Pion in vivo dosimetry using aluminum activation

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Abstract
The method of aluminum activation to 24Na has been shown feasible as a high-LET, in vivo dosimeter for clinical pion beams at the Clinton P. Anderson Meson Physics Facility in Los Alamos. A 3×3 in. φ NaI (Tl) well detector measures the 24Na activity following exposure by windowing the 2.75 MeV photopeak. Calculations of the 24Na activity agree well with experiment if one assumes a production ratio of 0.075 24Na/stopped π− in aluminum, and an in-flight cross section of 26 mb. The activity is produced primarily by stopping pions although 15–25% of the activity is the result of neutrons. Thus, the induced activation is a good measure of high-LET dose. By comparison with high-LET dose measured by a 7.6 μ silicon detector and a Rossi chamber, the amount of high-LET dose per activation is found to be 1.35×10−6 rad/(24Na/gm Al). A clinical setup has been installed and a sample patient measurement is compared with high-LET dose calculated by treatment planning programs.

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