¹²⁴I-PET dosimetry in advanced differentiated thyroid cancer: Therapeutic impact

Summary

Purpose: This study evaluated the impact of ¹²⁴I-positron emission tomography (PET) dosimetry on post-primary surgery therapy in radioiodine-naive patients with advanced differentiated thyroid cancer (DTC). Patients, material, methods: In each of 28 thyroidectomized patients with high-risk DTC (one or more of pT4, pN1 or pM1), we gave 23.50 MBq of ¹²⁴I as an oral capsule and performed PET dosimetry to calculate the individualized therapeutic ¹³¹I activity that would, insofar as possible, achieve a radioiodine dose ≥100 Gy to all metastases without exceeding 2 Gy to the blood (a surrogate for bone marrow toxicity). We thus determined the absorbed lesion dose per GBq of administered ¹³¹I activity (LDpA) based on serial PET (4, 24, 48, 72 and 96 h after oral ¹²⁴I intake) and PET/computed tomography (25 h after ¹²⁴I intake) and the critical blood activity (CBA) based on blood and whole-body radiation counting (2, 4, 24, 48, 72, 96 h after ¹²⁴I intake). We compared the dosimetry-based interventions with our standard empirical protocol. Results: 25 patients had a total of 126 iodine-positive metastases. 18 (72%) of the 25 had solely iodine-avid metastases, while seven (28%) had both iodine- avid and -non-avid metastases. In two patients (8%), none of the iodine-avid metastases could have been practically treated with a sufficient radiation dose. Relative to the empirical protocol, ¹²⁴I-PET dosimetry findings changed management in 7 (25%) patients, e. g. allowing application of activities >11 GBq ¹³¹I. Further changes included implementation of hematological back-up in a patient found to be at risk of life-threatening marrow toxicity, and early multimodal therapy in 9 (32%) patients. Conclusion: ¹²⁴I-PET dosimetry is a useful routine procedure in advanced DTC and may allow safer or more effective radioiodine activities and earlier multimodal interventions than do standard empirical protocols.

Zusammenfassung

Ziel: In dieser Studie wurden die Implikationen der ¹²⁴I-Positronenemissionstomographie(PET)-Dosimetrie bei Patienten mit fortgeschrittenem differenzierten Schilddrusenkarzinom (DTC) fur die erste Radioiodtherapie (RIT) untersucht. Patienten, Material, Methoden: Bei 28 thyroidektomierten Patienten mit Hochrisiko-DTC (pT4, pN1 und/oder M1) wurde eine ¹²⁴I-PET-Dosimetrie nach oraler Gabe einer ¹²⁴I-Kapsel (23.50 MBq) durchgefuhrt. Ziel war, die individuelle ¹³¹I-Aktivitat zu errechen, die eine Radioioddosis von ≥100 Gy in allen Metastasen erzielt, ohne die Blutdosis von 2 Gy (Surrogat fur Knochenmarktoxizitat) zu uberschreiten. Die Berechnung erfolgte durch Bestimmung der absorbierten Lasionsdosis pro verabreichter ¹³¹I-Aktivitat (LDpA) auf dem Boden serieller PET- und PET/ CT-Messungen (4, 24, 25, 48, 72 und 96 h nach ¹²⁴I-Applikation) sowie der Bestimmung der kritischen Blutdosis (CBA) mittels Blutproben und Ganzkorpermessungen. Wir verglichen die Dosimetrieergebnisse mit Vorgaben der Standardprotokolle. Ergebnisse: 25 von 28 Patienten hatten 126 iodspeichernde Metastasen. 18 der 25 Patienten (72%) hatten ausschlieslich iodspeichernde Metastasen, sieben Patienten hatten zusatzlich nicht iodspeichernde Metastasen. Bei zwei Patienten (8%) hatte keine der Metastasen mit einer suffizienten Strahlendosis behandelt werden konnen. Im Vergleich zum Standardprotokoll, wurde das therapeutische Vorgehen bei sieben Patienten (25%) geandert (z. B. >11 GBq ¹³¹I appliziert). Weitere Anderungen betrafen ein hamatologisches Back-up bei einer Patientin mit dem Risiko einer lebensbedrohlichen Knochenmarktoxizitat und die Einleitung einer fruhen multimodalen Therapie bei neun Patienten (32%). Schlussfolgerung: Die ¹²⁴I-PET-Dosimetrie ermoglicht die Planung individualisierter RIT und zeigt bei fortgeschrittenen DTC deutliche Anderungen des therapeutischen Vorgehens im Vergleich zu Standardtherapieprotokollen.

Keywords

Thyroid cancer - dosimetry - PET - PET/CT - ¹²⁴I - ¹³¹I

Schlüsselwörter

Schilddrüsenkarzinom - Dosimetrie - PET - PET/CT - ¹²⁴I - ¹³¹I

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