Frankenstein Conquers the World (1965) on Netflix

The heart of Frankenstein's monster is stolen from Dr. Frankenstein and taken by U-boat to Japan. The heart is immortal. When caught in the atomic bombing of Hiroshima, it begins to grow. Several years later a disfigured wild-boy is discovered stealing peoples livestock and eating a school's pet rabbit. Drs. Bowen and Togami keep the boy under surveillance, handcuffed. He grows at an enormous rate and eventually escapes his chains by chewing off his hand, which survives and crawls around until it dies of malnutrition. Frankenstein (as Bowen and Togami choose to call him) is blamed for destruction of the countryside near Mount Fuji, but Akira Takarada swears he saw another monster at the scene of the disaster.