1. Metabolic Studies of ^11C in Rabbit Thigh Muscle Implanted by Secondary Beams of HIMAC

Takehiro Tomitani, Joerg Pawelke¹, Mitsutaka Kanazawa, Kyosan Yoshikawa, Katsuya Yoshida², Mikio Sato², Toru Takami² Masahisa Koga, Yasuyuki Futami, Atsushi Kitagawa, Eriko Urakabe³, Mitsuru Suda, Tatsuaki Kanai, Hajime Matsuura⁴ Ikuhiro Shinoda⁴ and Shigeo Takizawa⁴ (¹Forschungszentrum Rossendorf, Germany ; ²School of Medicine, Chiba Univ ³Institute for Chemical Research, Kyoto Univ ⁴Simens-Asahi Medical Technologies Lid.)

Keywords: radioactive ion beam, metabolism, biological half-life

The accuracy of dose distribution of heavy ion therapy depends on the range estimation of ions in the target medium. Heavy ion range is estimated from the measured CT number by looking up a measured conversion table. The heavy ion range is a func tion of the electron density of the medium, while CT number is related to X-ray absorption coefficient and is a complex function of electron density, chemi cat composition and atomic numbers of the constituent elements. The empirical conversion leaves some ambiguities and some experimental checktng means is needed. With ⁺ emitting ion beams, we can measure the end-points of heavy ions by measuring annihilation pair 7 rays with a positron emission tomography (PET) or a positron camera. In our institute, ¹²C beams produced by HIMAC have been in use for heavy ion therapy. The secondary beam generator/separator was built in 1998. The ¹¹C beam was selected, since its LET is the same as that of "C and its half-life is 20.39 minutes, which is appropri ate for the measurement. One ambiguity with the use of the positron emitter is the metabolism of implanted ¹¹C inside living objects. We have already measured the metabolism of ¹¹C generated through autoactivation of the ¹²C beam, in which the activity level is quite low, since ¹¹C is the product of the frag mentation reaction of ¹¹C beams. With the same dose, the ¹¹C beams allow us about 50 times higher activity and thus the ¹¹C metabolism can be measured with high accuracy. We measured metabolic rates of ¹¹C inside the thigh muscle of rabbits with radioactive ¹¹C ion beams with an off-site PET and a in situ positron camera. The latter instrument altowed measurement of the earlier transition and the result was compared with that obtained with PET

Irradiation of ¹¹C beams and PET emission measurement of the rabbit were performed under anesthesia to avoid movement. The rabbit was sacrificed after the emission measurement, since anesthesia can be maintained at most for 2 only hours. After the emission measurement, five markers loaded with ¹⁸F were attached on the body surface and measured with PET then the transmission measurement was performed. The rabbit with markers was transferred to the X-CT device and the CT scan was performed to see uts anatomical structure. The markers were used to adjust two kinds of images. The same procedures were used for live and dead conditions. The ex perimental plan of rabbit experiments was approved by the committee for ethics in animal experiments in our institute.

The results of time-activity analyses measured with PET are shown in the left graph of Fig.1. Measured half-life of the dead rabbit is 20.3 minutes, while that of the live rabbit is 16.0 minutes from which biological half-life is 74 minutes, or about 4 times longer than the physical half-life. The estimated initial activity of the live one is about l/3 that of the dead one, which suggests the existence of a faster component. Due to the time for transportation of the rabbit from the irradiation site to PET site, the earlier transition cannot be measured with off-site PET. With an in situ positron camera, the earlier clearance can be observed. Counts in regionof-interest (ROI) versus time are shown in the right graph of Fig. 1. The existence of the fast component is revealed. Biological half-life of the slow component is 89 minutes. Biological half-life of the fast component is 3.6 minutes. The initial activity is 0.74 of that under dead condition. This suggests the existence of an even faster third component that could not be measured due to the finite irradiation time of about 100 seconds.

Fig.1. Time-activity curves. The left figure shows time-activity curves inside the ROI under live (lower) and dead (upper) conditions measured with off-site PET. The right figure shows time-activity curves under live (lower) and dead (upper) conditions measured with an in situ positron camera.

Publications:
1)Tomitani, T., Pawelke, J., Kanazawa, M. Yoshikawa, K., Yoshida, K., Sato, M., Takami, T., Koga, M., Futami, Y., Kitagwa, A., Urakbe, E., Suda, M., Kanai, T., Matuura, H., Shinoda, I. and Takizawa, S.: Jap. J. Med. Phys. 19, 192-195 1999.
2)Tomitani, T., Futami, Y., Iseki, Y., Kouda, S., Nishio, T., Murakami, T., Kitagawa, A., Kanazawa, IVI., Urakabe, E., Shinbo M. and Kanai, T.: Conference Record of 1999 IEEE NSS/ MIC M7-29 1999.