4. Spot Scanning Using Radioactive ¹¹C Beams for Heavy-Ion Radiotherapy

Eriko Urakabe, Tatsuaki Kanai, Mitsutaka Kanazawa, Atsushi Kitagawa, Koji Noda, Takehiro Tomitani, Mitsuru Suda, Yasushi Iseki¹, Katsushi Hanawa¹, Kohsuke Sato¹, Munefumi Shimbo², Hideyuki Mizuno³, Yoichi Hirata⁴, Yasuyuki Futami⁵, Yoshihisa Iwashita⁶ and Akira Noda⁶ (¹Toshiba Co.; ²Nat. Cancer Center; ³Saitama Cancer Center; ⁴AEC Co.; ⁵Shizuoka Pref.; ⁶Kyoto Univ.)

Keywords: spot scanning, heavy-ion radiotherapy, 3-D conformal irradiation, radioactive beam, positron emitter, momentum spread

A scheme for therapeutic irradiation with ¹¹C beams has been developed in order to form and verify a three-dimensionally (3-D) conformal irradiation field for cancer radiotherapy. The stopping points of positron emitter, such as ¹¹C can be directly measured after irradiation using a positron emission tomography (PET). Yield of ¹¹C beams through projectile fragmentation process is less than 1% of the primary beams and is considerably low. We have adopted a spot scanning method, which provides high beam-utilization efficiency.

The spot scanning method at HIMAC uses no ridge filter, which is an insertion device controlling the momentum distribution, and supress the further growth of the beam size inducing the scattering at devices located in the beam line. This method adopts spot beams whose dose distributions are 3-D localized around their Bragg peak. The spot beam is scanned stepwise over the tumor region in the direction lateral to the beam by horizontal and vertical scanning magnets. In parallel to the beam direction, the range is adjusted by inserting PMMA plates (range shifter). Thus we can form a 3-D conformal irradiation field. One of the features of the scanning system is that the beam delivery is stopped during the transition time between spots for precise dose control.

Produced ¹¹C beam has a momentum spread of a few %, and picking out its momentum can control the depth profile of spot beam. Since a wide momentum gives high beam intensity and a wide distal falloff, we compromised yield and momentum spread. The selected momentum spread between 97.5% and 99.5% gives a distal falloff of 3mm. The beam-utilization efficiency is 0.4% of the primary ¹²C beam.

An irradiation field of 35 x 35 x 43 mm³ was optimized and spot scanning using ¹¹C beams was carried out. The dose distribution was measured as shown in Fig. 4. by a multi-strip parallel-plate ionization chamber (MuSIC) and acrylic plates. The flatness of + 2.3%, including an error of 1% in the detector resolution, could be obtained.

In spot scanning, it is essential to estimate the 3-D dose distribution of each spot beam. In contrast to the primary beam, estimating the dose distribution is complicated because of the wide momentum spread of ¹¹C beams. A spot beam with a wide momentum spread is considered to consist of monochromatic beam fractions of various momenta, which lead to various ranges in the human body. Each beam fraction has a different lateral beam size because of the chromatic aberration of the lens system used in beam delivery. Based on a measurement of the beam size at each momentum, we estimate the lateral and depth-dose distributions of the spot beam. The reconstructed dose distribution of the irradiation field was in good agreement with the experimental results, i.e., within + 0.2%.

Publications:
1) Urakabe, E., Kanai, T., Kanazawa, M., Kitagawa, A., Noda, K., Tomitani, T., Suda, M., Iseki, Y., Hanawa, K., Satoh, K., Shimbo, M., Mizuno, H., Hirata, Y., Futami, Y., Iwashita, Y., and Noda, A.: Jpn. J. App. Phys., 40, 2540-2548, 2001.

Fig.4. Dose distributions of the irradiation field. (a) Calculated physical and biological ones in the depth direction. Physical ones measured by the X and Y electrodes of MuSIC. (b) Physical one in the lateral directions measured at 0, 150 and 245 mm-depth in acrylic phantom by MuSIC.

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