Early Skin Reactions

Microscopic Structure of the Skin

The epithelium of the skin is arranged into several layers, with the basal layer containing the proliferating stem cells. The daughter cells of these cells ascend through the stratum spinosum, stratum granulosum, stratum lucidum and stratum corneum before shedding as dead, keratinised cells. The cells of the basal layer are the target cells in radiation driven early effects; the cells in other layers are relatively resistant. Cells take about 2 weeks from creation in the basal layer to shedding from the surface.
The dermis lies beneath the skin, containing fibroblasts, endothelial cells and connective tissue.
Numerous skin appendages are also present:

  • Hair follicles are invaginations of the epidermis, containing the bulb at its base (in the dermis or hypodermis). The bulb is responsible for the creation of hair, which consists of a progressively keratinising strand of cells.
  • Sebaceous glands are found opening into the hair follicle, and function through holocrine secretion. They lie in the dermis.
  • Merocrine (Eccrine) sweat glands are present over most of the skin (except the genitals, lips, the tympanic membrane and the nipple), and are involved in releasing water fluid in response to thermal and other stimuli. They consist of a spiraling duct that extends through the epidermis into the dermis, where it forms a coiled ball with no branching.
  • Apocrine 'sweat' glands produce a thicker secretion, and are only found on the skin of the axilla, groin, nipple and genitals. They empty into hair follicles.

Phases of Early Skin Reactions

Erythema

Erythema occurs within hours of doses over 5 Gy to the skin; it is in response to release of inflammatory cytokines from the cells of the epidermis and dermis. With fractionated radiation, erythema may not become apparent for one - two weeks.

Dry Desquamation

Dry desquamation occurs after doses of 40 Gy, and is characterised by dry, itchy skin. The skin may appear pigmented or scaling. This situations is due to a loss of function of the merocrine sweat glands as well as complete dysfunction of the basal skin layer. Dry desquamation typically heals within 1 - 2 weeks after cessation of radiotherapy.

Moist desquamation

Moist desquamation occurs when there is complete loss of the epidermis and exposure of the dermis. It is characterised by redness of the skin, with prominent ooze / slough. Infection is likely in this scenario as the protective epidermal layer has been completely lost. Moist desquamation usually requires doses in excess of 45 Gy but may occur at lower doses. Doses over 60 Gy lead to a 50% risk of a non-healing ulcer of the skin.

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