Radiobiological Effectiveness (RBE) of Megavoltage X-Ray and Electron Beams in Radiotherapy

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Abstract
Recent experiments indicate that significant differences exist in the microdosimetric properties (i.e., linear energy distributions) of megavoltage X-ray and electron beams used in radiation therapy. In particular, dose averaged values of lineal energy for 18 MeV electrons are 10-30% lower than for 10 MeV bremsstrahlung X rays, which in turn are 30-60% lower than for 250 kVp X rays. Differences of this magnitude may manifest themselves in observable radiobiological effectiveness (RBE) differences between these radiations. Cell survival data have been obtained for line DLD-1 human tumor cells on all three of the above radiation sources. Results clearly demonstrate an RBE difference between orthovoltage and megavoltage radiation (P = 0.001). A small difference is also measured in RBE between megavoltage photons and megavoltage electrons, but the difference is not statistically significant (P = 0.25). All biological, dosimetric, and microdosimetric data were obtained under nearly identical geometric conditions. These data raise interesting questions vis a vis the applicability of microdosimetric theories in the interpretation of biological effects.