the respiration. They can move in all three spatial directions and even twist or tilt. Biophysicists at GSI work on a new method to include these motions of the tumour in ion beam therapy by using four-dimensional CAT scans.
In radiotherapy it is important to aim at the tumour and spare the healthy tissue. But to distribute the irradiated dose homogenously in the tumour is equally crucial to ensure that all areas get the same amount of radiation. Irradiating one spot of the tumour twice and another not at all would result in the tumour not being destroyed and continuing to grow.
Info box - Compensation of movement
The movement of the tumour is monitored during the treatment. The beam is adjusted to the motion in real-time so that it always irradiates the right spot. Due to the complexity of the task to detect the movement of the patient and to steer the accelerator this technique is the most challenging. But it is also fast and very precise.
Especially ion beam therapy only works when the dose is applied homogenously. In the established technology developed at GSI the tumour is divided into layers. The beam scans each layer point by point, thus each millimetre-sized spot of the tumour gets the same dose. This technique has been successfully used on tumours that can be immobilized. The head of a patient can, for example, be fixed with a mask and a brain tumour can be precisely aimed at.
A comparable solution for moving tumours is the goal of medicine physicists at GSI. They examine different techniques that might offer a solution (see infobox). Dr. Christian Graeff, head of "Medicinal Physics" in the GSI biophysics department, works on a new method for compensation of movement. "Up to now a singular dot matrix for scanning the moving tumour was generated by a CAT scan. These data were converted for the different phases of the breathing motion", he explains. "But this data lacks the information about the twisting or tilting of the tumour due to the movement. The irradiation can be optimized by generating a dot matrix for every phase instead."
Info box - Rescanning
The tumour is irradiated several times with small doses in different phases of the breathing motion. "Hot spots" and not irradiated areas are supposed to average out to a homogenous dose distribution. But if areas remain not irradiated the tumour could survive and continue to grow.
Necessary for the method is a four-dimensional CAT scan. The fourth dimension next to the three spatial directions is the time. The breathing motion is divided into a series of pictures and a dot matrix is generated for every single picture. During the irradiation the control system in the computer knows which respiratory phase applies and adjusts the accelerator to aim for the corresponding dot matrix. With an experiment using the GSI accelerator facility the scientists could prove the functionality of the method. They irradiated a film strip mounted on a moving slide. The system compensated the motion and the dose distribution was comparably homogenous to a non-moving film.
Info box - Gating
The position of the tumour in a certain phase of the breathing motion, e. g. the short pause after exhaling, is determined. Only in this moment the irradiation is applied. Gating is technically easy to realize, but the treatment takes longer. Also if the patient breathes irregularly or very shallow the treatment moment cannot be identified.
Using data from CAT scans of lung cancer patients Graeff shows that patients could really profit from the new method in their therapy. A treatment plan for the patients was generated with the conventional technique as well as with his 4D-optimization. "The calculations show the superiority of the optimized plan. It would allow a better treatment", says Graeff. "We hope that in the future we can not only experiment with the method but really make it available to patients."