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Increasing Bony Anatomy Set Up Tolerances for Low Risk Prostate Treatments Using Pencil Beam Scanning Proton Therapy

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D Hecksel

D Hecksel*, H Ramirez , M Gao , M Pankuch , Northwestern Medicine Chicago Proton Center, Warrenville, IL


SU-I-GPD-T-156 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Prostate treatments utilizing pencil beam scanning (PBS) do not utilize a smearing concept to selectively reduce dose conformality in an attempt to make the plan robust against interfractional prostate movement relative to bony anatomy. This constraint has led to tighter anatomical set up tolerances during prostate treatments using PBS. However, it may be possible to relax the anatomical set up tolerances and focus on aligning fiducial markers placed within the prostate.

Methods: The CTV contour for four low risk prostate patients was shifted in the anterior and superior direction simultaneously by 7, 9, 12 and 14 mm. This process was repeated in the posterior and inferior directions. The isocenter for each treatment field was shifted in the same magnitude and direction, and the dose was recalculated. The V95%, V90%, D95% and max dose evaluated after the shift. This procedure mimics a treatment situation where the prostate has shifted relative to the bony anatomy, but the fiducial markers can be aligned using orthogonal K-V x-rays for image guided radiation therapy (IGRT).

Results: There was no significant change in the tracked metrics for any scenario. Even for the 14 mm shift in either direction the D95% changed by less than 1.5%

Conclusion: It may be possible to increase the anatomical tolerances for daily IGRT for low risk prostate patients. The dose coverage of the plans tested showed no significant difference from the original plan. However, further study is warranted. The study requires a larger sample size before definitive tolerances can be concluded, and the case where the patient anatomy is pitched or rolled relative to the prostate must also be tested. Critical structure dose should also be evaluated to avoid a systematic treatment error leading to unwanted dose to healthy tissue.

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