ESTRO 36 Abstract Book

S74 ESTRO 36 _______________________________________________________________________________________________

and range deviations are quantified for two different treatment sites. Material and Methods Based on a database of more than 1000 clinical DECT scans acquired with a single-source DECT scanner (Siemens Somatom Definition AS), 10 prostate cancer and 54 head tumor patients were selected to assess intra- and interpatient tissue diversity and its impact on SPR prediction. To evaluate age- and sex-dependent variability, the head tumor cohort was divided in children, women and men. DECT scans were converted in 79 keV pseudo-monoenergetic CT scans (MonoCTs) and SPR datasets derived by voxelwise calculations of electron density and effective atomic number using syngo.via (Siemens Healthineers). In XiO (Elekta) clinical proton treatment plans were recalculated (a) on MonoCTs using the clinical HLUT and (b) on SPR datasets to quantify range and dose differences. Results The voxelwise correlation of SPR and CT number is similar for men and women, but differs considerably between adults and children in bony tissue, likely due to the amount of calcium embedded in bones, which increases with age. Based on voxelwise SPR comparisons, the clinical HLUT predicts on average (2.2 ± 0.6) % larger SPRs for head tumor patients and (1.7 ± 0.3) % larger SPRs for prostate cases. The impact of both approaches on dose distributions is shown in Fig. 1 and 2 for an exemplary head tumor and prostate cancer patient. In the head case, the HLUT predicts a 1.7 % shorter range (2.4 mm) resulting from a 0.7 mm range underestimation in water-filled ventricles (not precisely predicted by the HLUT) and different SPR predictions for brain. A range deviation of up to 3.0 % (7.1 mm) is obtained in the prostate case, which is mainly caused by different SPR predictions for bone marrow and muscle. These range differences in single beams are not compensated in the overall treatment plan.

Conclusion In contrast to a generic HLUT, a DECT-based SPR prediction can individually consider age- and sex- dependent tissue variability in proton treatment planning. This diversity information can also provide suggestions for subgroup-specific improvements of the heuristic CT calibration. The assessment of relative SPR and dose