Decay of enamel following radiotherapy is initiated due to:
Decrease in salivary flow
Direct radiations contacting the enamel
Dessication of tooth structure
Hypocalcification and pitting of enamel
Radiation caries is a rampant form of dental decay that may occur in individuals who receive a course of radiotherapy that includes exposure of the salivary glands. Patients receiving radiation therapy to oral structures have increase in Streptococcus mutans, Lactobacillus, and Candida. Caries results from changes in the salivary glands and saliva, including reduced flow, decreased pH, reduced buffering capacity, increased viscosity, and altered flora. 
White and Pharoah, pg-22