Radiotherapy is one of the major options currently for cancer treatment. Radiotherapy causes cellular damage inducing cell death, which is expected to be selective for tumor cells. However, side effects that alter the surrounding normal tissue are often hard to be avoided. When radiation involves the hypothalamic–pituitary axis, growth hormone deficiency (GHD) is frequently induced, causing developmental and metabolic-related diseases in childhood cancer survivors. Growth hormone (GH) replacement therapy has been used for these patients and has been shown to be safe in general. However, there are some debating for its long-term safety due to the known roles of GH in inducing cell growth, which could be related to cancer recurrence. In addition, studies have shown that GH is involved in the development of resistance to chemotherapy and radiotherapy through various mechanisms. In this review, we will first discuss the effects of GHD induced after radiotherapy and the safety of the GH replacement treatment. Then, we will discuss the role of the GH–IGF-1 axis in radioresistance via a mechanism of improving DNA repair.