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BEST IN PHYSICS (JOINT IMAGING-THERAPY)- Proton Beam Range Verification Using Proton Activated Fiducials and Off-Site PET

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J Cho

J Cho1*, G Ibbott1, M Gillin1, C Gonzalez-lepera2, U Titt1, O Mawlawi3, (1) Department of Radiation Physics, MD Anderson Cancer Center, Houston, Texas, (2) Department of Experimental Diagnostic Imaging, UT MD Anderson Cancer Center, Houston, TX, (3) Department of Imaging Physics, MD Anderson Cancer Ctr., Houston, TX.

WE-G-500-2 Wednesday 4:30PM - 6:00PM Room: 500 Ballroom

Purpose:
Proton range uncertainty places limits on the accuracy of proton therapy. PET imaging of activated endogenous tissue following proton treatment has been suggested as a tool for proton range verification. However, this approach is limited due to the weak activation of tissue near the proton distal fall off and the need for expensive in room PET scanners. To overcome these limitations, we investigated the use of patient implantable proton activated elements such as Zn68(p,n)Ga68 which is strongly activated by low energy protons and decays by positron emission with a 68min half life. This study compares PET signals from irradiated Zn68 enriched foil and a tissue substitute polycarbonate,(C16O3H14).

Methods:
Zn68 (>97%) enriched foils simulating implanted fiducials of 10 mm3 (10 x 10 x 0.1 mm thick) and polycarbonate sheets simulating tissue volumes of 305 mm3 (20 x 20 x 0.76 mm thick) were irradiated with a 160 MeV, 10-cm SOBP proton beam. 2 foils and 2 sheets were placed at each of four depths over the distal fall-off range (100% ~ 40% of SOBP dose). A dose of 12.5 Gy was delivered, simulating thoracic SBRT. 47 min following activation a 2.5 hour PET scan was acquired and the corresponding images were reconstructed. The PET signal intensity of each foil and sheet was then measured and normalized to the results of the shallowest depth. Monte Carlo simulation was also performed for comparison.

Results:
Normalized mean activity concentration for Zn68 (polycarbonate) at the distal fall off region at 100~99%, 99~98%, 93~81%, 62~40% dose were 100±8% (2.6±0.4%), 83±11% (1.4±0.5%), 96±48% (1.4±0.8%), 64±2% (2.4±1.0%). Monte Carlo simulation agreed with measurements and were 82% (6%), 100% (2%), 84% (1%), 38% (0%).

Conclusion:
The higher activation of Zn68 over the distal fall-off suggests the possibility of using Zn68 as proton activated fiducials for proton range verification.

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