Similar to x-rays. Gamma rays are more powerful than conventional x-rays. Gamma rays can be directed from multiple sources to a point where the cancer happens to be thus sparing the normal tissue the highest dose delivered to the tumor. This form of radiation treatment is called gamma knief.
Powerful penetration. Of the three electromagnetic radiation energies , alfa rays hardly pass through a paper, beta rays will penetrate, gama rays are most powerful radiation energy even will pas through a metal, could be focused to a pin point, and kill cancer cells with out injuring adjacent normal tissue. Gamma knife is such device used to treat brain tumors. This is also called radio surgery etc.