Multi-centre calibration of an adaptive thresholding method for PET-based delineation of tumour volumes in radiotherapy planning of lung cancer

Summary

Purpose: To evaluate the calibration of an adaptive thresholding algorithm (contrastoriented algorithm) for FDG PET-based delineation of tumour volumes in eleven centres with respect to scanner types and image data processing by phantom measurements. Methods: A cylindrical phantom with spheres of different diameters was filled with FDG realizing different signal-to-background ratios and scanned using 5 Siemens Biograph PET/CT scanners, 5 Philips Gemini PET/CT scanners, and one Siemens ECAT-ART PET scanner. All scans were analysed by the contrast-oriented algorithm implemented in two different software packages. For each site, the threshold SUVs of all spheres best matching the known sphere volumes were determined. Calibration parameters a and b were calculated for each combination of scanner and image-analysis software package. In addition, “scanner-typespecific” calibration curves were determined from all values obtained for each combination of scanner type and software package. Both kinds of calibration curves were used for volume delineation of the spheres. Results: Only minor differences in calibration parameters were observed for scanners of the same type (Δa ≤ 4%, Δb ≤ 14%) provided that identical imaging protocols were used whereas significant differences were found comparing calibration parameters of the ART scanner with those of scanners of different type (Δa ≤ 60%, Δb ≤ 54%). After calibration, for all scanners investigated the calculated SUV thresholds for auto-contouring did not differ significantly (all p > 0.58). The resulting sphere volumes deviated by less than –7% to +8% from the true values. Conclusion: After multi-centre calibration the use of the contrast-oriented algorithm for FDG PET-based delineation of tumour volumes in the different centres using different scanner types and specific imaging protocols is feasible.

Zusammenfassung

Ziel: Anhand von Phantommessungen in elf Zentren sollte überprüft werden, ob der kontrast- orientierte Algorithmus zur Volumenkonturierung in der FDG-PET nach Kalibrierung multizentrisch eingesetzt werden kann. Methodik: Phantommessungen eines Zylinderphantoms mit integrierten Glaskugeln verschiedener Durchmesser wurden an fünf Siemens- Biograph-PET/CT-Scannern, fünf Philips- Gemini-PET/CT-Scannern und an einem Siemens-ECAT-ART-PET-Scanner durchge - führt, wobei verschiedene Signal-zu-Hintergrund- Verhältnisse simuliert wurden. Die Auswertung erfolgte unter Anwendung des Kontrast- orientierten Algorithmus in zwei Software- Systemen. In jedem Zentrum wurden die Schwellenwert-SUVs ermittelt, die die wahren Kugelvolumina am besten wiedergaben. Hieraus wurden „zentrumsspezifische“ Werte für die Konstanten a und b der Kalibrierkurven der einzelnen Scanner nach Auswertung in beiden Software-Systemen bestimmt. Zusätzlich wurden aus allen Messwerten „scannerspezifische“ Kalibrierkurven für jede Kombination aus Scannertyp und Auswertesoftware ermittelt. Beide Arten der Kalibrierung wurden zur Konturierung der Kugelvolumina eingesetzt. Ergebnisse: Unter der Voraussetzung, dass übereinstimmende Akquisitions- und Auswerteprotokolle eingesetzt wurden, unterscheiden sich die Werte der Parameter a und b für Scanner des gleichen Typs nur wenig (Δa ≤ 4%, Δb ≤ 14%). Im Vergleich hierzu wurden für den ARTScanner signifikant unterschiedliche Werte der Parameter a und b beobachtet. Nach Kalibrierung waren die mittels Kontrastorientiertem Algorithmus errechneten SUVSchwellenwerte der verschiedenen Scanner statistisch nicht signifikant unterschiedlich (alle p > 0,58). Die konturierten Kugelvolumina zeigten Abweichungen von den wahren Werten zwischen –7% und +8%. Schlussfolgerung: Der Kontrast-orientierte Algorithmus eignet sich nach Kalibrierung der Scanner-Typen einschließlich der Akquisitions- und Auswerteprotokolle gut zur FDG-PET-basierten Zielvolumenkonturierung und ist multizentrisch einsetzbar.

Keywords

Volume delineation - 18F-FDG-PET - lung cancer - GTV definition - multi-centre trial - calibration

Schlüsselwörter

Volumenbestimmung - 18F-FDG-PET - Lungenkrebs - GTV-Definition - Multicenter-Studie - Kalibrierung

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