H. Schöller (1,2) , M. Blaickner (3),
W. Hofmann (4) , H. Deutschmann (1,2) , M. Kopp (1)
, K. Wurstbauer (1) , F. Sedlmayer (1,2)
Combined 3D
Segmentation of PET- and CT-Datasets
concerning Dosimetry,
Tumour Staging and Treatment Management in Targeted Radionuclide Therapy and
External Beam Radiotherapy
1) University Clinic for Radiotherapy and Radio-Oncology,
2) radART – Institute for
research and development on Advanced Radiation Technologies at the Paracelsus
Medical University Salzburg,
3) Medical Physics
Department, Radiation Safety and Applications Seibersdorf, Austrian Research
Centers GmbH – ARC,
4) Division of Physics and Biophysics, Department of
Materials Engineering and Physics,
Volume
segmentation is a wide-spread method to identify functional and anatomical
entities on volume datasets of various imaging devices as MRI, CT, PET, or US. Diagnosis,
planning, the treatment and its evaluation are topics based on the combined
analysis of both anatomical (e.g. CT or MRI) and functional (PET, SPECT) data.
Segmentation aims in defining anatomical structures and tumours. Precise
estimation of their volumes is crucial for the accuracy of calculated absorbed
doses, as well for External Beam Radiotherapy (EBRT), as for Targeted
Radionuclide Therapy (TRT).
In TRT the
situation is somehow different compared to EBRT. Calculation of absorbed dose
is not only based on 3D-electron-density-distribution, but also on temporal and
spatial distributions of administered activities which are monitored in dynamic
PET-studies. Analysis of the characteristics provides diagnostic insight and is
supportive for planning and dosimetry of TRT. The treatment itself is performed
by administering the same tracer, now coupled to a
Beta Minus
emitting component. As TRT is an inherently multidisciplinary approach and
volume segmentation is a shared central task for TRT- and EBRT, this project is
designed as collaboration of multiple institutes.
Positron
emission tomography (PET), mainly using [18F]fluoro-Deoxyglucose (FDG) as
tracer, is widely accepted as diagnostic tool in oncology due to itsability to
provide functional information by extending spatial information from
diagnostic- or planning– CT.
Figures 1 and 2: Ecat-Exact HR and Biograph 6 PET
Scanner, Siemens Medical, Malvern (US)
Staging: Clinical assessments of PET and PET/CT showed
differences, particulary in cases of NSCLC [Baar06], in both staging and
treatment management, that influence clinical target volumes to a considerable
extent.
Planning and Evaluation of Treatment Response:
Examinations about the influence of PET/CT on treatment-planning, target
volume delineation and evaluation of EBRT have been performed for the sites
Head and Neck, Brain, Lung, Lymphoma, Gynaecology and Rectum.
One of the latest clinical studies evaluated the feasibility to
correlate intratumour-heterogeneity as visualized of 18F-FDG PET with histology
for NSCLC. It is suitable for correlating intra-tumour heterogeneity in tracer
uptake with histology [Baar08] .
Application II:
Dose-Calculation for Targeted Radionuclide Therapy (TRT)
In Targeted radionuclide therapy (TRT) unsealed radioactive compounds, consisting
of a ‘carrier’-component and an attached radionuclide enrich in tumour tissue
due to the metabolic properties of the carrier and its kinetics.
Figure 3: Rendered view of absorbed dose distribution
from SPECT/CT
segmentation of I-131 mIBG therapy of neuroblastoma [Flux06]
Segmentation of combined PET and CT is a
prerequisite for treatment planning in TRT. By
using positron- and electron-emitting radionuclides of the same element
attached to the same tracer the Beta Plus component (e.g. I-124) is suitable
for visualization of the agents biodistribution via PET and the Beta Minus
component (e.g. I-131) can be administered for therapy since it experiences the
same kinetics.
To implement patient-specific dosimetry for
targeted radiotherapy, calculations have to be performed based on spatial and
temporal distribution of the radioactive agent and established
electron-densitydistribution in the nearest neighbourhood of spots with
relevant
source-concentrations.
The Medical Physics Department, Radiation Safety
and Applications Seibersdorf of the Austrian Research Centers GmbH develops new
and deterministic calculation algorithms which are validated by
The current project aims to perform segmentation
of clinical data in order to provide the input for dose calculations in TRT.
Methods for Automatic Segmentation
A main shortcoming of
PET is that exact tumour-borders are not well defined, making visual
delineation error-prone. Available software is mainly restricted to
threshold-based-strategies, calculating either thresholds relative to maximum
or related to source-to-background-ratios. Combining processing of
co-registered CT and PET and coordination of specialized analysing-methods of
different modalities can enhance usability of PET/CT.
Although documented
segmentation-algorithms offer a great variety of methods, clinical
implementations of automatic PET-related methods are based on mainly two
approaches: The majority are (adaptive) threshold-oriented and only a minority
uses gradient-oriented procedures.
