Only skin and mucous membranes have a more significant boron-10 uptake than cancer cells, so the treatment plan must direct patient positioning in such a manner that these tissues are irradiated as little as possible.
Good responses to BNCT treatment after one fraction
Because we can modulate the neutron field in accelerator-based BNCT devices, we can often achieve a relatively homogenous dose in large tumours. On the other hand, we currently do not yet know the optimal BNCT dose for killing cancerous cells. Hence, clinicians prescribe relatively high doses based on the tolerance dose of normal tissues.
Nevertheless, Dr Koivunoro emphasises that – in contrast with traditional radiotherapy – patients with cancer types indicated for BNCT have typically excellent responses to BNCT treatment even after one fraction, dose-volume histograms showing parts of the tumour receiving, for example, doses of 10 Gy to 35 Gy, while the spinal cord irradiated with less than 4 Gy.
Status of Boron Neutron Capture Therapy in Japan
Dr Akira Matsumura is the president of Ibaraki Prefectural University of Health Sciences and a neurosurgeon who entered the radiation therapy field early on and has considerable experience with both proton therapy and boron neutron capture therapy.
He recalls how, for years, Japanese oncologists used two reactors in Japan, the Tokai (JAEA JRR-4) and Kyoto (KURRI, KUR), of which the former was shut down due to the earthquake, and the latter will be shortly closed.
Japan has several accelerator-based BNCT facilities currently up and running, the most of any country in the world. Two clinics and one university have a cyclotron-based BNCT device, specifically the Osaka Med & Pharm University, Southern Tohoku BNCT research center and Kyoto University.
The second type of a clinically operating BNCT device is the one in the National Cancer Center Hospital in Tokyo, which has a high current of low-energy protons. This device has, interestingly, a vertical beam. It uses a LINAC to generate its protons which slam into a lithium target to produce epithermal neutrons.
The third class of BNCT system is installed at Dr Matsumura’s “home base” at the University of Tsukuba. It is also based on a LINAC, with a medium proton energy of eight MeV and a medium proton current of 2 mA. The device employs a barium target and a horizontal beam.
New Japanese BNCT Facilities Under Construction
As proof of what a BNCT hotbed Japan really is, three additional facilities are currently under construction – or nearing it. These are the Edogawa Hospital BNCT Center, the Shonan-Kamakura Hospital and the Nagoya University.
The Shonan-Kamakura device uses an electrostatic accelerator to produce a low-proton beam with a very high proton current and has a lithium target. The Nagoya University BNCT facility will also use an electrostatic-based device with a lithium target, but with half of the Shonan-Kamakura’s proton current, namely 15 mA.
Summary of ongoing Japanese clinical trials
With all its investment in BNCT technology, it is little wonder that Japanese oncologists probably lead in the number and scope of BNCT clinical trials.