4. Improvement of the HIMAC Treatment System with the Layer-Stacking Conformal Irradiation Method

Nobuyuki Kanematsu, Tatsuaki Kanai, Yasuyuki Futami, Ken Yusa and Masahiro Endo

Keywords: heavy ion radiotherapy, 3D conformal radiotherapy, dynamic multi-leaf collimation, spread-out Bragg peak

Even though charged particles have ideal characteristics for radiotherapy, the actual particle treatments may not always be perfect in terms of dose distribution. One of the major limitations in the conventional particle therapy, where used is a range-modulation device such as a ridge filter, is that a fixed width of the spread-out Bragg peak (SOBP) has to cover the 3D target volume. Therefore, it is usually inevitable for the fixed SOBP to extend to healthy tissues. The layer-stacking method was proposed to resolve this problem by producing a variable SOBP without requiring a drastic modification of the conventional beam delivery system.

In the layer-stacking method, a target volume is virtually divided into thin layers in the depth direction and those individual layers are treated sequentially with irradiations with a common small SOBP, different beam ranges, and conformal fields. In order to deliver the sequence of irradiations automatically as a dynamic beam for daily clinical practice, the beam-monitor/device-control system was modified and an independent device-monitor/beam-interlock system was added to shut off the beam during the transition times in a sequence and in case of any control failures for safety. In addition to the range shifter and the multi-leaf collimator, the wobbler magnets are dynamically controlled in order to keep the field uniformity during the delivery. The treatment planning system was also modified to automatically determine the control sequence of the beam. Since the conventional ridge-filter method and the layer-stacking method should flawlessly coexist on the same treatment system, great care was taken for continuity and integrity with the ongoing treatments to provide the clinical-level quality assurance.

The layer-stacking method will be routinely used at NIRS in a complementary manner with the conventional method. Furthermore, it should be a reasonable upgrade option for many other facilities with a conventional particle radiotherapy machine.

Publications:
1) Kanai, T., Kawachi, K. , Matsuzawa, H. and Inada, T.: Med. Phys., 10, 344-346 (1983).
2) Futami, Y., Kanai, T., Fujita, M., Tomura, H., Higashi, A., Matsufuji, N., Miyahara, N., Endo, M. and Kawachi, K.: Nucl. Instrum. Methods A, 430, 143-153 (1999).
3) Schaffner, B., Kanai, T., Futami, Y., Shimbo, M. and Urakabe, E.: Med. Phys., 27, 716-724 (2000).
4)Kanematsu, N., Endo, M., Futami, Y., Kanai, T., Asakura, H., Oka, H. and Yusa, K.: to be published in Med. Phys..
5)Kanai ,T., Yusa, K., Kanematsu, N. and Y. Futami: submitted to Int. J. Radiat. Oncol. Biol. Phys..

Fig.4. Principle of the layer-stacking irradiation shown with the relevant devices and a patient: For each layer of the target, the thin ridge filter produces a small SOBP, the range shifter adjusts the beam range, and the multi-leaf collimator defines the field. The wobbling radius is always adjusted to keep the field uniformity. These devices are controled according to the integrated output of the beam monitor to coform a variable SOBP to the target volume (red region).