Pion in vivo dosimetry using aluminum activation.

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 X 3 in. phi 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 pi- 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 mu silicon detector and a Rossi chamber, the amount of high-LET dose per activation is found to be 1.35 X 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.