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

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Abstract
The response of a rhabdomyosarcoma in an inbred strain of rat was measured after irradiation by charged-particle beams of He and Ne ions. Tumor volume regression and regrowth and tumor cure were measured and compared with results from 220-k Vp X-irradiation. RBE [relative biological effectiveness] for 20-day radiation-induced growth delay (RBE20), 50-day radiation-induced growth delay (RBE50) and 50% tumor cure probability (RBETCD50/180) were measured in the plateau (high energy) and spread-out Bragg-peak (low energy) ionization regions of both charged-particle beams. In the Ne ion peak region, RBE20, RBE50 and RBETCD50/180 values (with SD) 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 [linear energy transfer] contribution in the beam and possibly the presence of a significant hypoxic fraction in this tumor. In the Ne plateau region, RBE20 and RBE50 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 the plateau regions of the beam. This characteristic may make the Ne beam a candidate for possible use in the radiotherapy of human tumors. For the He ion spread-out Bragg-peak position, RBE20, RBE50 and RBETCD50/180 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 He ion spread-out peak region. For the He ion plateau region, RBE20 and RBE50 values of 1.0 .+-. 0.2 and 1.1 .+-. 0.2 indicate that this region can be considered a region of low-LET radiation.

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