This type of growth pattern is characterized by gradual evolution and involution to the time of puberty. An example of an organ that follows this pattern is the thymus. It reaches its greatest relative weight at birth, but its absolute weight continues to increase until the onset of puberty. In fact in the renewal phase of adulthood, it is estimated that the thymus gland is only about 5–10 % of its original size. No significant late effects from RT occur in the lymphoid system. In particular, there are no age-specific effects, perhaps because the inherent radiation sensitivity of lymphoid tissues outweighs any differential that could be observed due to age.

This type of growth pattern is characterized by rapid postnatal growth which slows down and is almost complete in adolescence. Naturally, the head and neck also follow this pattern of growth.

The brain is most sensitive to ionizing radiation in the early fetal period, but postnatally during the first few years of life. Brain growth during the first 3 years of life is not secondary to increase in number of neurons but due to axonal growth, dendritic arborization, and synaptogenesis. Myelinization, though well developed at about the second year of life, is not complete until the second to third decade of life (Dobbing and Sands 1963). By 6 years of age, the child’s brain has reached adult size. Several studies have shown greater cognitive impairment in younger children compared to older children receiving cranial irradiation (Ris et al. 2001; Conklin et al. 2008). It is for this reason that for many years prior to the era of conformal radiotherapy, in an effort to avoid the deleterious effects of radiotherapy, chemotherapy after surgery was the treatment for children <3 years of age with medulloblastoma, ependymoma, and high-grade gliomas (Duffner et al. 1993). A report from St. Jude Children’s Research Hospital showed cognitive decline after conformal radiotherapy for children under 12 years of age with low-grade glioma (Fig. 2). In fact, age at time of irradiation was more important than radiation dose in predicting cognitive decline. Children <5 years old show the most cognitive decline (Merchant et al. 2009). Consistent with this, most children who are affected with radiation-induced Moyamoya syndrome were irradiated when they were <5 years of age, a time when brain growth is rapid (Desai et al. 2006).

One study attributed a greater degree of deficient development with loss of white matter, and this was presumptively correlated with the impaired cognitive outcome of younger children (Mulhern et al. 2001). This description of white matter changes is consistent with the classic finding after radiation insult of the normal brain of a focal or diffuse area of white matter necrosis (Martins et al. 1977). It is also consistent with the premise that radiotherapy would have more effect during the time when there is greater growth of the brain. Table 3 shows the growth and development of the brain according to time of irradiation.

In adulthood, late effects of RT to the brain involve neurocognitive dysfunction which is discussed in greater detail elsewhere in this textbook.