Response of skin to radiation. The skin is composed of the outer layer, the epidermis, which is the site of early radiation reactions, and the deeper layer, the dermis, which is the site of late radiation reactions. The epidermis (30-300 μm thick) is derived from a basal layer of actively proliferating cells, which is covered by several layers of nondividing differentiating cells to the surface, at which the most superficial keratinized cells are desquamated. From the time a newly formed cell leaves the basal layer to the time it is desquamated from the surface is about 14 days. The target cells for radiation damage are the dividing stem cells in the basal layer. Hair folicle Sweat gland Skin Layer Epidermis Dermis Subcutaneous tissue (Fat) Muscle } 0 - 0 mm Figure 10: Diagram showing skin layers The dermis is a dense connective tissue, 1 to 3 mm thick, within which scattered fibroblasts produce most of the dermal proteins. The vasculature (blood supply) of the dermis plays a major role in the radiation response. The target cells are thus the fibroblasts and the vascular endothelial cells. A few hours after doses greater than 2 Gy (200 rad) there is an early erythema similar to sunburn, which is caused by vasodilation, edema, and loss of plasma constituents from capillaries. Reactions resulting from stem cell death take longer to develop. Examples of skin responses following a course of radiation therapy include: • Initial erythema develops in the second to third week of a fractionated regimen of radiation therapy, • followed by dry or moist desquamation resulting from depletion of the basal cell population. At lower doses, islets of skin may regrow from surviving stem cells. • At higher doses, at which there are no surviving stem cells within the area treated, moist desquamation is complete, and healing must occur by migration of cells from outside the treated area. • The clinical appearance of radiation fibrosis results from atrophy leading to contraction of the irradiated area. Skin appendages are a special case. Within a few days after irradiation, the death of germinal cells results in hair dysplasia (i.e., short, thin hair). The proportion of dysplastic hair is dose-dependent. Epilation (hair loss) occurs during the third week, and regrowth may occur after 1 to 3 months. Sebaceous glands are as sensitive as hair, but sweat glands are less radiosensitive. Regenerated skin may be dry and hairless. While these effects are common-place in radiation oncology, where large doses are used...
