Earliest feature of radiation toxicity is

Ans. a (Erythema). (Ref. Harrison, Principles of Medicine, 15th ed., 2562).RADIATION TOXICITY TO SKIN# Skin reaction can be seen with two weeks of fractionated radiotherapy (RT).# Erythema is the earliest observed feature followed by desquamation (dry/moist).# Chronic reaction can be seen starting at 6-12 months after irradiation.# Atrophic, easily damageable epidermis, hyperpigmentation, thin skin with hair loss also occur.# Rarely malignant change can occur.Educational points:Low-energy ORTHOVOLTAGE beams (150 to 400 kV) scatter when they strike the body, much like light diffuses when it strikes particles in the air resulting in more damage to adjacent normal tissues and less radiation delivered to the tumor.MEGAVOLTAGE radiation (1 MeV) has very low lateral scatter; this produces# a skin-sparing effect,# more homogeneous distribution of the radiation energy, and greater deposit of the energy in the tumor, or target volume.