Download Target volume delineation with PET: two case

Investigations and research
Target volume delineation with PET:
two case studies
W.A. Weber
M. Mix
Medical Director, Department of Nuclear Medicine, University Medical Center Freiburg, Freiburg, Germany.
Head of Medical Physics, Department of Nuclear Medicine, University Medical Center Freiburg, Freiburg,
Germany.
Objective
1a
The objective of the study is to evaluate the accuracy
of different approaches for target volume delineation
on amino acid PET images in clinical studies.
Methods
G
Figure 1a. Recurrence of a grade II oligodendroglioma in a 47-year-old female patient.
There is markedly increased FET uptake in the cingulate cortex (white arrows) without a
clear corresponding abnormality in the T2-weighted MRI. Conversely, there is an area of
increased T2 signal (red arrows) in the MRI without increased FET uptake.
1b
Three different approaches for target delineation
were compared:
• Manual contouring by an experienced nuclear
medicine physician
• A n algorithm based on absolute SUV thresholds or
SUV thresholds based on maximum tumor uptake
• A source/background algorithm.
Parameters of the two automated approaches were
optimized in the first patients to be studied. The
impact of high-resolution PET reconstruction
techniques using 2 mm slices on target volume
delineation was compared with T2-weighted MRI.
Trajectories of stereotactic biopsies were fused with
PET/CT images and the intensity profile along the
trajectory was correlated with histopathologic findings.
Illustrative case
G
Figure 1b. The trajectory of the
stereotactic biopsy projected into
the maximum intensity projection
of the FET PET scan (above: anterior
projection; below: lateral projection).
The graph shows the intensity of
the PET signal along the trajectory
of the stereotactic biopsy. Biopsies
taken at positions marked in red all
demonstrated the presence of
recurrent oligodendroglioma.
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Case 1: Recurrent
oligodendroglioma demonstrated
by amino acid (FET) PET
Background
Several studies have indicated that PET imaging with
radiolabeled amino acids such [11C] methionine (MET)
or [18F]fluorethyltyrosine (FET) can improve the
detection and characterization of gliomas. Amino acid
PET can detect tumor tissue in areas that show no
or unspecific abnormalities on MRI [1]. Conversely,
abnormalities on amino acid PET are more specific
for tumor tissue than contrast enhancement or edema
on MRI [2]. Based on these data, amino acid PET is
increasingly used to define tumor extension for
planning radiation therapy or surgical interventions
[3]. However, validated algorithms for automatic
delineation of tumor extension on amino acid PET
are currently lacking.
A 47-year-old female patient with suspected
recurrence of a grade II oligodendroglioma was
studied by FET PET and MRI (Figure 1a). There
was markedly increased FET uptake in the cingulate
cortex without a clear corresponding abnormality in
the T2-weighted MRI; conversely, there was an area
of increased T2 signal lateral to the PET finding
without increased FET uptake.
The trajectory of the stereotactic biopsy was projected
onto the maximum intensity projection of the FET
PET scan (Figure 1b) and the intensity of the PET
signal was measured along the trajectory. Biopsies
taken close to the target point all demonstrated the
presence of recurrent oligodendroglioma.
Conclusion
Amino acid PET is more specific for tumor tissue
than contrast enhancement or edema on MRI. There
is a close correlation between the intensity of the PET
signal and the presence of tumor tissue, as confirmed
by biopsy. Based on the data obtained to date, amino
acid PET offers an effective method for target volume
delineation for planning radiation therapy or
surgical interventions.
Case 2: Planning stereotactic radiotherapy of lung metastases with PET
Background
2
Stereotactic radiotherapy is increasingly used for
treatment of lung metastases and small primary lung
tumors [4,5]. However, radiation treatment planning
on routine CT scans is limited by the respiratory
motion of the metastases.
Objective
The objective of the study is to evaluate the use of
PET for definition of target volumes for stereotactic
radiotherapy of lung cancer and lung metastases, and
compare the results with CT.
Methods
Patients underwent an ungated PET scan followed
by a respiratory gated CT (Figure 2: A1 and B1).
Tumor volumes were delineated on all gates of the CT
scan (Figure 2: A2, A3) using a threshold method
combined with manual corrections to eliminate vessels
and other structures. The combination of all respiratory
gates represents the volume which was covered by the
lung lesion during the whole respiratory cycle (VBT).
Tumor volume on PET (VPET) was calculated by a
thresholding technique (50% of background corrected
maximum FDG uptake, Figure 2: B2, B3). The sum
of volumes of all respiratory CT gates provides a map
(VPM) of the probability of tumor location during
the breathing cycle (Figure 2: A/B4, upper part).
The coverage of VPM by the PET-based volume
delineation was calculated by integrating the VPM
encompassed by VPET (Figure 2: A/B4, lower part).
In addition the volume of VPET which does not
overlap with VBT is calculated (VPET-O). For
comparison, tumor volume on the mid-inspiratory CT
scan, plus a safety margin of 5 mm, was determined
(VCT). The latter approach represents the current
standard for target volume delineation.
G
Figure 2. Ungated PET and respiratory-gated CT in a 74-year-old female with known
metastases. Tumor volume calculated on the PET images covered 87% of the tumor location
during the respiratory cycle, while 22% did not include tumor tissue. Tumor volume
calculated from the CT images covered 93% of the tumor location, but a much larger
percentage (160%) did not include tumor tissue.
Illustrative case
A 74-year-old female with known metastases, who was scheduled for stereotactic radiation
therapy, was examined by ungated PET and respiratory gated CT (Figure 2).
During the respiratory cycle 10 studied metastases encompassed a volume of 5.8 ± 6.0 ml
(0.6-16.5 ml, VBT). Tumor volume calculated on the PET images (VPET) was 5.7 ± 5.8 ml
(0.9-17 ml) and covered 87% of the tumor location during the respiratory cycle (VPM).
0.5 ± 0.4 ml of VPET (22% of VBT) did not include tumor tissue.
Tumor volume calculated from the CT images (VCT) covered a comparable percentage of
the tumor location (93%), but a much larger percentage did not include tumor tissue (6.3
± 4.7ml, 160% of VBT).
Conclusion
FDG-PET study can improve target volume delineation for stereotactic radiotherapy of
lung metastases [6]. The smaller target volumes provided by PET may reduce the toxicity
of stereotactic radiotherapy and may thus allow more patients to become eligible for this
non-invasive therapy.
References
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usthoven KE, Kavanagh BD, Burri SH, Chen C, Cardenes H,
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[6]Mix M, Nestle U, Grosu A, Weber WA. FDG-PET for Radiation
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