ESTRO 37 Abstract book

S1093

ESTRO 37

EP-2003 On the impact of dose variations in contrast to mean dose on EQDx and α/β estimation N. Niebuhr 1,2,3 , A. Pfaffenberger 1,3 1 German Cancer Research Center DKFZ, Medical Physics in Radiation Oncology, Heidelberg, Germany 2 Heidelberg University, Department of Physics and Astronomy, Heidelberg, Germany 3 National Center for Radiation Research in Oncology, Heidelberg Institute for Radiooncology, Heidelberg, Germany Purpose or Objective Multiple factors in radiotherapy cause local deviations between delivered and planned doses. Daily imaging enables the estimation of the delivered dose, yet spatial and temporal accumulation and averaging of the dose leads to a loss of the newly gained information of the impact of (fractional) dose variations on clinical parameters. Here, we investigate the impact of dose variations on the delivered EQDx as well as on caused uncertainties in the estimation of α/β values. Material and Methods The analyses theoretically investigate the impact of dose variations of different amplitudes and in different fractionation schemes (conventional vs. hypofractiona- tion) on the EQDx and the α/β estimation. The EQD2 formalism was adapted to compare a constant (mean) x Gy per fraction schedule to one that accounts for each fraction dose. This enables the comparison of treatment with constant dose x with that of varying doses around a mean of x (Figure 1). Furthermore, assuming a constant α-value, the ratio of the accumulated mean dose to the fractional delivered dose was studied as an estimate of the impact of dose variation on the prediction accuracy of α/β (Figure 2). Results Figure 1 shows that dose variations induce deviations in EQDx that increase with higher variation amplitude and mean dose, and with lower α/β as well as for hypofractionation scenarios. The difference from EQDx from constant mean dose to that from the same mean dose but with a standard deviation of 50% exceeds 15%. For lower mean doses, the effect is smaller, yet it is in the same order of magnitude (Figure 1, right).

Assuming a constant mean dose in contrast to accounting for dose variations systematically underestimates the resulting α/β (Figure 2). The estimated impact on the α/β estimation is independent of the mean fraction dose and the number of fractions.The underestimation is by 2%, 5% and 20% for 12.5%, 25% and 50% dose standard deviation, respectively, and represents an additional factor in the uncertainty of α/β estimation to the uncertainty from deviations of delivered to planned total dose (Figure 2, right).

Conclusion Neglecting occurring dose variations by assuming a constantly delivered planned dose or averaging of the delivered dose leads to deviations and uncertainties in EQDx and α/β predictions. This leads to an underestimation of the actually applied biological dose. Furthermore, the assumption of a mean dose (spatial or temporal) causing a certain observed outcome rather than considering dose variation, results in an underestimation of the derived α/β value. Both factors can especially affect normal tissues in dose gradient regions, and our findings suggest that the magnitude of these effects should be investigated in patient cases. We conclude that local analysis of the dose can reduce uncertainties in dose response modeling which are induced by spatial as well as temporal averaging and this may create an additional benefit from daily imaging information. EP-2004 Cardiac Radiation Dose Reconstruction in the Study of Late Effects:A Comparison of Different Methods G. Ntentas 1 , M.C. Aznar 2 , D.C. Hodgson 3 , R.M. Howell 4 , M.V. Maraldo 5 , S. Ahmed 3 , A. Ng 3 , S.C. Darby 1 , D.J. Cutter 1 1 University of Oxford, Nuffield Department of Population Health, Oxford, United Kingdom 2 The University of Manchester, School of Medical Sciences - Faculty of Biology- Medicine and Health, Manchester, United Kingdom 3 Princess Margaret Cancer Centre, Department of Radiation Oncology, Toronto, Canada 4 The University of Texas, Radiation Dosimetry Services, Houston, USA 5 Rigshospitalet, Department of Oncology, Copenhagen, Denmark Purpose or Objective Radiation-related cardiovascular disease can occur several decades after radiotherapy. Cardiac radiation dose information is not easily available for most historic patient cohorts. Therefore, to investigate radiation- related cardiac late effects, it is necessary to reconstruct the doses delivered to the heart retrospectively, often without individual CT planning scans. Several reconstruc-