Radiotherapy is one of the most important methods used for the treatment of cancer. Irradiating a moving target is also one of the most challenging tasks to accomplish in modern radiotherapy. We have developed a tracking system by modifying an add-on collimator, the Siemens Moduleaf, for realtime applications in radiotherapy. As the add-on collimator works nearly completely independently of the linear accelerator (LinAc), no modifications to the latter were necessary. The adaptations to the Moduleaf were mainly software-based. In order to reduce the complexity of the system, outdated electronic parts were replaced with newer components where practical.Verification was performed by measuring the latency of the system as well as the impact on applied dose to a predefined target volume, moving in the leaf's travel direction. Latency measurements in the software were accomplished by comparing the target and current positions of the leaves. For dose measurements, a Gafchromic EBT2 film was placed beneath the target 4D phantom, in between solid water plates and moved alongside with it. Comparing the dose distribution on the film with a moving target between 'tracking disabled' towards 'tracking enabled' functions resulted in penumbra widths of 23 mm to 4 mm for 0.1 Hz sinusoidal movements with an amplitude of 32 mm, respectively. The maximum speed was therefore 20 mm s(-1). Latency was measured to be less than 50 ms for the signal runtimes. Based on the results, a tracking-capable add-on collimator seems to be a useful tool for reducing the margins for the treatment of small, slow-moving targets.

Clinical evaluation of an optical three-dimensional surface scanning (3D-SS) system for patient positioning and monitoring during radiotherapy (RT) for breast cancer.

PURPOSE Solid state flat panel electronic portal imaging devices (EPIDs) are widely used for megavolt (MV) photon imaging applications in radiotherapy. In addition to their original purpose in patient position verification, they are convenient to use in quality assurance and dosimetry to verify beam geometry and dose deposition or to perform linear accelerator (linac) calibration procedures. However, native image frames from amorphous silicon (aSi:H) detectors show a range of artifacts which have to be eliminated by proper correction algorithms. When a panel is operated in free-running frame acquisition mode, moving vertical stripes (periodic synchronization artifacts) are a disturbing feature in image frames. Especially for applications in volumetric intensity modulated arc therapy (VMAT) or motion tracking, the synchronization (sync) artifacts are the limiting factor for potential and accuracy since they become even worse at higher frame rates and at lower dose rates, i.e., linac pulse repetition frequencies (PRFs). METHODS The authors introduced a synchronization correction method which is based on a theoretical model describing the interferences of the panel's readout clocking with the linac's dose pulsing. Depending on the applied PRF, a certain number of dose pulses is captured per frame which is readout columnwise, sequentially. The interference of the PRF with the panel readout is responsible for the period and the different gray value levels of the sync stripes, which can be calculated analytically. Sync artifacts can then be eliminated multiplicatively in precorrected frames without additional information about radiation pulse timing. RESULTS For the analysis, three aSi:H EPIDs of various types were investigated with 6 and 15 MV photon beams at varying PRFs of 25, 50, 100, 200, and 400 pulses per second. Applying the sync correction at panels with gadolinium oxysulfide scintillators improved single frame flood field image quality drastically [improvement of the signal-to-noise ratio (SNR) up to 66.1 dB for 6 MV and 66.0 dB for 15 MV]. Also for the EPID with a caesium iodide scintillator, the noise for the lower PRFs could be reduced (SNR at 6 MV of up to 56.3 dB and at 15 MV up to 46.7 dB). However, the simplistic readout interference model fails at higher PRFs, where image lag and ghosting effects due to trapped charges in the thin film transistor and scintillator postglowing require additional corrections. CONCLUSIONS The presented free-running sync correction method improves SNR of single frames and enables imaging applications, like low-dose rate imaging at increased image frame rates (e.g., to track moving gold fiducials in the lung). Adaptive image guided radiotherapy protocols become even feasible in VMAT plans. Also simultaneous kilovolt and MV imaging applications can benefit from new possibilities of MV scatter removal in x-ray images.

Amorphous silicon (aSi:H) flat panel detectors are prevalent in radiotherapy for megavoltage imaging tasks. Any clinical and dosimetrical application requires a well-defined dose response of the system to achieve meaningful results. Due to radiation damages, panels deteriorate and the linearity of pixel response to dose as well as the stability with regard to changing operating temperatures get worse with time. Using a single level gain correction can lead to an error of about 23% when irradiating a flood field image with 100 MU min(-1) on an old detector. A multi-level gain (MLG) correction is introduced, emending the nonlinearities and subpanel-related artifacts caused by insufficient radiation hardness of amplifiers in the read-out electronics. With rising temperature, offset values typically increase (up to 300 gray values) while the response at higher dose values per frame remain constant for a majority of pixels. To account for temperature-related image artifacts, two additional temperature correction methods have been developed. MLG in combination with temperature corrections can re-establish the aSi:H image quality to the performance required by reliable medical verification tools. Furthermore, the life span and recalibration intervals of these costly devices can be prolonged decisively.

