The present invention provides a therapeutic system that is widely applicable to general solid cancers, can achieve both a reduction in side effects of cancer therapy and suppression of cancer recurrence and metastasis, and requires no expensive drug such as an antibody; and a nanoparticle for internal radiation therapy. A system for internal radiation therapy of a vascular lesion site comprising: a device comprising a means for acquiring image data showing a position of a vascular lesion site, and a means for positioning a needle, which should be punctured into the vascular lesion site, at the vascular lesion site based on the image data; and a nanoparticle comprising an amphiphilic block polymer comprising a hydrophilic block having a sarcosine unit and a hydrophobic block having a lactic acid unit, and a substance labeled with a β-ray emitting nuclide. The nanoparticle for internal radiation therapy.