(Wednesday, 7/17/2019) 10:30 AM - 11:00 AM
Room: Exhibit Hall | Forum 1
Purpose: Site-specific and/or beam path specific reduction of beam range uncertainty could reduce range margins and decrease complications. This study investigates the proton range uncertainty introduced by CT HU to RSP conversion for prostate cancer treatment with biological phantoms.
Methods: A 26×14×12 cm³ biological phantom, made of fresh animal tissues mimicking the pelvic anatomies in prostate patients, was scanned with single energy CT scanner. A passive scattering beam perpendicular to phantom surface was irradiated through the phantom laterally. The beam with range of 29 cm with 10 cm SOBP was chosen so that exiting doses can be recorded by ion-chamber array device (MatriXX PT). Various thickness of solid water slabs was inserted between the phantom and MatriXX PT to obtain the information of distal dose falloffs on SOBPs. The water equivalent path lengths (WEPLs) of the phantom were calculated in 2-dimensional plane using the dose extinction method. The measured WEPLs were then compared to the predicted values from CT-based ray tracing method.
Results: For the pelvic-like phantom, range uncertainty varies with tissue types traversed by proton beams. An underestimation of (1.2%, 2.5%) and (2.4%, 3.5%) in WEPL was observed for soft tissues and bone edges, respectively. An overestimation of 0.8% to 1.3% was observed in WEPL when soft tissues and bones coexist.
Conclusion: In this study, proton range uncertainty between CT prediction and range measurements ranges from 1.2% up to 3.5% depending on tissue types. In case of soft tissue and bone mixing, the uncertainty is only 0.8% to 1.3%. This suggests that the current 3.5% proton range uncertainty used in clinic can be reduced by at least 1% even for single energy CT in prostate treatment.
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