113. Cellular survival as the common denominator between heavy-ion and photon beam radiotherapy

Abstract

Experimental data on cellular survival in vitro often serve as a radiobiological basis for modeling and interpreting the clinical results of “classical” cancer radiotherapy, using external photon and electron beams. Since the radiobiological ettectiveness (RBE) of such beams is 1, the isodose distribution, the basis for optimizing the radiotherapy planning procedure, could readily be interpreted as the distribution of surviving cells in the tumour region, if a suitable radiobiological model were available. In the case of heavy-ion beam radiotherapy where various techniques are used to spread the Bragg peak over the tumour volume, RBE is known to vary widely along the depth of the beam, and to depend on many parameters, such as the physical characteristics of the ion beam (particie energy-fluence depth distribution, ion charge, dose or fluence of initial particles) and the radiobiological descriptors of tumour and healthy tissue cell lines (characterized, e.g., by in vitro survival curves and appropriate RBE-LET dependences). To clinically evaluate results of heavy-ion radiotherapy, typically, a “clinical RBE” value is sought – a single factor by which the physical dose of the heavy ions should be multiplied to equate the clinical effect of the “classical” (i.e. photon or electron) and heavy ion beam modalities, in a similar clinical situation. The cellular track structure theory developed by Katz and co-workers, a four-parameter analytical model, has been extremely successful in quantitatively describing and predicting RBE for cellular survival in vitro after heavy ion bombardment, whereby RBE is referred to a beam of Co-60 gamma rays. Survival curves after a dose from a beam of heavy ions (specified by the charge, energy and fluence of these ions) can be calculated, once the four parameters have been simultaneously fitted to a set of experimentally measured cellular survival curves. While the basic track structure calculation concerns “track-segment” irradiation, the model provides for mixed-field irradiation along the beam depth, thus cell survival-beam depth dependences can be readily calculated for realistic treatment conditions, as a function of initial particie fluence (representing entrance dose), say, at the patient's skin. Thus, rather than multiply the local dose from heavy ions by an appropriate value of RBE to arrive at “iso-biological dose” distributions, iso-survival distributions after “conventional” treatment (represented in the model by a Co-60 beam) and after treatment with beams of heavy charged particles, may be compared. This approach and its possible advantages against the usual comparison of “physical dose*RBE = biological dose” distributions, will be discussed.