Can radiation therapy cause cancer?
Yup. Radiation induced secondary cancers exist and vary in their site of incidence and risk of incidence based on the site of treatment. In breast cancers treated with radiation, for example , there is upto a 3 fold increase in relative risk relative to the general population for leukaemias within five years and 1.5 times the relative risk for secondary lung cancers within 15 years. 

The literature also reports 1 in 10 malignancies as being of a secondary nature, and radiotherapy amongst others is probably responsible for a fraction of those.

Despite this risk, radiotherapy is very often worth using because without it the risk of dying from the primary cancer is vastly greater than the risk of a secondary malignancy from therapy, and secondly, unlike systemic chemotherapy, the side effect profiles of radiation are more tolerable and localised. 

It is also possible to "shape" fields of irradiation to maximise the dose delivered to the tumour and to minimise exposure to normal tissue; this is called intensity modulated radiotherapy (IMRT). 

Also, a lot of the cancers radiotherapy is used for occur in older people, and the benefits of saving someone from their primary tumour again outweighs the risk of secondary malignancies later on. It is especially of concern in paediatric patients because that brings with it the risk of secondary malignancies in middle-age, but advances in intensity modulation I think has the potential to minimise that risk.