Program Information
PBS Prostate Plan Robustness: A Tool for Patient Specific Setup Tolerance
S Tang1*, L Song1 , C Chen1 , C Chang1 , B Chon1 , H Tsai1 , E Soffen1 , O Cahlon1 , D Mah1 , (1) Procure Proton Therapy Center, Somerset, NJ
Presentations
SU-E-T-603 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose: Fiducial markers are commonly used for setup of prostate patients using orthogonal radiographs. After aligned with the markers, the displacement of the bony anatomy relative to the planned DRR can be up to 10 mm. Such offset can potentially have significant dosimetric effects because it changes the radiological path length of protons in differing amounts of bone. It is imperative to develop a method to evaluate its impact on target coverage and hence establish patient specific setup tolerance for prostate proton PBS treatment.
Methods: Prostate patients were planned in RayStation according to the PCG protocol with bi-lateral beams. The primary planning objectives are: (1) 100% of CTV receives full prescription dose; (2) 98% of the prescription dose covers at least 98% of the PTV; (3) OARs meet criteria per protocol. For each patient 108 dose perturbations were automatically generated using an in-house script, which considered the isocenter shifting in S-I and A-P directions (up to ±15 mm with an interval of 6mm) as well as the range uncertainty (±3.5%). The target coverage was evaluated on the contour shifted along with prostate to mimic the daily treatment.
Results: The minimum CTV coverage as a function of offsets in S-I and A-P directions is presented in a 2D contour map. The offsets along A-P direction generally have greater impact than along S-I direction. Both the CTV D98%>98% or CTV V98%>98% are achievable for most patients if the offset is <10 mm in either direction despite of range uncertainties.
Conclusion: We developed a method to evaluate the plan robustness for proton PBS prostate treatment. It can provide patient specific setup tolerance of bony structure offset. For our current planning approach, a 1 cm displacement is acceptable. This approach can be generalized to other target structures that move relative to bony anatomy.
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