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Effect of Residual Errors On TCP and NTCP Values in H&N Irradiation in Photon and Proton Therapy: A Treatment Planning Study

C Cheng

C Cheng1,2*, S Srivastava3 , J Yuan1,2 , D Mattson1 , F Jesseph1 , D Mansur1,2, M Awan1,2, M Yao1,2, (1) University Hospitals Cleveland Medical Center, Cleveland, OH, (2) Case Western Reserve University, Cleveland, OH, (3)University of Arizona Cancer Center, Phoenix, AZ,


SU-I-GPD-T-173 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Immobilization and IGRT are generally employed for patients in H&N irradiations. In this study, we compare the effects of residual positioning errors from the context of TCP and NTCP calculations between VMAT and proton plans. This is the first study to look at the radiobiological effects of residual errors in H&N irradiation.

Methods: The CT image sets and contours for six H&N patients treated with 6MV VMAT were chosen for the study and re-planned with double scattering proton beams (DS). Since most random setup errors would be corrected from daily IGRT, the residual errors are expected to be in the order of 1-2 mm but also random in nature. The residual errors are simulated by isocenter shifts in the range ± 1-2 mm in the treatment plans without changing the beam parameters. DVH were calculated as well as the equivalent uniform dose (EUD), the tumor control probability (TCP) and the normal tissue complication probability (NTCP) for selected OARs for each plan and compared.

Results: For 2mm residual errors, the DS proton plan has a large change in PTV coverage compared to the VMAT plan. As a result, the TCP for DS suffers more than the VMAT plan. On the other hand, the DVH variation in the spinal cord, the L/R parotids are more pronounced for VMAT than for DS. The EUD, however, are significantly higher for VMAT than for DS.

Conclusion: Residual errors may have impact on PTV coverage and OARs doses for both the VMAT and proton plans. For small structures, the effect of residual errors may be even larger for VAMT than for proton plans as shown in this study, and hence the radiobiological effects.

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