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Technological Innovation in Radiation Therapy and E=mc²
— On the Occasion of the 131st Annual Meeting of the Japanese Society of Medical Physics —

　It was not so long ago. According to the recollections of Dr. Masahiro Endo, during Japan’s economic bubble era, the situation of radiation therapy was as described below:
“Until around 1990, the cancers that could be cured by radiation therapy were limited to cervical cancer treated with intracavitary brachytherapy and some early-stage head and neck cancers (such as early pharyngeal cancers). Most of the cancers treated with radiation therapy were advanced cancers, making cure difficult, and even if initially effective, recurrences were common.”¹⁾
It is not hard to imagine that radiation therapy was perceived as an undesirable last resort when no other options were available.

　Over 30 years have passed since then, and the situation has changed dramatically. Today, radiation therapy stands alongside surgery and pharmacotherapy as one of the three major standard cancer treatments. This transformation was pioneered by the development and advancement of diagnostic imaging modalities such as CT, PET, and MRI beginning in the 1970s. Thanks to advances in diagnostic imaging engineering, it became possible to precisely understand the three-dimensional extent of tumors, enabling the implementation of highly-precision radiation therapy.

　High-precision radiation therapy focuses radiation doses precisely on the tumor while minimizing damage to surrounding normal tissues. Techniques such as particle therapy utilizing the Bragg peak, intensity-modulated radiation therapy (IMRT), and stereotactic radiation therapy (SRT) have been established to concentrate doses (i.e., energy deposition) on tumor sites. As a result, radiation therapy outcomes for many cancers have come to be recognized as comparable to or even superior to those of surgical treatment, indicating that “spatial selectivity of dose” has reached a certain level of maturity. Physics has greatly contributed to improving this “spatial selectivity of dose.” By the 2000s, the share of cancer patients receiving radiation therapy had expanded significantly, supported mainly by the pursuit of this spatial selectivity.

　A quarter-century has already passed since the start of this new century. If the achievements of high-precision radiation therapy can be considered the result of pursuing spatial dose selectivity, what will be the next technological innovation (paradigm shift) in radiation therapy? While preparing for this 131st Annual Meeting of the Japanese Society of Medical Physics, I contemplated this question. My thoughts turned to two therapies that have recently shown signs of widespread adoption in Japan: boron neutron capture therapy (BNCT) and targeted radionuclide therapy (TRT). Both therapies share a common feature: the combination of “cellular selectivity of dose” through drug delivery systems and “efficient tumor cell killing using nuclear energy.” The perspective shifts to a more microscopic level than ever before.

　BNCT involves first accumulating a boron-10 compound that is selectively taken up by cancer cells, then irradiating from outside the body with neutron beams. The nuclear reaction (boron neutron capture reaction) generates high-energy alpha particles and lithium particles, which destroy only the cancer cells.
TRT involves labeling drugs that are selectively taken up by cancer cells with radioactive isotopes. The emitted alpha or beta particles from radioactive disintegration then kill the cancer cells. This therapy is often referred to as nuclear medicine therapy.
While BNCT had been studied clinically in Japan since the 1960s using nuclear reactors and achieved results in treating glioblastoma, the development of accelerator-based neutron sources has rapidly accelerated its adoption.
Japan had long lagged in nuclear medicine therapy, but in recent years, there has been active research and development of therapeutic agents, especially those containing alpha-emitting isotopes, which are drawing great attention.

　The radiation used to attack cancer cells in these two therapies arises from nuclear energy derived from the mass defect before and after nuclear reactions, as expressed by Einstein’s famous equation E = mc² (radioactive disintegration can be considered a continuous nuclear reaction). The typical energy is an order of several MeV, and depending on the particle, most of this energy is deposited within the drug-accumulated cells, thereby destroying only the cancer cells — this is the “cellular selectivity of dose.”

　At JSMP131, we have planned a symposium titled “The Achievements of BNCT,” considering that about five years have passed since accelerator-based BNCT became covered by insurance in Japan. This symposium will summarize clinical experiences thus far and discuss future prospects. Additionally, we will hold a session titled “The Cutting Edge of Nuclear Medicine Therapy,” inviting experts from Japan and abroad to provide an overview of the current status and discuss future research directions.

　Moreover, with such short-range, high-energy heavy charged particles, the radiation quality, represented by the ionization density along the track, changes significantly with particle energy. Therefore, precise evaluation of the relative biological effectiveness (RBE) per unit dose becomes crucial. This requires close collaboration with radiation biology, demanding reliable models describing the relationship between dose and biological effects. Although we have long relied on the classical LQ model, we plan to present bold hypotheses to re-examine this approach at this opportunity. We also plan a session titled “Rethinking RBE Today” to explore dose evaluation that accounts for various biological modification factors affecting RBE. Given the short range of these particles, dose evaluation itself is one of the major challenges.

　I firmly believe that the development direction of radiation therapy is now at a significant turning point. It seems likely that the equation E = mc², that is, nuclear energy, will play a critical role. What transformations will radiation therapy undergo a quarter-century from now, in 2050? With great expectations, we hope this meeting will be a place where we can share dreams and discuss the future together. We sincerely look forward to your participation.

Reference
1) Masahiro Endo, “Cancer Radiation Therapy and the Role of Physics,” Iryo Kagaku Sha, 2024.

The 131th Scientific Meeting of JSMP (JRC2026)

The 131th Scientific Meeting of the Japan Society of Medical Physics will be held as follows. The conference will be held in a hybrid style of on-site and webcast. Details will be announced on the conference website as they become available.

1. Date

2. Venue

PACIFICO Yokohama

3. Contents

• JRC joint symposium
• JSMP committee program
• Educational lecture
• Oral presentations
• 5th International Conference on Radiological Physics and Technology (ICRPT)

4. Registration of participation

Executive Committee

Program Committee

Scientific Meeting

Pacifico Yokohama Conference Center
1-1-1 Minatomirai, Nishi-ku, Yokohama-shi, Kanagawa 220-0012
TEL: +81-45-221-2155

Equipment Exhibition

Pacifico Yokohama Exhibition Hall

Access map

Access to Pacifico Yokohama

Coming soon

Coming soon