SU-E-T-153: Dosimetric Impact in Moving Tumor Under Irradiation Dose by Chasing Its Motion with DMLC

Abstract

Purpose: The dosimetric error in a moving tumor by delivering the dose with the real-time tumor tracking was evaluated to evidence the clinical efficacy of its new technique. Methods: A spherical tumor phantom (2, 3 and 4cm-diam.), in which a small ionization chamber or a radiation film on coronal plane were inserted, was sinusoidally moved by using the Washington University 4D Phantom. A dynamic MLC motion pattern same with a tumor motion was independently prepared by considering the PTV margin of 0, 0.5 and 1cm, and this chased tumor phantom synchronously by the trigger of RPM system (Varian Medical Systems, Ltd. Co.). The absolute dose at center of tumor and the dose distribution were measured under the delayed dMLC chasing, which was induced by changing the beam-on trigger in RPM, and they were evaluated by comparing with its in static. Results: The significant dosimetric error was not observed with a sinusoidal motion of the amplitude less than ±0.5cm, even if the chasing delay time went over 1sec. A small PTV margin and a large tumor motion caused a large dosimetic error at the range of −5% ∼ −50%. The penumbra of dose distribution at the edge of a tumor was blurred at the error of 5% under the delayed dMLC chasing with 1sec. Conclusion: The dosimetric error in a moved tumor by chasing with dMLC was only observed in an unlikely situation, such as 1sec chasing error, which corresponded to a maximum geometrical discrepancy of 1cm between dMLC and tumor if the tumor moved with an amplitude of 2cm and a period of 4sec. The same evaluation will apply with a human respiratory motion furthermore. Japan Science Promotion Foundation, No:24601001