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Plan Quality Assessment of Intensity Modulated External Photon and Proton Beam Radiation for the Treatment of Borderline Resectable Pancreatic Cancer

T De La Fuente Herman*, T Nguyen, S Ahmad, T Herman, Z Richards, University of Oklahoma Health Sciences Center, Oklahoma City, OK

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose:
To examine the feasibility of improving treatment planning with intensity modulated photon and proton beam for pancreatic cancer with the aim of reducing dose to healthy organs and increasing the therapeutic gain.


Methods:
Data from ten pancreatic cancer patients treated previously with photon radiation were used to generate IMRT, VMAT, and IMPT plans. The IMRT and VMAT plans were generated with Varian Eclipse (Varian Medical Systems) with 6 MV beams and seven to nine beams or two full arcs, respectively. The IMPT plans were generated with RayStation 9A (RaySearch) using the Clinical Monte Carlo v.4.4 and were prescribed to the CTV with an added 3.5% range uncertainty + 5mm setup uncertainty. The prescription was 4950 cGy in 18 fractions. Target coverage evaluations were based on uniformity (UI), conformity (CI), and heterogeneity indices (HI). Doses to normal tissues (kidneys, liver, spinal cord, and bowel) were evaluated by dose-volume metrics. Comparisons used two-tail’s student t-test (P< 0.05).

Results:
The PTV/CTV coverage between IMRT, VMAT, and IMPT plans were similar, with IMPT having a statistically reduced conformity index due to the added 5 mm setup uncertainty margin. Dose sparing to organs at risk was significantly improved in the IMPT plans for all OARs except for the total kidneys, where VMAT plans showed statistically significant sparing of total kidney. This is due to beam orientation in IMPT plans, which were not allowed to go through bowel due to range uncertainties.

Conclusion:
The use of protons was limited by favorable anatomic geometry, which allowed at least two beam pathways to the tumor not traversing extensive small bowel with changing gas patterns.

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