Preliminary clinical observations after extended Bragg peak helium ion irradiation

R.b.e. and o.e.r. of extended-Bragg-peak helium ions: survival and development of rat embryos

Rats were exposed under aerobic or hypoxic conditions to 200-1200 rads of 60Co gamma-rays or extended-Bragg-peak helium ions on the eighth day of gestation. Uterine contents were examined on the twentieth day of gestation. At the 50 per cent embryonic survival level, helium ion r.b.e. was 1(.0) (aerobic) and 1(.2) (hypoxic). Maximum attainable gamma-ray and helium-ion o.e.r.s. were 2(.2) and 1(.7) respectively, indicating an oxygen-effect gain (o.e.g.) of 1(.2). At the 10 per cent survival level helium ion r.b.e. was 1(.1) (aerobic) and 1(.4) (hypoxic). Gamma-ray and helium-ion 0.e.r.s. were 2(.0) and 1(.5) respectively, indicating a helium ion o.e.g. of 1(.3). These data demonstrate that the small fraction of high-LET radiation present in this helium ion beam has a neglible effect on the aerobic r.b.e., but lowers the effective o.e.r. of the beam approximately 25 per cent relative to that of gamma-rays. Helium ions were significantly more effective than gamma-rays in killing embryos under hypoxic conditions, in producing congenital abnormalities under aerobic conditions, and in stunting foetal growth under both conditions.

Variation of the relative biologic effectiveness with tumor size using accelerated helium ions

The RBE of an accelerated helium ion beam was determined on murine osteosarcomas ranging in size from single cells to visibly detectable lung colonies of approximately 1 X 10(-2) mm3. That the RBE increased as tumor size increased provides some biologic basis for the selection of advanced cases of neoplasia for clinical trials using relatively high LET radiation sources.