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Monitoring Tumor Motion Using Beam-Eye-View EPID Imaging During IMRT: A Gradient-Based Fiducial Detection Algorithm

X Chen

X Chen, J Fan, L Chen, C-M Ma, Radiation Oncology Department, Fox Chase Cancer Center, PHILADELPHIA, PA

SU-C-BRA-6 Sunday 1:30:00 PM - 2:15:00 PM Room: Ballroom A

Purpose: Intrafractional tumor motion introduces significant dose uncertainty for radiotherapy. Using EPID imaging during IMRT delivery will provide a beam-eye-view (BEV) tumor motion observation and it also provides a cost-efficient tumor tracking method, avoiding additional imaging system. The purpose of this study is to develop a fiducial detection algorithm for tumor motion monitoring using beam-eye-view (BEV) EPID imaging during external beam IMRT treatment.

Methods: A gradient-based fiducial detection algorithm was developed which includes steps: (1) determine IMRT segment; (2) define a search region; (3) Image gradient, grad(I), within the search region was calculated and a threshold was applied to find candidates; (4) the size and orientation (obtained using radon transform) of each candidate was determined, and compared to expected values (from treatment plan) to find the fiducial. To test this algorithm, external beam breast IMRT fields were delivered to a breast phantom (size: ~13x10cm) with a gold fiducial (1x5 mm) on the Varian Trilogy. Portal images were acquired using Varian aSi1000 EPID system with the following settings: continuous mode, 3 averages per image, image size 1024x768 with resolution 0.392mm per pixel, EPID at 140cm SID (source-imager-distance).

Results: Twenty portal images acquired at different time during IMRT delivery, of which 10 images were with couch shift (4mm), have been tested and initial results show that the algorithm is able to detect the fiducial successfully (100%). The gradient-based method, which basically uses the edge information, has the advantage over intensity-based method for MV portal images since generally MV images have lower contrast compared with KV images.

Conclusion: A gradient-based fiducial algorithm has been developed and tested on breast IMRT treatment using a breast phantom. Further studies are warranted for other tumor sites such as prostate IMRT treatment.

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