Radiobiological Aspects in Determination of Residual Normal Tissue Tolerance Doses for Various Re-irradiation Scenarios

Abstract

<i>Introduction. </i>The current state of re-irradiation in radiation oncology is characterized by the high heterogeneity of re-irradiation practices between institutions. The implementation of imaging methods and new irradiation techniques has created scope for the development and application of more accurate re-irradiation procedures associated with the use of radiobiological modelling, that are allowing often the replacement of palliative intent by radical. Therefore, the preparation of a planning protocol for re-irradiation is a significantly more complex process than for primary treatment planning. It requires quantified dose-volume records from primary and second series, radiobiological knowledge of the regeneration capacity of organs at risk (OaR) and using an appropriate SW-tool for modelling tumour control probability (TCP) versus normal tissue complication probability (NTCP) from individual DVH and pause between series taking into account significant differences in OaR regeneration capacity. Significant restoration takes place within 3-6 months e.g. in the skin, spinal, cord, brain, brain stem and lungs. Other tissues, e.g. kidneys, heart, bladder, have only a small regenerative capacity. This knowledge should be included in the process of preparing a re-irradiation protocol for an individual patient. <i>Purpose: </i>In this contribution we present - an overview of residual tolerance doses for selected OaR in the measure% EQD2_cum (biologically equivalent dose of 2 Gy in percents) for 15 - the most critical OaR extirped from retrospective studies (e.g.%EQD2_cumfor brain stem, spin cord and bladder are 170%, 140%, 125%, respectively). <i>Material and methods:</i> A description of simultaneous determination of residual doses in re-irradiation with an original OaR regeneration model (REG_pause) by the authors of paper included into the calculation of the normal tissue complication probability (NTCP) for individual irradiation scenarios of re-irradiation using the “BioGray” program developed in the workplace of authors. <i>Results:</i> A demonstration of the benefits of the tumour control probability (TCP) versus NTCP prediction depending on the location and volume of the clinical tumour volume (CTV) in the primary and second series. <i>Conclusion: </i>The use of the methodology of radiobiological modelling brings a shift from paradigm of verbalism and estimations in the management of re-irradiation to quantitative evaluation of these processes and utilization of translation research knowledge linked to the current technological possibilities of application IMRT, VMAT, SRS/SBRT and proton therapy.