On-line Adaptive Planning System for Prostate IMRT Treatment

Article Outline

• Purpose/Objective(s)
• Method/Materials
• Results
• Conclusion
• Copyright

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Purpose/Objective(s) 

To develop a novel on-line adaptive planning system for the prostate IMRT treatment. The system directly re-optimizes the intensity map to re-conform the original optimal dose distribution to the “anatomy-of-the-day”.

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Method/Materials 

The system includes following major steps: (1) Optimize IMRT plan based on treatment planning CT prior to treatment; (2) Acquire on-board 3D images with either CT-on-rail or CBCT and delineate “structures-of-interest (SOIs)”; (3) Perform deformable registration between the planning and the on-board CT images. Transform the original optimal dose distribution to the “anatomy-of-the-day” using the deformation matrix. Assign the deformed dose distribution as “prescription dose distribution” and the original intensity map as initial solution; (4) Re-optimize the intensity map using linear programming; (5) Convert the new intensity maps to dynamic MLC sequences using model-based sequencing algorithm.

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Results 

The system was tested with a clinical patient case. Figure 1 shows (a) the original IMRT plan, (b) the uncorrected plan delivered to the “anatomy-of-the-day” and (c) the re-optimized plan. The GTV, bladder and rectum are shown as white, green and yellow contours, respectively. Comparing (c) to (b), the under-dose and overdose regions have been effectively corrected by re-optimizing the intensity maps. Overall, the dose distribution closely follows the “anatomy-of-the-day” in regions where SOIs' position and shape changed, and highly resembles the original plan in regions where either SOIs remain unchanged or only normal tissues exist.

The DVHs of the 3 plans is also shown in the figure. The D95 (min/max) doses to the GTV are 99.3% (90.0%/104.1%), 83% (57.6%/104.1%) and 98.3% (90.0%/103.8%), for the original, uncorrected and re-optimized plans, respectively. The DVH to the rectum appears better for the uncorrected plan, due to the gross under-dose to the nearby GTV regions. The D33 and median dose to the rectum are comparable between the original (55.4%, 14%) and re-optimized (61.0%, 16.0%) plans. The D20 (median dose) to the bladder are 49.1% (14%), 61% (18.8%), and 50% (16.4%) for the original, uncorrected and re-optimized plans, respectively.

The re-optimization process took less than 1 min. on a desktop PC and the dMLC sequencing took another min.

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Conclusion 

It is technically feasible to perform on-line re-optimization based on the “anatomy-of-the-day” and to achieve results similar to the original dose distribution. For clinical implementation, a quality assurance step is also needed.

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