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Evaluation of Single Field Uniform Dose (SFUD) Proton Pencil Beam Scanning (PBS) Planning Strategy for Lung Tumor

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G Liu

G Liu1*, X Li1 , D Yan1 , Quan2 , X Ding1 , (1) Beaumont Health System, Royal Oak, Michigan, (2) Wuhan University, Wuhan, Hu Bei


PO-BPC-Exhibit Hall-21 (Saturday, March 5, 2016)  Room: Exhibit Hall

Purpose: We quantitatively evaluate the actual dosimetric outcome based on four different SFUD PBS planning strategies

Methods: A virtual lung patient 4DCT images were generated by inserting a sphere with diameter of 3 cm representing a rigid motion target(GTV) to the right lung of a patient 4DCT. The target motion is set in superior-inferior(SI) direction from -5mm to 5mm with step size of 1mm to simulate a rigid tumor motion. CT-Average, Maximum Intensity Project(MIP) were generated. Four proton SFUD planning strategies were evaluated based on: (1) MIP-CT; (2) CT_average with ITV override to muscle tissue(CTavg_Muscle); (3) CT average with ITV override to tumor density(CTavg_Tumor); (4) CT_average without any override density(CTavg_only). Dose distributions were recalculated on each individual phase and accumulated together to assess the ‘actual’ treatment. To estimate the impact of proton range uncertainties, +/-3.5% CT calibration curve was applied to the 4DCT phase images. Dose-Volume-Histogram (DVHs) of GTV and Heart were analyzed.

Results: Comparing the dose from initial plan with the dose accumulation: The 'actual' accumulated GTV D98 was 57.68Gy; 53.48Gy; 59.73Gy ; 60.4Gy; Heart dose D1 increases from 1.88Gy to 8.07Gy; from 2.69Gy to 2.96Gy; from 3.74Gy to 5.98Gy; from 4.38Gy to 7.82Gy; In the presence of proton range uncertainties of +/- 3.5%, CTavg_tumor based plan’s accumulated GTV D98 degraded to 57.99 Gy(+3.5%) 59.38Gy (-3.5%) CTavg_Muscle based plan’s accumulated GTV D98 degraged to 59.37Gy(+3.5%) 59.37Gy (-3.5%)

Conclusion: CTavg_Tumor and CTavg_Muscle provide the most robust GTV coverage. However, clinicians need to be careful about the dose to OARs at distal end of beam directions because the proton might stop further than the initial plans especially if plan is based on CTavg_Muscle. The study also indicates that the current SFUD PBS planning strategy might not be sufficient to compensate the CT calibration uncertainty.

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