Why beta radiation can be used in treating skin cancer but not cancers inside the body?

Beta radiation is not bone penetrating like gamma radiation. The beta has a difficult time even penetrating the epidermis layer of skin and the dosage must be great to succeed. Gamma is penetrating and kills a great amount of cells. Beta is low level and kills only local cells. You can use gamma radiation on skin cancer; but there will be excessive skin death, as well as cells in muscle and bone being damaged or killed.

I work with doctors of nuclear medicine and I also worked for the United States Department of Energy at the Savannah River Site.

I have use beta radiation to treat cancer spread through the abdomen. It is not used on skin cancers..
They are...Beta particles can be used to treat health conditions such as eye and bone cancer, and are also used as tracers. The main element for these jobs is Strontium-90. Beta particles are also used in quality control to test the thickness of an item coming through a machine (paper coming through a roller being the most notable example). Some of the beta radiation is absorbed while passing through the product. Should the product get too thick then more radiation will be absorbed. A sensor will detect this and transfer more power to the rollers which will be pushed closer together to keep the material thin. Likewise if the material gets too thin the opposite will happen.

Inverse beta decay of a radioactive tracer isotope is the source of the positrons used in positron emission tomography.

Technetium-99
Technetium-99, a gamma-free source of beta particles, is also used as a tracer in medicine. The half-life of Technetium-99 is relatively short, which limits damage to the body.