The most common type of radiation caries is:
Clinically, three types of radiation caries exists.

The most common is widespread superficial lesions attacking buccal, occlusal, incisal, and palatal surfaces. Another type involves primarily the cementum and dentin in the cervical region. These lesions may progress around the teeth circumferentially and result in loss of the crown. 
The third type appears as a dark pigmentation of the entire crown. The incisal edges may be markedly worn. 
Combinations of all these lesions develop in some patients. The location, rapid course, and widespread attack distinguish radiation caries. 
There is also evidence that radiation caries is more likely to lead to periapical inflammatory lesions, if the periapical bone received a high dose of radiation.

Reference: White and Pharoah's Oral Radiology Principles and Interpretation 7th edition page no 22
Cervical surface involvement of teeth
Widespread superficial lesion attacking buccal, occlusal, incisal and palatal surfaces
Spreading to occlusal and palatal surfaces
Dark pigmentation of the entire crown