Radiotherapy plays an important role in the treatment of lung cancers. Highly precise radiotherapy enables the concentration of radiation dose to the tumor with sparing surrounding normal lung tissues. However, in radiotherapy for lung cancer, physicians should take caution to radiation-related lung toxicities such as radiation pneumonitis and fibrosis. The tolerant dose for normal lung in the case of whole lung irradiation is reported to be around 20 Gy, which is much lower compared to the curative dose for lung tumor. Boron neutron capture therapy can yield a large dose gradient between the tumor and the normal lung. In this chapter, we present a case report on BNCT for pleuritis carcinomatosis from lung cancer. In this clinical experience, it is deduced that BNCT has the potential to be applied to multiple or disseminated pleural metastases.