The impact of radiotherapy on the hypothalamo-pituitary axis: old vs new radiotherapy techniques

Multimodality cancer therapy has led to remarkable improvements in survival of childhood and young adult cancer, with survival rates exceeding 85%. Such remission rates come with their own adverse sequelea or 'late effects'. Although the cause of these late effects is multi-factorial, radiation-related adverse effects are one of the most prevalent. Hypopituitarism is a recognised complication of irradiation of brain tumours distant to the hypothalamo-pituitary (HP) axis when the axis is included within the exposed field. Much of the data concerning the development of hypopituitarism, however, relate to early forms of photon-based radiotherapy. In this narrative review, we discuss advances in individual radiotherapy techniques currently used in treating brain tumours and their theoretical benefits based primarily on dosimetric studies. Increasingly precise radiation techniques, including advances in the delivery of photons (i.e. intensity-modulated radiotherapy) and proton beam therapy, are now available options. The premise behind these newer techniques is to reduce the dose and volume of normal tissue irradiated whilst maintaining an effective radiation dose to target tissue. When treating brain tumours distant to the HP axis the expectation, based upon dosimetric studies, is that newer forms of radiotherapy will less frequently involve the HP axis in the exposed field, and where incorporated within the field it will be exposed to a lower radiotherapy dosage. Intuitively the dosimetric studies should translate into significant reductions in the prevalence of HP dysfunction. These data are promising; however, to date there are minimal robust clinical data to determine if the theoretical benefits of these newer techniques on HP dysfunction is to be realised.