500 / 1096
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).
Figure:
GTV delineations by the eight observers (each a
different color) for 3DCT+MRI and 3DCT.
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.