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A Novel Expansion Method for MRI Based Target Delineation in Prostate Radiotherapy

B Ruiz

B Ruiz1,2*, Y Feng2, R Shores3 , C Fung4, (1) Alliance Oncology, Newburyport, MA, (2) East Carolina University, Greenville, NC, (3) Alliance Oncology, Greenville, MS, (4) Beth Isreal Deaconess Medical Center, Radiation Oncology, Newburyport, MA


SU-E-J-221 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To compare a novel bladder/rectum carveout expansion method on MRI delineated prostate to standard CT and expansion based methods for maintaining prostate coverage while providing superior bladder and rectal sparing.

Methods: Ten prostate cases were planned to include four trials: MRI vs CT delineated prostate/proximal seminal vesicles, and each image modality compared to both standard expansions (8mm 3D expansion and 5mm posterior, i.e. ~8mm) and carveout method expansions (5mm 3D expansion, 4mm posterior for GTV–CTV excluding expansion into bladder/rectum followed by additional 5mm 3D expansion to PTV, i.e. ~1cm). All trials were planned to total dose 7920 cGy via IMRT. Evaluation and comparison was made using the following criteria: QUANTEC constraints for bladder/rectum including analysis of low dose regions, changes in PTV volume, total control points, and maximum hot spot.

Results: ~8mm MRI expansion consistently produced the most optimal plan with lowest total control points and best bladder/rectum sparing. However, this scheme had the smallest prostate (average 22.9% reduction) and subsequent PTV volume, consistent with prior literature. ~1cm MRI had an average PTV volume comparable to ~8mm CT at 3.79% difference. Bladder QUANTEC constraints were on average less for the ~1cm MRI as compared to the ~8mm CT and observed as statistically significant with 2.64% reduction in V65. Rectal constraints appeared to follow the same trend. Case-by-case analysis showed variation in rectal V30 with MRI delineated prostate being most favorable regardless of expansion type. ~1cm MRI and ~8mm CT had comparable plan quality.

Conclusion: MRI delineated prostate with standard expansions had the smallest PTV leading to margins that may be too tight. Bladder/rectum carveout expansion method on MRI delineated prostate was found to be superior to standard CT based methods in terms of bladder and rectal sparing while maintaining prostate coverage. Continued investigation is warranted for further validation.

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