This is a two-year progress report on a life span dose-response study of brain 
tumor risk at moderate to high doses of energetic protons. It was initiated 
because a joint NASA/USAF life span study of rhesus monkeys that were irradiated 
with 55-MeV protons (average surface dose, 3.5 Gy) indicated that the incidence 
of brain tumors per unit surface absorbed dose was over 19 times that of the 
human tinea capitis patients whose heads were exposed to 100 kv x-rays. 
Examination of those rats that died in the two-year interval after irradiation 
of the head revealed a linear dose-response for total head and neck tumor 
incidence in the dose range of 0-8.5 Gy. The exposed rats had a greater 
incidence of pituitary chromophobe adenomas, epithelial and mesothelial cell 
tumors than the unexposed controls but the excessive occurrence of malignant 
gliomas that was observed in the monkeys was absent in the rats. The estimated 
dose required to double the number of all types of head and neck tumors was 5.2 
Gy. The highest dose, 18 Gy, resulted in high mortality due to obstructive 
squamous metaplasia at less than 50 weeks, prompting a new study of the relative 
biological effectiveness of high energy protons in producing this lesion.