Malignant pleural mesothelioma (MPM) has a complicated tumor shape, and it is difficult to radically treat advanced cases even with modern high-precision radiation therapy. In BNCT, a boron drug automatically searches tumor cells and accumulates in them, so that in principle, tumor-selective treatment is possible. BNCT was experimentally attempted on an advanced case of MPM, but the treatment had to wait until the tumor/blood (T/B) ratio of the boron drug concentration reached a sufficient value due to the influence of preceding chemotherapy. Although the lung contains air, since it is a large organ, the difference between the maximum and minimum tumor dose is larger than that of other organ tumors. As stated above, it is difficult in many cases to administer a dose to control the tumor. New ideas in physical engineering are indispensable.