Purpose: We developed and evaluated a correction strategy for prostate rotations using direct adaptation of segments in intensity-modulated radiotherapy (IMRT).Method and Materials: Implanted fiducials (four gold markers) were used to determine interfractional translations, rotations, and dilations of the prostate. We used hybrid imaging: The markers were automatically detected in two pretreatment planar X-ray projections; their actual position in three-dimensional space was reconstructed from these images at first. The structure set comprising prostate, seminal vesicles, and adjacent rectum wall was transformed accordingly in 6 degrees of freedom. Shapes of IMRT segments were geometrically adapted in a class solution forward-planning approach, derived within seconds on-site and treated immediately. Intrafractional movements were followed in MV electronic portal images captured on the fly.Results: In 31 of 39 patients, for 833 of 1013 fractions (supine, flat couch, knee support, comfortably full bladder, empty rectum, no intraprostatic marker migrations >2 mm of more than one marker), the online aperture adaptation allowed safe reduction of margins clinical target volume-planning target volume (prostate) down to 5 mm when only interfractional corrections were applied: Dominant L-R rotations were found to be 5.3 degrees (mean of means), standard deviation of means +/- 4.9 degrees, maximum at 30.7 degrees. Three-dimensional vector translations relative to skin markings were 9.3 +/- 4.4 mm (maximum, 23.6 mm). Intrafractional movements in 7.7 +/- 1.5 min (maximum, 15.1 min) between kV imaging and last beam's electronic portal images showed further L-R rotations of 2.5 degrees +/- 2.3 degrees (maximum, 26.9 degrees), and three-dimensional vector translations of 3.0 +/- 3.7 mm (maximum, 10.2 mm). Addressing intrafractional errors could further reduce margins to 3 mm.Conclusion: We demonstrated the clinical feasibility of an online adaptive image-guided, intensity-modulated prostate protocol on a standard linear accelerator to correct 6 degrees of freedom of internal organ motion, allowing safe and straightforward implementation of margin reduction and dose escalation. (C) 2012 Elsevier Inc.

In EBRT of prostate cancer, safety margins can be reduced by the means of interfractional correction following IGRT, hence reducing doses to organs at risk. An innovative method for further rectal dose reduction was successfully introduced by spacer materials injected between prostate and rectum. Aim of the study was the investigation of the spacer's influence on inter- and intrafractional movements of the prostate. In a prospective matched-pair comparison, eight patients were investigated up to now. In all of them, four gold marker fiducials were implanted perineally into the prostate under endorectal sonographic guidance. Four of the patients additionally received a rectal spacer consisting of 10 mL polyethylene glycol (PEG) hydrogel. Fiducials were used to determine translations and rotations of the prostate. During 7 field IMRT, we captured MV portal images on the fly and registered them by means of a panel flex correction procedure. Two-dimensional marker positions were automatically detected by means of a marker kernel convolution algorithm. Three-dimensional marker positions were reconstructed from two oblique projections. Daily analysis of first pair of MV images (gantry angles 220° and 265°) was done to determine interfractional movements. Intrafractional movements were derived from shifted marker positions in the last pair of MV portal images (gantry angles 95° and 140°). The time gap in between the two reconstructions was 4.4 min on average. Eight hundred eighty-four images gained during 221 fractions (supine, flat couch, knee support, comfortably full bladder, empty rectum, no intraprostatic marker migrations >2 mm of more than one marker) were analyzed: 95 fractions in the spacer group and 126 fractions in the control group, respectively. Interfractional 3D vector translations and L-R rotations were found to be 9.4 +/- 4.5 mm / 5.5° +/- 6.4° with spacer, 7.5 +/- 3.5 mm / 2.2° +/- 2.9° without spacer, respectively.(mean of means). Intrafractional movements were 1.9 +/- 2.1 mm / 2.3° +/- 2.7° in the spacer cohort, 1.9 +/- 1.1 mm / 2.0° +/- 2.8° in the control cohort. Differences were statistically not significant. Our findings compare to previously published 9.3 +/- 4.4 mm / 5.3° +/- 4.9° inter- and 3.0 +/- 3.7 mm / 2.5° +/- 2.3° intrafractional movements of 39 patients / 833(1013) fractions without spacer. Distensions of the anterior rectal wall could successfully be provided in all spacer patients. Spacers so far did not show a stabilizing effect on motion degree and direction between and during treatment fractions. Both cohorts will be completed to a sample size of 20 each. Whereas spacer application leads to a significant dose reduction in the anterior rectal wall, inter- and intrafractional mobility of the prostate was not affected, which has to be taken into account in safety margin design.

Image-guided alignment procedures in radiotherapy aim at minimizing discrepancies between the planned and the real patient setup. For that purpose, we developed a 2D/3D approach which rigidly registers a computed tomography (CT) with two x-rays by maximizing the agreement in pixel intensity between the x-rays and the corresponding reconstructed radiographs from the CT. Moreover, the algorithm selects regions of interest (masks) in the x-rays based on 3D segmentations from the pre-planning stage. For validation, orthogonal x-ray pairs from different viewing directions of 80 pelvic cone-beam CT (CBCT) raw data sets were used. The 2D/3D results were compared to corresponding standard 3D/3D CBCT-to-CT alignments. Outcome over 8400 2D/3D experiments showed that parametric errors in root mean square were <0.18° (rotations) and <0.73 mm (translations), respectively, using rank correlation as intensity metric. This corresponds to a mean target registration error, related to the voxels of the lesser pelvis, of <2 mm in 94.1% of the cases. From the results we conclude that 2D/3D registration based on sequentially acquired orthogonal x-rays of the pelvis is a viable alternative to CBCT-based approaches if rigid alignment on bony anatomy is sufficient, no volumetric intra-interventional data set is required and the expected error range fits the individual treatment prescription.