Response of a Rat Rhabdomyosarcoma to Neon- and Helium-Ion Irradiation

Abstract

The response of a rhabdomyosarcoma tumor in an inbred strain of rat has been measured after irradiation by charged-particle beams of helium and neon ions. Tumor volume regression and regrowth and tumor cure were measured and compared with results from 220-kVp x irradiation. RBEs for 20-day radiation-induced growth delay (RBE/sub 20/), for 50-day radiation-induced growth delay (RBE/sub 50/), and for 50% tumor cure probability (RBE/sub TCD50/180/) were measured in the plateau (high energy and spread-out Bragg-peak (low energy) ionization regions of both charged-particle beams. In the neon-ion peak region, RBE/sub 20/, RBE/sub 50/, and RBE/sub TCD50/180/ values (with standard deviations) of 2.9 -+ 0.7, 2.6 -+ 0.5, and 3.1 -+ 0.6, respectively, were obtained. These high values reflect the effect of a high-LET contribution in the beam and possibly the presence of a significant hypoxic fraction in this tumor. In the neon plateau region, RBE/sub 20/ and RBE/sub 50/ values of 1.7 -+ 0.4 and 1.8 -+ 0.3 were obtained. These values are significantly lower than those in the peak region and indicate a marked difference in beam quality between the peak and plateau regions of the beam. This characteristic may make the neon beam a candidate for possible usemore » in the radiotherapy of human tumors. For the helium-ion Bragg-peak position, RBE/sub 20/, RBE/sub 50/, and RBE/sub TCD50/180/ values of 1.5 -+ 0.3, 1.4 -+ 0.3, and 1.4 -+ 0.2, respectively, were obtained. These values indicate that there is a significant increase in biological effectiveness in the helium-ion spread-out peak region. For the helium-ion plateau region, RBE/sub 20/ and RBE/sub 50/ values of 1.0 -+ 0.2 and 1.1 -+ 0.2 indicate that this region can be considered a region of low-LET radiation.« less