Adaptive and image guided radiation therapy
The goal of modern radiotherapy (RT) is to deliver a lethal amount of dose to cancerous target volumes while sparing the surrounding tissue. Advanced techniques such as intensity-modulated RT (IMRT) allow the delivery of highly conformal dose distributions to static targets. However, patient setup errors, tumor delineation uncertainties and especially organ motion can compromise the clinical benefits of IMRT. In particular, the dose-blurring effects due to target motion undermine the benefits of dose distributions exhibiting steep dose gradients to protect normal tissues. Adaptive RT aims to compensate for these motion effects and to restore the high dose conformity of static IMRT deliveries.
Organ motion is classified into inter- and intra-fractional motion. Inter-fractional organ motion describes relatively slow movements between daily radiotherapy fractions, which are caused by tumor shrinkage, patient weight loss or bladder filling. Intra-fractional organ motion occurs on timescales below one minute, i.e. during the actual radiation delivery, and is caused by respiration or digestion (figure 1). Inter-fractional organ motion can be compensated by repositioning the patient prior to treatment. The more challenging management of intra-fractional organ motion requires to monitor the target motion continuously during treatment and to adapt the dose delivery process in real-time to the reported target position.
Figure 1: Inter-fractional organ motion (top): prostate CTs acquired prior to daily radiotherapy fractions. Intra-fractional organ motion (bottom): 4D CT of a lung cancer patient.
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Monitoring of internal organ motion
On-line x-ray imaging
The Siemens Artiste research linear accelerator (linac) at our department is equipped with on-board x-ray imaging capabilities. The x-ray tube is mounted at a 180°-position with respect to the treatment beam. A Flat-Panel Imager (FPI) is installed between Multi-Leaf Collimator (MLC) and patient table (with its front plane pointing towards the x-ray tube). This unique in-line geometry (figure 2) allows real-time simultaneous monitoring of both the patient anatomy being irradiated and the treatment field. A special post-processing algorithm is employed to separate megavoltage (MV) treatment field signal and kilovoltage (kV) x-ray image. Automatic detection of prominent structures such as markers within the x-ray image then helps to determine the position of the anatomy with respect to the treatment beam allowing for ad-hoc interventions if necessary.
Figure 2: Experimental set-up for intra-fractional x-ray imaging. Phantom motion is detected with the aid of metallic markers embedded in the phantom
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Electromagnetic tracking
Another possibility to monitor the movement of a tumor is electromagnetic tracking. At our department this is done with the Calypso Tumor Tracking System. This system provides real-time position verification of the tumor and gives no additional dose to the patient. To monitor the movement of the tumor during the therapy, three transponders (called beacons) are implanted into the tumor prior to treatment. These beacons are passive resonant circuits with three different resonance frequencies and their position can be detected with a mobile antenna positioned over the patient. The deviation between the actual position of the beacon centroid and the planned position is shown on a monitor for direct interventions and recorded for later data analysis. We use two Calypso systems: a clinical version with 10 Hz update rate for prostate patients in a clinical trial and a research version at our research linac, which provides an update rate of the beacon centroid position of 25 Hz and transfers it as real-time output to other devices.
Figure 3: The Calypso System. Beacon positions relative to the mobile antenna are detected through electromagnetic tracking. The antenna position is detected by an infrared camera system.
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Adaptive dose deliveries
The treatment beam shape of modern linacs is formed by multileaf collimators (MLC). We have developed a research software platform, which enables real-time control of the leaf dynamics of a Siemens 160 MLC mounted on an research Artiste linac [1]. With aid of this control system, we investigate alternative RT techniques like dynamic IMRT or arc-modulated RT, where the collimator leaves or even the complete linac gantry move continuously while the treatment beam is turned on.
Furthermore, we use the system for intra-fractional tumor motion tracking: the collimator aperture is shifted in synchrony with the tumor motion, which is monitored by one of the aforementioned techniques [2]. We verify and test our developments in phantom studies. The experimental setup is displayed in figure 4. A computer controlled motion platform moves the phantom on predefined trajectories, such as prerecorded breathing patterns. Based on the reported target motion, optimum leaf positions are recalculated and the MLC is actuated in real-time. Target motion prediction is applied to compensate for the system latency.
Figure 4: Experimental set-up for tumor tracking. Phantom motion is detected with the Calypso System. The MLC of the research Siemens Artiste linac is adapted in real-time to the reported target motion
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People involved
- Martin Fast, PhD student
- Andreas Krauss, PhD student
- Daniela Schmitt, PhD student
Selected publications
[1] Tacke MB; Nill S; Krauss A; Oelfke U: Real-time tumor tracking: Automatic compensation of target motion using the Siemens 160 MLC. Med. Phys., 37 (2) (2010) 753-761.
[2] Krauss A; Nill S; Tacke MB; Oelfke U: Electromagnetic real-time tumor position monitoring and dynamic multileaf collimator tracking using a siemens 160 MLC: Geometric and dosimetric accuracy of an integrated system. Int. J. Radiat. Oncol. Biol. Phys., 79 (2011) 579-587.