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Image Guided Radiotherapy (IGRT) – in-room imaging

Investigations on in-room IGRT performed on linacs equipped with kV imaging systems and electronic portal imaging devices (EPID).

Scope and outline
Investigations on in-room IGRT are performed both on the robotic Cyberknife unit and on linacs equipped with kV imaging systems and electronic portal imaging devices (EPID). Here we focus on linac IGRT. MV imaging of the treatment beams is performed with CCD-based EPIDs (Theraview NT, Cablon Medical), also used in our EPID dosimetry program. The kV systems (Synergy, Elekta AB) can be used for acquisition of 2D (planar) images, and 3D or even 4D Cone Beam CT (CBCT) scans. All images are collected and processed in an in-house developed IGRT software platform, used throughout the clinic. Fruitful collaboration with industrial partners has aided both development and clinical introduction of novel techniques.

The main research objective is development of systems and procedures for a guaranteed high positioning accuracy during radiotherapy dose delivery, without significant increases in 1. treatment time, 2. workload, and 3. radiation dose. An important aspect of the research is assessment of residual uncertainties and required planning margins. For set-up corrections, both off-line (NAL, eNAL) and on-line protocols have been developed.

For prostate cancer, a system, designated StereoGraphic Targeting (SGT), has been built for fast and accurate on-line set-up corrections. Gold markers implanted in the prostate are imaged in a crossfire of kV and MV beams. 3D marker positions are then automatically extracted from these orthogonal images, and deviations from the intended position are immediately transferred to the Theraview Couch Set-up Assistant (TCSA, Cablon Medical) for remote couch adjustments. Daily, on-line patient re-positioning based on SGT drastically reduces positioning errors (Fig. 1), and takes less than one minute.  For the cervix cancer, deformation of the target area and surrounding healthy tissues is quantified using both multi-2D imaging of dedicated implanted markers as well as CBCT scans. In an on-going project, prediction of the target position from ultrasound measured bladder volumes is being investigated. For stereotactic liver treatments, tumor motion and average position is found from tracking markers in fluoroscopic movies. (see also Stereotactic Body Radiation Therapy (SBRT) for Treatment of Liver Tumors). IGRT techniques are also being developed for highly focused irradiation of only the involved vocal cord in early glottic carcinoma. Here, 4D imaging of the cords’ breathing motion is combined with CBCT-based on-line set-up corrections to achieve a daily positioning accuracy better than 1 mm. In a recent breast study, it was demonstrated that surgical clips in the lumpectomy cavity, imaged with kV, are representative for position verification of both the tumor bed and the whole breast in simultaneously integrated boost (SIB) treatments. In 30% of the patients, time trends larger than 3 mm were present over the course of treatment, resulting in superior off-line correction with the eNAL protocol, which was especially designed for coping with time trends. 
 

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Fig. 1.Cumulative distribution of daily positioning errors of the prostate, before and after the SGT procedure.


Contact
M. Hoogeman, PhD and J.J.M.E Nuyttens, MD, PhD