Lowest effective ¹³¹I activity for thyroid remnant ablation of differentiated thyroid cancer patients

 

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Summary

Aim: A theoretical dosimetry-based model was applied to estimate the lowest effective radioiodine activity for thyroid remnant ablation of low-risk differentiated thyroid cancer patients. Patients, methods: The model is based on the distribution of the absorbed (radiation) dose per administered radioiodine activity and the absorbed dose threshold of 300 Gy for thyroid remnants, the level believed to destroy most thyroid remnants. For this purpose, 124I PET/CT images of 49 thyroid-ectomised patients were retrospectively analysed to measure the distribution of the (average) absorbed doses to thyroid remnant per administered ¹³¹I activity. The fraction of thyroid remnants that received at least 300 Gy was determined for standard activities between 0.37 and 5.55 GBq. The lower activity was considered to be equally effective to that obtained with higher activity if the (absolute) fraction difference was below 5%. Results: A total of 62 thyroid remnants were included. The medians and ranges (in parentheses) for the absorbed dose per unit ¹³¹I activity were 359 Gy/GBq (34 to 1825 Gy/ GBq). The fractions of thyroid remnants receiving more than 300 Gy at different therapy activities (within parentheses) were 60% (1.11 GBq), 76% (1.85 GBq), 79% (2.22 GBq), and 81–82% for activities between 2.59 and 3.70 GBq. The therapy activity of 1.11 GBq is considerably less effective than that of 1.85 or 2.22 GBq; therapy activities were equally effective in the range between 2.22 to 3.70 GBq. Conclusion: On the basis of the model and the patients' data included, the lowest effective therapy activity appears to be approximately 2.2 GBq to ablate thyroid remnants. The results of this study may help to guide the design of prospective clinical studies.

Zusammenfassung

Ziel: Ein theoretisches Dosimetrie-basiertes Modell wurde angewandt, um die niedrigste effektive Radioiodaktivität zur Ablation des Schilddrüsenrestes bei Niedrig-Risiko-Patienten abzuschätzen. Patienten, Methode: Das Model basiert auf die Energiedosisverteilung pro applizierte Radioiodaktivität und die allgemein akzeptierte Schwellenwertdosis von 300 Gy für Schilddrüsenrestgewebe. Zu diesem Zweck wurden 124I-PET/CT-Bilder von 49 thyreoidektomierte Patienten retrospektiv ausgewertet, um die mittleren Energiedosen in Schilddrüsenreste pro applizierte ¹³¹I-Aktivität zu ermitteln. Unter der Annahme einer hohen Ablationsrate bei einer Zieldosis von 300 Gy wurde der Anteil der Schilddrüsenreste, die mindestens 300 Gy deponiert haben, für Standardaktivitäten zwischen 0,37 bis 5,55 GBq errechnet. Betrug für verschiedene Aktivitäten die (absolute) Anteilsdifferenz weniger als 5%, so wurde die geringere Aktivität als gleichermaßen effektiv wie die höhere angesehen. Ergebnis: Insgesamt wurden 62 Schilddrüsenreste eingeschlossen. Der Median und die Spannbreite (innerhalb der Klammern) für die Energiedosis pro applizierte ¹³¹I-Aktivität betrugen 359 Gy/GBq (34–1825 Gy/GBq). Die Anteile der Schilddrüsenreste oberhalb der 300-Gy Schwellenwertgrenze in Abhängigkeit von der applizierten Therapieaktivität (innerhalb der Klammern) waren 60% (1,11 GBq), 76% (1,85 GBq), 79% (2,22 GBq) und 81–82% zwischen 2,59 und 3,70 GBq. Die Therapieaktivität von 1,11 GBq war deutlich weniger effektiv als die Therapieaktivitäten von 1,85 oder 2,22 GBq; Therapieaktivitäten zwischen 2,22 bis 3,70 GBq waren gleichermaßen effektiv. Schlussfolgerung: Auf der Grundlage des Modells und der eingeschlossenen Patientendaten kann vermutet werden, dass die niedrigste effektive Radioiodaktivität zur Ablation des Schilddrüsenrestes etwa 2,2 GBq beträgt. Die Ergebnisse dieser Arbeit können für die Planung prospektiver klinischer Studien von Nutzen sein.

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