Room: Room 209
Purpose: To evaluate plan quality gains when transitioning from two-dimensional (2D) planning with point-based standard loading prescriptions to three-dimensional (3D) planning with Inverse-Planning-Simulated-Annealing (IPSA) optimization algorithm to the high risk volume (HR-CTV) for Tandem-and-Ring and Vaginal Cylinder brachytherapy treatments.
Methods: Six Tandem-and-Ring and three Vaginal Cylinder plans were evaluated. Prescriptions for Tandem-and-Ring were 600cGy/fraction to points A (ICRU #38) for 2D plans and to the HR-CTV for 3D plans; for Cylinders, it was 700cGy/fraction to 5mm depth from the edge of the applicator for both cases. Plans were created with Oncentra TPS. Although patients were treated with 2D plans, they underwent CT-scanning with instruments inserted prior to treatment. The 2D plans were recreated on the 3D image sets by matching the dwell positions and times, and calibration with original values; hence, 3D dose data was acquired for the 2D plans. New 3D IPSA optimized plans were created to conform the HR-CTV but restricted by normal tissue dose thresholds. Comparisons were based on the percentage dose covering 90% volume (%D90) and doses to 2 cmÂ³ (D2cc) volumes.
Results: For 2D Tandem-and-Ring plans, two patients had larger pear-shape isodose coverage than actual HR-CTV size, thus the target was overdosed by more than 50%; one patient had larger HR-CTV and the 2D isodose coverage was 33% of the prescription. For two patients, the HR-CTVs were too large to conform the target dose without penalizing the normal structures so the 2D point and IPSA optimized plans performed similarly. On one patient, the 2D and 3D plans adequately conformed the dose although the IPSA plan calculated higher doses to the normal structures. For Cylinders, both techniques produced similar results.
Conclusion: Volumetric target for inverse 3D planning renders consistency in target coverage, and depending on the size of HR-CTV, less possible detriment to critical structures.