Ion therapy guideline (Version 2020)

Abstract

Charged particle radiotherapy can be traced back to 1954 when Lawrence Berkeley National Laboratory launched proton therapy. After experimentation with different kinds of particles, including neutrons, mesons, helium ions, and neon ions, the National Institute of Radiological Sciences in Japan started using carbon ions for cancer treatment. Proton therapy has the physical advantage of the Bragg peak, which can well realize the high-dose distribution in the tumor target volume and the low-dose distribution in surrounding normal tissue, so proton therapy has found wide applications in the field of ion radiotherapy. Nevertheless, the physical dose distribution and biological characteristics of carbon ions are significantly superior to those of other particles. Compared with the conventional photon radiotherapy, carbon ion radiotherapy stands out with its favorable radiophysical and biological advantages.1 In the current clinical practice, heavy ion radiotherapy mainly refers to the carbon ion radiotherapy. So far, although some textbooks and publications have provided references for standardized applications of ion radiotherapy, there has not yet been any consensus to guide clinical practices. With the rapid development of ion radiotherapy in China, and the increase of proton and heavy ion therapy centers, ion radiotherapy, which serves as a promising radiotherapy technology, has been applicable to more and more indications. Nevertheless, there has not yet been a guideline to guide ion therapy clinical practices based on national circumstances and the current situation of ion radiotherapy. The Chinese Medical Doctor Association Radiation Oncology Physicians Branch has organized domestic relevant experts to formulate the guidelines for ion therapy with reference to the latest research evidence, which should guide the clinical practice, and promote the popularization and application of ion therapy technology in China, so as to benefit the majority of cancer patients. From the physical point of view, photons (Χ-, γ-rays) have no charge and no mass, but ions, such as protons and heavy ions, are charged and have a certain mass. The energy release of photon therapy is the largest near the tissue surface, and the energy gradually decreases with the depth of the tissue structure. However, ion therapy will show a low-dose plateau area in the initial irradiated tissue, and the maximum energy; that is, Bragg peak, will be deposited when reaching a certain depth of tissue. According to the location and size of the tumor, the spread out Bragg peak can be modulated to accurately cover the tumor target volume, so as to achieve a higher dose of radiation to the tumor, while the surrounding normal tissue is better protected.2 Due to the linear energy transfer difference between proton and carbon ions, they have their own physical characteristics. The physical characteristics of protons are as follows: (i) nearly three-dimensional dose distribution can be formed in longitudinal and transverse directions; (ii) the penumbra edge is very sharp due to the proton energy deposition track being an approximate straight line; and (iii) the proton beam hardly deposits any dose outside the far edge of the Bragg peak by limiting the range.3 The physical characteristics of carbon ions are as follows: (i) compared with the conventional photon line, carbon ions have the reverse dose distribution characteristics; (ii) the multiple scattering effect of the carbon ion beam in the incident tissue is small, and the transverse scattering of the beam is also small; (iii) the beam distribution is flexible, because the charged particles can deflect under the action of magnetic field, so flexible and diverse beam distribution systems can be adopted according to the actual situation; and (iv) when the medium- and high-energy carbon ion beam penetrates the target material, it can produce radioisotopes by colliding with the target nucleus, and it decays and releases positron