Algorithms for
automatic segmentation can be classified into three categories:
• structural techniques
, based on local structural information
• statistical
(stochastic) techniques
• hybrid techniques,
combining structural and stochastic methods
Other common
classification systems divide segmentation methods in
• Point-oriented
methods
• Edge-detection based
strategies
• Region-based
procedures
• Texture-based trials
• Model-based
strategies and object-recognition
• Atlas-guided
approaches
Our specific approach
combines PET- and CT-oriented techniques merging the CT’s high spatial
resolution and the PETs functional information.
Overlay-techniques, and
volume-projections in „Beams-Eye-View“ are available through our inhouse
developed ROKIS-Software RT² and allow for proper evaluation and clinical
assessment of new developed algorithms.
First Results from Applying
Thresholds at PET-Data
Building PET-volumes by
applying absolute thresholds allow for individual analysis of PET/CT-datasets.
Relative thresholds of about 40% of the maximum SUV-values or maximum
activity-levels are proper settings for segmenting true tumour-volumes [Erdi95],
[Erdi97], [Cier05], [Vall93], [Davi02], [Bent04]. Overlay-functions give spatial
orientation in combining PET-findings with anatomical information.
Threshold-based volumes can be projected by Maximum-Intensity-Projection (MIP)
in various treatment-field-geometries and visualized in Beams-Eye-View.
Examples shown in figs. 4 and 5 are concerning bronchus carcinoma and
gynaecological findings.
Figure
4: Threshold-limited Maximum-Intensity-Projection (MIP) of PET-Study of
bronchus carcinoma of the medial lobe T2 N2 M0 and corresponding GTV/PTVs: A:
ventral field, B: corresponding left view and C: Overlay with Sum-Projection of
co-registered CT-dataset.
Figure
5: Threshold-limited MIP of PET-Study of Gynaecological finding A: left view,
B: Corresponding ventral view , C: Corresponding ventral overlay with threshold-filtered
sum-projection of co-registered CT-dataset and field geometry.
Literature: [Bent04]: S.M. Bentzen, High-tech in radiation oncology:
should there be a ceiling? Int J Radiat Oncol Biol Phys 2004;58:320–30, [Baar06]: A.v. Baardwijk, B.G. Baumert, G. Bosmans,
M.v. Kroonenburgh, S. Stroobants, V. Grégoire, Ph. Lambin, D.De Ruisscher,
The current status of FDG-PET in tumour volume definition in radiotherapy
treatment planning, Cancer Treatment Reviews (2006) 32, 245-260, [Cier05]:
I. F. Ciernik, M. Huser, C. Burger, J. Bernard Davis, G. Szekely, Automated
functional image-guided Radiation Treatment Planning for rectal cancer,
Int. J. Radiation Oncology Biol. Phys., Vol. 62, No 3, 893-900, 2005, [Baar08]:
A. v. Baardwijk, G. Bosmans, R.J.v. Suylen, M.v. Kroonenburgh, M.
Hochstenbag, G. Geskes, P. Lambin, D.de Ruysscher, Correlation of
intra-tumour heterogeneity on 18F-FDG PET with pathologic
features in non-small cell lung cancer: A feasibility study, Radiotherapy
and Oncology 87 (2008) 55-58, [Davi02]: J. B. Davis, B. Reiner, A. Dusserre, J.Y.
Giraud, M. Boll, Quality assurance of the EORTC trial 22911. A phase III
study of post-operative external radiotherapy in pathological stage T3N0
prostatic carcinoma: the dummy run, Radiotherapy and Oncology 64 (2002)
65–73, [Erdi95]: Y. E. Erdi, Barry W. Wessels, Murray H. Loew, and
Alev K. Erdi, Threshold Estimation in Single Photon Emission Computed
Tomography and Planar Imaging for Clinical Radioimmunotherapy,
CANCERRESEARCH(SUPPL.)55. 5823s.-5826s, December 1995, [Erdi97]: Y. E. Erdi, O. Mawlawi, Steven M.
Larson, M. Imbriaco, H. Yeung, R. Finn, John L. Humm, Segmentation of Lung
Lesion Volume by Adaptive Positron Emission Tomography Image Thresholding,
Cancer, Volume 80 Issue S12 , Pages 2343 - 2753 (15 December 1997), [Krem07]:
R. Kremslehner, Computer-Assisted Localization of Mice Organs in Micro
Positron Emission Tomography, Diplomarbeit am Institut für Festkörperphysik
der Technischen Universität Wien und Austrian Research Centers, Seibersdorf,
Wien 2007. [Rein07]: D. Reiner, Deterministic algorithms for the
calculation of tumour and organ doses in targeted radionuclide therapy and
their implementation with voxel models, unveröffentlicht, Austrian Research
Centers Gmbh., Seibersdorf 2007, [Vall93]: J.F.Valley, M. Ro,
Comparison of treatment techniques for lung cancer. Radiother Oncol,
1993;28:168–73, [Flux06]: G. Flux et al, The Impact of PET and SPECT
on Dosimetry for Targeted Radionuclide Therapy. Z. Med Phys. 16 (2006)
47-59
