ESTRO 36 Abstract Book

S485 ESTRO 36 _______________________________________________________________________________________________

measure of overlap of the delineated volumes (1=full overlap, 0=no overlap), was calculated. Furthermore, the local observer variation was calculated for approx. 32,000 points on the median delineated surface (i.e. the surface of the volume that ≥50% of the observers included in their delineation). Local observer variation was defined as the standard deviation (SD) over the perpendicular distances between delineated surfaces at that point and is also known as local SD. The overall observer variation was defined as the root-mean-square of all local SDs. These parameters were compared between CT-only and CT+MRI delineations, for 3DCT and 4DCT (Wilcoxon signed-rank test; significance level α=0.05). Results Delineations differed substantially between observers in both CT and CT+MRI, as illustrated for the GTV in the figure. For both GTV and iGTV, the mean volume on CT+MRI was 32% smaller than on CT only (p<0.0005) (Table). Although smaller volumes were delineated on CT+MRI, the CI gen was similar in both studies (CT+MRI: 0.33, CT: 0.32). Furthermore, CT+MRI showed smaller overall observer variations (average SD=5.9 mm) in six out of eight delineated structures compared to CT only (average SD=7.2 mm). The median volumes from the (i)GTV on CT+MRI were included for 97% and 92% in the median volumes from GTV and iGTV on CT, respectively. Finally, iGTV delineation on 4DCT increased uncertainty with and without MRI, compared to GTV delineation on 3DCT. Both CT and CT+MRI delineations had regions of large local observer variation (SD>0.8) close to biliary stents and enlarged lymph nodes. This was largely due to ambiguous instructions (near stents) and poor protocol compliance (near lymph nodes).

Conclusion The availability of MRI images during target delineation of pancreatic cancer on 3DCT and 4DCT improved the interobserver variation. Furthermore, delineated volumes are smaller on CT+MRI than on CT only. An approach of 3DCT GTV delineation with margins may be preferable to 4DCT iGTV delineation since the latter increases uncertainty.

Poster: Physics track: (Quantitative) functional and biological imaging

PO-0885 Assess Tumor Voxel Dose Response (SF2) Using Multiple FDG PET Images D. Yan 1 , S. Chen 2 , D. Krauss 2 , P. Chen 2 , G. Wilson 2 1 Beaumont Health System, Radiation Oncology, Royal Oak MI, USA 2 Beaumont Health system, Radiation Oncology, Royal Oak- MI, USA Purpose or Objective To determine human tumor voxel dose response with using bio-marker (SF2) derived from multiple FDG PET images and evaluate the pattern of tumor local radioresistance and failure. Material and Methods Multiple FDG PET/CT images obtained at the pre- and weekly during the treatment (35x2Gy) for 15 HN cancer patients were used for the study. from 0 to 70Gy, for each tumor voxel, v, such that TMR(v, d) = SUV(v, d)/SUV(v, 0). TMR of each patient was constructed following voxel-by- voxel deformable PET/CT image registration. Assuming ln(TMR(v, d)) = k . ln(SF(v, d)), the optimal linear regression with unknown break-points was used to determine the tumor voxel survival fraction, SF at different treatment dose levels. Therefore, SF2(v, d), the tumor voxel survival fraction after 2Gy, was obtained and used as the tumor voxel dose response marker. Tumor voxel SF2 distribution at different treatment dose level was analyzed to evaluate the pattern of tumor voxel dose response, specifically the tumor local radioresistant pattern; meanwhile the effects of tumor voxel reoxygennation and proliferation with respect to tumor local failure were also evaluated. The tumor volumes with different SF2 cutoffs were also correlated to treatment outcome. Results Human tumor has significant inter- and intra-tumor variations in their voxel dose response (Table). Tumor voxels with the mean SF2 > 0.7 shows significant prediction power (p < 0.01) after the treatment dose of 20~40Gy on tumor local failure. Tumor local recurrence showed strong correlation to the tumor local radioresistance determined using SF2 (Figure). Two of 3 failures showed a clear reoxygennation effect after 20Gy, therefore could be potentially controlled by escalating dose to destroy the local radioresistant niches. One failure had a small (< 1cc) local radioresistant sub-volume and showed the minimal reoxygennation, therefore it may need a local ablation.

Figure: GTV delineations by the eight observers (each a different color) for 3DCT+MRI and 3DCT.