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Physiologically Gated Microbeam Radiation Therapy Using Electronically Controlled Field Emission X-Ray Source Array

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P Chtcheprov

P Chtcheprov*, M Hadsell, L Burk, R Ger, L Zhang, H Yuan, Y Lee, S Chang, J Lu, O Zhou, University of North Carolina, Chapel Hill, NC

SU-D-144-4 Sunday 2:05PM - 3:00PM Room: 144

Purpose:
Microbeam radiation therapy (MRT) is an experimental pre-clinical technique using arrays of microscopically-thin, low-energy X-ray radiation to treat radio-resistant, deep-seated tumors. All previous MRT experiments were performed using synchrotron radiation. We have developed a compact MRT small animal system using carbon nanotube (CNT) field emission x-ray technology. Our purpose is to incorporate respiratory gating to the MRT system, minimizing motion blurring and increasing the peak-to-valley-dose ratio (PVDR), which is important for normal tissue sparing.

Methods:
A prototype CNT MRT system was employed. The intrinsically divergent radiation is collimated into ~300um microbeams using an external collimator. Parallel microbeam planes were delivered by translating the object perpendicular to the microbeam plane in a step-and-shoot fashion. The device generated an average entrance dose rate of 1Gy/minute. For respiratory-gated irradiation, electron field emission from CNT cathodes was synchronized with the respiratory cycle of the mouse so x-ray radiation is only extracted during the motionless phase of respiration.
A phantom simulating mouse respiration used a servo motor pushing on a pressure sensor in a typical breathing pattern. A control program used the signal to trigger x-ray irradiation during the motionless phase. Two experiments were run: a single line irradiation at 160 kVp delivering 0.9 Gy/min for 5 minutes, and three such lines of irradiation separated by 900um. Dose profiles were measured with EBT2 Gafchromic dosimetry films.

Results:
A single beam showed a FWTM increase from 638 to 1088um with un-gated motion. With gating, the FWTM was 669um, similar to the motionless case.
PVDR quantifies MRT effectiveness in the case of multiple lines. Motion reduces PVDR 50 percent; with gating PVDR is almost entirely recovered, 5.5 percent higher than without motion.

Conclusion:
Gated MRT largely eliminates microbeam broadening from physiological motion, maintaining high PVDR. The technique increases precision and will be tested during MRT treatments.

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