Pitfalls in der [¹⁸F]-FET-PET-Diagnostik von Hirntumoren

Zusammenfassung

Die strukturelle Bildgebung mit der Magnetresonanztomografie (MRT) ist derzeit das Verfahren der ersten Wahl in der Diagnostik von Hirntumoren. In vielen Situationen sind jedoch die Möglichkeiten der Standard-MRT durch eine eingeschränkte Beurteilung des Tumorgewebes (z. B. Identifikation von kontrastmittelnegativen Tumoranteilen) und in der Differenzierung von Tumorgewebe zu unspezifischen Gewebeveränderungen (z. B. perifokales Ödem, therapieassoziierte Gliosen) eingeschränkt.

Die Positronenemissionstomografie (PET) mit radioaktiv markierten Aminosäuren, insbesondere mit O-(2-[¹⁸F]-Fluorethyl)-L-Tyrosin (FET) und [¹¹C]-Methionin (MET), haben sich in Kombination mit der MRT in den letzten Jahren in einer Vielzahl von Studien als sehr leistungsstark erwiesen. Der entscheidende Vorteil der Aminosäure-PET ist die Darstellung des stoffwechselaktiven Hirntumors unabhängig von der Permeabilität der Blut-Hirn-Schranke. Dadurch ist in der Diagnostik von Gliomen eine sehr gute Beurteilung der Lokalisation, Ausdehnung und Heterogenität des Tumors möglich. In der Therapieplanung wird die Aminosäure-PET daher zur Festlegung des Biopsieorts und zur Planung des Resektionsausmaßes sowie des Strahlentherapiefelds verwendet. Während des Therapiemonitorings ist nach einer Radio-/Chemotherapie eine Unterscheidung zwischen einer Tumorprogression von einer posttherapeutischen Veränderung (z. B. Pseudoprogression, Radionekrose) und einer Response von einer Pseudoresponse im Verlauf einer antiangiogenen Therapie möglich.

Die Vorteile von FET im Vergleich zu MET liegen in der deutlich längeren Halbwertszeit von ¹⁸Fluor, wodurch FET an verschiedene Zentren verteilt werden kann. Zusätzlich zeigt FET eine geringere Aufnahme in entzündliche Läsionen bzw. inflammatorische Zellen als MET.

Durch die rasch zunehmende Bedeutung und Verwendung der Aminosäure-PET in den meisten neuroonkologischen Zentren sollen in dieser Übersichtsarbeit die „Pitfalls“ insbesondere in der FET-PET-Diagnostik von Hirntumoren systematisch dargestellt werden. Insbesondere bei der Erstdiagnostik von solitären Raumforderungen mit oder ohne (ringförmige) Kontrastmittelaufnahme in der MRT sind neben hirneigenen Tumoren auch entzündliche, ischämische, hämorrhagische oder traumatische Läsionen zu berücksichtigen.

Abstract

In clinical neuro-oncology structural magnetic resonance imaging (MRI) is currently the investigation of choice for diagnosing brain tumors. In many situations, however, the capacity of MRI identifying non-enhancing tumor or differentiating neoplastic tissue from unspecific treatment-related changes is limited.

In the last years positron emission tomography (PET) using radiolabeled amino acids such as O-(2-[¹⁸F]-Fluoroethyl)-L-tyrosine (FET) and [¹¹C]-methionine (MET) in combination with MRI has shown a great potential for a more accurate diagnosis of gliomas. The decisive advantage of amino acid PET is a tumor-specific tracer uptake independent from the blood-brain-barrier permeability. In the diagnostics of primary brain tumors the amino acid PET is able to describe the localization, extent and heterogeneity of the metabolic active tumor, which can be used for improved targeting of biopsy as well as planning of resection and radiotherapy by better visualization of tumor margins. Furthermore, following chemo-/radiotherapy amino acid PET can be used for distinguishing tumor recurrence from pseudoprogression and tumor response from pseudoresponse during antiangiogenic treatment.

The main advantage of FET compared to MET is the longer half-life of the [¹⁸F]-label, which allows a distribution on a wide clinical scale. In addition, FET uptake appears to be more specific for tumor tissue, because there is a higher uptake of MET in inflammatory cells and tissues.

Due to the rapidly growing importance and regular use of amino acid PET in many neuro-oncology centers the aim of this review is to highlight the “pitfalls” especially of FET PET diagnostics of brain tumors. The differential diagnosis of newly diagnosed solitary cerebral lesions with or without (circular) contrast enhancement on MRI includes different types of malignant brain tumors, as well as various benign, non-neoplastic lesions such as inflammatory, ischemic, hemorrhagic or traumatic lesions.

Schlüsselwörter

Aminosäure-PET - Hirntumoren - O-(2-[¹⁸F]-Fluorethyl)-L-Tyrosin (FET)

Key words

amino acid PET - brain tumors - O-(2-[¹⁸F]-Fluorethyl)-L-Tyrosin (FET)

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