Definition

Radiation therapy is a form of cancer treatment using ionizing radiation. Every irradiation of a patient is preceded by a sophisticated treatment planning to determine the optimal treatment parameters for this individual patient. The subject of adaptive radiation therapy is a modification in any of the initially planned parameters during the treatment course to re-optimize the treatment as reaction to occurring unavoidable changes.      

Motivation

The goal of an optimized radiation therapy is to apply as much as possible radiation dose to tumor volumes and simultaneously to spare adjacent healthy tissue and organs at risk. Safety margins are applied to account for positioning uncertainties as well as anatomical deformations to ensure dose coverage of the target volume. To reduce this additional safety margins, an adaptation is necessary to compensate the changes in the applied dose distribution caused by the occurring geometrical variations.

The detection and quantification of those occurring geometrical changes during the treatment course is frequently realized by accompanied imaging of the patient combined with image registration methods. All adaptive strategies using imaging modalities are referred to as image-guided radiation therapy (IGRT) techniques.

In fractionated radiation therapy, which is subdivided into multiple fractions over several weeks, inter-fractional uncertainties, like repeated re-positioning accuracy or daily organ position variations, are in the center of attention. In cases where the prescribed dose is applied in only few fractions or even in a single one intra-fractional uncertainty becomes crucial. This type of treatment is frequently used for lung or abdominal cancer treatment where the intra-fractional uncertainty is governed mainly by the breathing motion.

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Challenges - Interfractional motion

Typically, the correction parameter is a shift of the treatment table to re-position the shifted planned target point to the isocenter of the treatment device. The target point correction is a wide-spread IGRT strategy and has proven its superiority in treatments of tumor sites with less pronounced deformations. Under deforming anatomy, however, it is still unclear how to deduce an optimal rigid shift correction. Currently, only geometrical position information is consulted prior to TPC application. But the desired information is its impact on the changes of the dose distribution. 
We investigate inter-fractional variations in collectives of patients with treated tumor sites prone to deformations and their effect onto the difference between the prescribed and applied dose with and without correction strategies.

Dose accumulation over an extended period of time with underlying deformations in patient’s anatomy is still an intensively investigated issue.  It is unclear how uncertainties of deformation extraction translates to dose uncertainties and how volume changes coming along with deformations can appropriately be incorporated into the geometrical accumulation process. 
We investigate uncertainties associated with deformable image registration methods and explore their influence on dose accumulation methods.

Challenges - Intrafractional motion

Respiratory induced tumor motion poses a challenge in the application of modern three-dimensional conformal radiotherapy.
Based on fast 2-dimensional fluoroscopy scans the varying position of the tumor during the respiratory cycle can be assessed. A gating technique can be used to switch on the radiation beam only when the tumor is in a predefined region called “gating window”. This way the residual motion of the tumor is kept small.
We explore and develop a system to account for breathing induced tumor motion and by utilizing fluoroscopic images to generate trigger signals for a gated treatment.

The precise and immediate knowledge of the current tumor position is crucial to adapt the radiation beam according to occurring breathing motion. Tumor motion can be extracted from fluoroscopic video with registration techniques. This way the tumor is tracked by the beam and thus additional safety margins to cover the breathing induced motion of the tumor can be reduced. Tracking techniques may even help to spare healthy lung tissue, too.
We explore and develop methods to extract the motion of the tumor in fluoroscopic images of lung cancer patients.

Precise management of intrafractional motion needs to be accomplished by a good reproducibility of the tumor motion pattern. Although respiration consists of a cyclic repetition of inspiration and expiration, it should be regarded as spontaneous motion, since changes in the motion pattern (e.g. from coughing) need to be considered as well. Breathing coaching is a promising technique to improve reproducibility and stability of the breathing course. 
We explore and develop a system for breathing coaching, which provides patient-individualized instructions. Visual instructions are provided using video goggles.

People involved:
Rolf Bendl
Michael Schwarz
Kristina Giske
Hendrik Teske

Alumni:
Urban Malsch
Gerhard Lechsel
Sven Skobowski
Franziska Herget
Jens Schwanke
Christian Streibl

You want to know more? …. Get in touch with Prof. Dr. Rolf Bendl

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